The journey of iron-based electrocatalytic materials for nitrogen reduction reaction: from current status to future prospects

Abstract

Ammonia (NH₃) is the most important chemical and carbon-free energy carrier, which is currently mainly produced by the energy-intensive Haber–Bosch process at high temperatures and pressures. In the spirit of global carbon neutrality and sustainable development, electrochemical nitrogen reduction reaction (NRR) is expected as a promising alternative to the traditional Haber–Bosch process for industrial ammonia production due to both low energy consumption and zero emission of carbon dioxide. However, further breakthroughs in electrochemical NRR strongly depend on the efficiency and selectivity of electrocatalysts. The element Fe has been found to be an important component in nitrogenase enzymes, generating extensive research interest. As one of the most cost-effective and abundant transition metals, iron-based materials have presented great potential for electrochemical NRR. In this review, we summarize recent advances in iron-based materials (including its oxides, chalcogenides, borides, single Fe-atom catalysts, and bimetallic catalysts especially for Fe–Mo-based compounds) towards highly efficient N₂-to-NH₃ conversion under ambient conditions. Based on recent theoretical and experimental studies, we focus on the structure–property relationship. Strategies to boost NRR performances and perspectives for future developments are comprehensively presented in synthetic protocols, structure modification, activity/selectivity enhancement, and reaction mechanisms to provide insightful guidance for the field of NRR studies.

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Yi-Han Wang

Yi-Han Wang is pursuing his B.S. degree at Dalian University of Technology. His research focuses on the design and preparation of electrocatalysts for NRR under the supervision of Dr Xue-Zhi Song.

Ji-Hong Dong

Ji-Hong Dong received her B.S. degree in chemical engineering from Dalian University of Technology. Now, she is pursuing the M.S. degree under the supervision of Associate Prof. Xue-Zhi Song. Her research focuses on the design and preparation of electrocatalysts for NRR.

Zhenquan Tan

Zhenquan Tan is a Full Professor at School of Chemical Engineering in Dalian University of Technology (China). He received his B.S. degree in chemistry at Peking University (China) in 2002, and received his PhD degree in chemistry from Tohoku University (Japan) in 2008. His current research interests are focused on the inorganic nanomaterials and their applications in photo-/electrocatalysis, energy storage and conversion, and anti-tumor nanodrug delivery systems.

Xiao-Feng Wang

Xiao-Feng Wang is an associate professor in Dalian University of Technology, China. She received her PhD degree in Material Physics and Chemistry from Shandong University, China in 2014. She was a visiting scholar at A. J. Drexel Nanomaterials Institute, Drexel University, during 2018. Her research interests focus on nanomaterials and their applications in gas sensors and catalysis.

Xue-Zhi Song

Xue-Zhi Song received his B.S. degree from Lanzhou University (China) in 2009. He completed his PhD degree in Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (China) in 2015. He is currently an Associate Professor in Dalian University of Technology (China). His research interest focuses on functional inorganic nanomaterials for electrocatalysis in HER, OER, and NRR.

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1. Introduction

Ammonia is one of the most principal chemicals and carbon-free energy carriers and also plays an indispensable role in sustaining life.¹ Its worldwide annual production is estimated to be 150–200 million tons, providing a hint of insufficiency due to the booming human demands.² Despite the fact that N₂ makes up more than 78% of the atmosphere, it cannot be directly utilized by industry or living organisms.³ Modern industrial large-scale ammonia production still relies on the Haber–Bosch process, which was invented in the 20th century and has contributed to human society for hundreds of years.⁴ However, the Haber–Bosch technology entails thermocatalytic conversion of N₂ and H₂ at high temperature (400–500 °C) and intense pressure (200–300 atm).^5–7 These conditions determine that Haber–Bosch process is extremely energy-intensive, accounting for 2% of the global energy consumption, accompanied by large greenhouse gas emissions (300–400 million tons of CO₂ annually).^8,9 Therefore, a greener and more efficient NH₃ synthesis method under mild conditions is desired to support the world's sustainable development and carbon neutrality goals.^10,11

Currently, it has been proven that biocatalysis, photocatalysis, and electrocatalysis are clean pathways for ammonia synthesis from nitrogen reduction.^12–15 Electrochemical synthesis of ammonia is considered the most promising alternative to the Haber–Bosch process, due to its mild and stable reaction conditions, abundant hydrogen source (direct extraction from electrolyte instead of fossil fuels), zero CO₂ emissions, and flexible and easily-controlled production process by adjusting external parameters.¹⁶ In the electrochemical ammonia synthesis, NH₃ is produced from N₂ and H₂O under ambient conditions by inputting electrical energy, with the catalyst playing a decisive role. In fact, the breakthrough development of the electrochemical ammonia synthesis is hampered by some inherent limitations: (1) the ultra-low solubility of N₂ in water (6.8 × 10⁻⁴ M, 1.01 × 10⁵ Pa, 298 K) is not conducive to maintaining high reactant concentrations, resulting in the unsatisfying ammonia production rate. (2) Low Faraday efficiency (FE). The improvement of FE is hindered by competing hydrogen evolution reactions (HER).¹⁷ The electrochemical NRR and HER are both thermodynamically available under reaction conditions. Not only that but HER is preferred from a kinetic point of view because it is a feasible two-electron process. By comparison, electrocatalytic NRR is a multi-step kinetic retardation process, involving six protons and six electrons as well as the activation of inert nitrogen bonds.¹⁸ (3) The stability and ammonia yield rate of the catalyst is challenging to fulfill the requirements of industrial production.¹⁹

From the molecular mechanistic level, the primary challenge of NRR results from the inertness of the N≡N triple bond, which is the strongest among the diatomic molecules and lacks a permanent dipole.²⁰ A high energy input of 945 kJ mol⁻¹ is required to cleave the N≡N triple bond, which well explains the harsh conditions of the Haber–Bosch process.²¹ The kinetically difficult electron transfer reaction and the thermodynamically forbidden protonation of N₂ to N₂H⁺ also make the electrochemical NRR more difficult.²² Researchers have developed numerous catalysts with good performances to address the above challenges of electrochemical NRR, including noble metals, transition metal (TM) compounds, and metal-free materials.^23–33 Among these materials, iron-based catalysts stand out as a non-noble metal protagonist with the advantages of low cost, large-scale industrial synthesis, and high catalytic efficiency. Iron (Fe) is the only element that coexists among the three biological nitrogen-fixing enzymes, suggesting that iron plays an important role in NRR.²¹ Moreover, Fe is an active metal easy to be compounded with other elements, such as modifying it into nitrides, hydroxides, oxides, sulfides, and phosphides, which may exhibit distinguished catalytic activation and stability. More importantly, the electronic configuration of Fe provides good solutions to activate the N₂ molecule. As a transition metal, the Fe atom has an electronic configuration of 3d⁶4s², indicating four unpaired d-spin electrons. These abundant d-orbital electrons and unoccupied orbitals activate the inert N≡N triple bond by driving the well-known donation-acceptance process.^34–40 To be more precise, the empty or half-occupied orbitals in Fe allow σ donation to accommodate electrons from N₂ bonding orbitals (σ orbitals). Meanwhile, the affluent d-electrons in Fe atoms could be injected into the antibonding orbitals (π* orbitals) in N₂ by back-donation. As a result, the loss of electrons from the bonding orbitals and the injection of electrons into antibonding orbitals greatly destabilize N₂ and expedite the break of the N≡N bond. It is acknowledged that the NRR activity of the transition metals is usually closely related to the occupancy of e_g and t_2g orbitals in the catalyst. Fe atom has a manipulable local spin state of t2g4eg2. Both experimental and theoretical analysis indicate that the heightened e_g electrons penetration to the antibonding π-orbital of N₂ and regulation of the adsorption strength of nitrogen species can be realized by customized spin-polarization configuration.⁴¹ Moreover, the electron jump between high-spin iron and ferrous in the appropriate Fe lattice provides good electronic conductivity and poor adsorption of protons, which may lead to more ideal efficiency and selectivity towards NRR.^42,43 From a macroscopic view in the design of catalysts, Fe is easily modified for various construction strategies, including vacancy engineering, heteroatom doping, facet control, single atom dispersion, and even metal–organic frameworks (MOFs).^21,44–46 These strategies usually enhance N₂ adsorption by redistribution of charge density in the catalyst and provide more active sites to improve catalytic efficiency of NRR.^47,48

Based on these facts, this review introduces the recent progress of iron-based catalysts for electrocatalytic NRR under ambient conditions. Firstly, we systematically present the recent advances on iron catalysts for NRR according to the types of iron compounds, which are iron oxides, iron sulfides, Fe-containing bimetallic materials, especially iron-molybdenum compounds, iron borides, and single iron atoms catalysts (Fig. 1). Furthermore, we analyze the available approaches to design advanced electronic and interface structures for different types of iron-based catalysts and relationships between structures and properties from experimental and theoretical perspectives. Finally, we prospect the challenges and potential strategies for further enhancement of NRR performance in terms of FE, ammonia yield, and selectivity.

Fig. 1 Scheme organization of the main contents of this review. Iron oxides: reproduced with permission: copyright 2021, Elsevier.⁴⁹ Copyright 2022, Wiley-VCH.⁵⁰ Copyright 2022, Royal Society of Chemistry.⁵¹ Iron chalcogenides: reproduced with permission: copyright 2021, Elsevier.⁵² Copyright 2021, Wiley-VCH.⁵³ Fe–Mo-based catalysts: reproduced with permission: copyright 2019, Elsevier.⁵⁴ Copyright 2020, Royal Society of Chemistry.⁵⁵ Copyright 2020, American Chemical Society.⁵⁶ Iron borides: reproduced with permission: copyright 2020, Elsevier.⁵⁷ Copyright 2019, Royal Society of Chemistry.⁵⁸ Fe-based SACs : reproduced with permission: copyright 2022, Wiley-VCH.^59,60

2. Fundamental principles of the electrocatalytic NRR

Understanding the characteristics of nitrogen molecules and the reaction mechanism of NRR is the basis of superior NRR electrocatalyst design.

2.1 Properties of N₂

The very stable nitrogen molecule is formed by the linear combination of two N atoms, each of them has five valence electrons, arranged in the configuration of 2s²2p³. According to molecular orbital theory, the valent orbitals of N₂ molecules are composed of four bonding orbitals (σ_2s, σ_2p, and two π) and four antibonding orbitals , in which two π* orbitals and one orbital display an empty state, while other orbitals are fully occupied by the coupled spin electrons (Fig. 2A).⁶¹ The three occupied σ_2p and two π orbital bonds also contribute to the high-energy triple bond.⁶² The large energy gap (10.82 eV) between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of N₂, the high ionization potential (15.58 eV), and the negative electron affinity (−1.9 eV) all account for the resistance of N₂ to Lewis acid/base and electron-transfer processes.^63–68 In the view of thermodynamics, NRR is practicable with the overall negative Gibbs free energy. In fact, as the first step of the overall reaction, the adsorption and activation of N₂ are intractable since their non-polarity and high stability. The initial endothermic protonation of N₂ to form N₂H⁺ (ΔH⁰ = +37.6 kJ mol⁻¹) is thermodynamically forbidden. Another challenge is the kinetically complex nitrogen hydrogenation process, which involves multiple electron transfers and complex intermediates.⁴⁴ These intrinsic features render the N₂ fixation via electro-reduction of N₂ to NH₃ of great challenge and difficulty. Nevertheless, the N≡N bond can be weakened by σ donation and π back-donation between N₂ and Fe atoms (Fig. 2B).

Fig. 2 (A) Diagram of N atomic orbitals and N₂ molecular orbital diagram. Reproduced with permission: copyright 2018, Wiley-VCH.⁶⁶ (B) Binding modes of N₂ molecules with transition metals. Reproduced with permission: copyright 2018, American Association for the Advancement of Science.⁷⁹ (C) Reaction pathways for the electrochemical nitrogen reduction to ammonia. Reproduced with permission: copyright 2021, Elsevier.¹²

This will provide guidance for the rational design of catalytic active sites starting from addressing the intrinsic inertness of N₂.

2.2 NRR mechanisms

The net NRR can be expressed as the equations below:

In acidic media

N₂ + 6H⁺ + 6e⁻ → 2NH₃

In basic media

N₂ + 6H₂O + 6e⁻ → 2NH₃ + 6OH⁻

The detailed NRR process mainly involves the following three steps: (i) the chemisorption of N₂ molecules on the active sites; (ii) the dissociation and hydrogenation of nitrogen; and (iii) the desorption of the ammonia molecules from the catalyst surface. According to the adsorption configurations and the hydrogenation reaction sequences of nitrogen, two basic pathways have been proposed (i) the dissociative mechanism and (ii) the associative mechanism (Fig. 2C).^69–73

2.2.1 Dissociative mechanism. In the typical dissociative pathway, the nitrogen molecule is cleaved into two independent nitrogen atoms and adsorbed on the catalyst surface for subsequent hydrogenation. *NH₃ is derived from *N via *NH and *NH₂, after three successive hydrogenation steps. However, the direct breaking of the N≡N bond has a large energy barrier and usually occurs only at high temperatures (such as in the Haber–Bosch process), which is unlikely to be a favorable pathway for electrochemical NRR under ambient conditions.

2.2.2 Associative mechanism. In contrast, the gradual cleavage of the N≡N bond provides thermodynamically favorable pathways, namely, associative pathways, where the hydrogenation of N₂ occurs prior to the breaking of the N≡N bond. The two different adsorption modes (end-on and side-on) initiate varying associative pathways. It can be classified into three possible reaction pathways depending on the binding pattern of N₂ and the hydrogenation sequence. As for the end-on adsorbed N₂, it accepts an electron and proton to generate *N₂H, and then both the alternative pathway and the distal pathway can proceed.
Associative-alternative pathway. In the alternative pathway, both N atoms are reduced successively with alternative hydrogenation. The N≡N bond remains unbroken until the last hydrogenation and two NH₃ molecules are almost concurrently desorbed.
Associative-distal pathway. In the distal pathway, the distal N atom accomplishes hydrogenation to form and release an NH₃ molecule before any hydrogenation of the proximal N atom.
Associative-enzymatic pathway. In this particular case, N₂ is absorbed in a side-on adsorption mode and each nitrogen atom is hydrogenated enzymatically, which is similar to the alternating pathway. Notably, this pathway is favored in the presence of biological nitrogenases and some catalysts that have a strong bind with N₂. The hydrazine (N₂H₄), formed as an intermediate of the enzymatic pathway, is mostly a minor product thanks to the high energy barrier of its desorption from the catalyst surface.¹² The enzymatic routes that occur on iron-based catalyst surfaces were reported in several works.^74–76

In addition to the classical NRR pathways mentioned above, novel pathways have been discovered in practice by in situ spectroscopies.^77,78 Shao's group proposed a new two-step reaction pathway for NRR, based on surface-enhanced infrared-absorption spectroscopy and differential electrochemical mass spectrometry data.⁷⁸ In such a pathway, the absorbed nitrogen molecule reduces to by a two-electron transfer process, followed by its desorption and decomposition to form NH₃ (3N₂H₂ → 2NH₃ + 2N₂). These characterization techniques and findings provide important tools and new insights into the reaction mechanism of NRR on metal surfaces, which can inspire further understanding of the NRR mechanism and rational design of the more advanced electrocatalysts.

3. Iron-based electrocatalytic materials

Electrocatalysis is a surface-sensitive process, and surface and subsurface configurations directly affect catalytic performance.^80,81 Various material modification strategies, such as vacancies, defects, high-index facets (HIFs), and strains have been developed to design NRR catalysts, including iron-based catalysts.^44,82–86 These strategies will be introduced, respectively in the category/section to which they belong. Amongst them, defect and vacancy engineering (e.g., oxygen vacancies (OVs), sulfur vacancies (SVs), and tellurium vacancies (TeVs)) are more common and effective to generate active sites and enhance intrinsic NRR activity for the following reasons. Firstly, by creating defects and vacancies in the catalyst, the electronic structure of the electrocatalyst can be manipulated to improve conductivity and promote N₂ adsorption and activation, thereby reducing the activation energy barrier.^87,88 Besides, the coordination environment of the metal centers can be adjusted to produce low-coordinated and unsaturated metal sites, thus increasing the extrinsic catalytic activity of the electrocatalyst by increasing the number of active sites.⁸⁹

3.1 Iron oxide

Metal oxides are widely used in the field of chemical catalysis studies, due to their low cost, wide source, tunable activity, and environment benignity.⁹⁰ Interestingly, plenty of works about iron oxides and their derivatives for electrochemical ammonia synthesis have been reported, providing a wealth of experience for the further development and application of iron oxides as efficient NRR catalysts.^91–93 Iron oxides are suggested to offer good long-term durability and cycling stability in a neutral electrochemical environment.⁴⁹ Also, some works have pointed to the beneficial effects of pairing iron oxides with highly conductive catalyst substrates to accelerate electron transfer efficiency and achieve good electrochemical performance.^94,95 Iron oxides have the natural defect of OVs, tailoring their electronic structures and charge distribution for increasing the quantity of active sites with a modulated energy barrier. On the one hand, surface OVs could accommodate the electron-accumulating Lewis base as well as catalytically reactive unsaturated sites to guide NRR to proceed through pathways with low energy barriers. On the other hand, theoretical studies revealed that the electrons tend to accumulate around the valence band maximum of metal oxides with OVs, and thus facilitate N₂ adsorption and N₂ activation by stretching the N≡N triple bond.^96–99

Stoichiometric Fe₃O₄ and Fe₂O₃ and their derived structures have been extensively studied as iron oxides for NRR electrocatalysis. Hu and coworkers reported a Fe/Fe₃O₄ hybrid catalyst via thermal oxidization of Fe foil and subsequent in situ electrochemical reduction (Fig. 3B).¹⁰⁰ X-ray photoelectron spectroscopy (XPS) (Fig. 3C) revealed the composition and chemical state changes of the surface layer in the three samples. The Fe⁰ on the Fe foil surface was completely oxidized and then reduced to both Fe⁰ and oxidic Fe components by the subsequent electrochemical reduction. Furthermore, the presence of α-Fe and Fe₃O₄ crystallites in the as-prepared catalyst was confirmed by X-ray diffraction (XRD) (Fig. 3D). Such Fe/Fe₃O₄ catalyst demonstrated significantly better NRR selectivity than several commercial iron materials (Fig. 3A), reaching an NH₃ yield of 0.19 μg cm⁻² h⁻¹ and a FE of 8.29% in the phosphate buffer solution. Similarly, Xie et al. developed a one-step green synthesis of Fe–Fe₃O₄ catalyst via the reduction of FeSO₄ in an aqueous solution using KBH₄.⁵⁰ High-resolution transmission electron microscope (HRTEM) image (Fig. 3E) depicted the presence of atom-vacancy defects in Fe–Fe₃O₄ nanocrystals, which resulted in the excellent NRR performance by enhancing the nitrogen adhesive ability. Electrochemical impedance spectroscopy (EIS) suggested that Fe–Fe₃O₄ had a significantly enhanced charge transfer capacity. During five cycle tests, no obvious changes in the current density curves (Fig. 3F), NH₃ yield rate, and FE (Fig. 3G) were observed, indicating the excellent stability of Fe–Fe₃O₄ for NRR. Porous structures with high specific surface area have been built to enhance the catalytic performance by increasing active surface area. In a recent study, Ying et al. reported porous Fe₃O₄ nanosheets via regeneration from deep eutectic solvent (DES) and subsequent annealing (Fig. 3H).⁴⁹ A series of annealing temperatures was applied, sample Fe₃O₄-300, which was sintered at 300 °C had the largest specific surface area (88.30 m² g⁻¹), exhibiting an NH₃ yield rate of 12.09 μg h⁻¹ mg_cat⁻¹ and a remarkable FE of 34.38% (Fig. 3I).

Fig. 3 (A) FE of Fe/Fe₃O₄ and several commercial iron materials. (B) SEM images of the Fe foil, the 300 °C oxidized sample, and the sample after the reduction. XPS spectra of the Fe 2p peaks (C) and grazing-incidence XRD patterns (D) of the three samples. Reproduced with permission: copyright 2018, American Chemical Society.¹⁰⁰ (E) HRTEM images of Fe–Fe₃O₄ catalyst. Chronoamperometry curves (F), NH₃ production rate and FE (G) of Fe–Fe₃O₄ catalyst during the 5 cycles tests. Reproduced with permission: copyright 2022, Wiley-VCH.⁵⁰ (H) Synthetic route schematic of porous Fe₃O₄ nanosheets. (I) Average NH₃ production rate and FEs (at −0.1 V) of samples were annealed at different temperatures. Reproduced with permission: copyright 2021, Elsevier.⁴⁹

Except for the widely studied aqueous electrolyte systems, advances of iron-based catalysts have also been made in non-aqueous electrolyte systems to address some inherent difficulties.^18,101,102 Ammonia electro-synthesis in aqueous systems suffer from low N₂ solubility and competitive HER.¹⁰³ Typically, the reduction of water to hydrogen occurs in the same region of potential with NRR, resulting in low FEs.¹⁰⁴ Calculations and experiments showed that nitrogen is much more soluble in some ionic liquids (ILs) than in water.^105,106 The improved N₂ solubility in IL electrolytes is an effective way to increase N₂ adsorption at the active sites.¹¹ Besides, ILs are hydrophobic and can serve as excellent non-aqueous substances because they can lower the water content at an optimal level to effectively restrain the evolution of H₂.¹⁰⁴ MacFarlane and coworkers practically explored the NRR performance of Fe deposited working electrode substrates (including fluorine-doped tin oxide glass, nickel foam, and stainless steel cloth) in a hydrophobic, high N₂-solubility IL electrolyte ([C₄mpyr][eFAP] and [P_6,6,6,14][eFAP], respectively).¹⁰⁴ The strong interaction between N₂ and [eFAP]⁻ was proposed by DFT calculations, which explained the high solubility of N₂. This work also suggests the influence of the viscosity of ILs on electrochemical performance, that lower viscosity can support higher mass transport to produce higher current densities and yields. Although this NRR system achieved an incredibly high FE of 60%, the reduction currents obtained on low surface area substrates are too small and the yield needs to be improved for practical applications. In another work, MacFarlane's group further focused on the rational electrode–electrolyte design and demonstrated the effectiveness of using highly fluorinated solvents to realize efficient electrochemical ammonia synthesis.¹⁰⁷ The mixture of 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether and [C₄mpyr][eFAP] were chosen and characterized to determine the optimum solvent–IL ratio to achieve a maximum conductivity of 1.95 mS cm⁻¹ and electrochemical window of 3.40 V, which was ideal for NRR. The core–shell α-Fe@Fe₃O₄ catalyst was applied in such an IL mixture and accomplished an ammonia yield rate of ∼2.35 × 10⁻¹¹ mol s⁻¹ cm_ECSA⁻² and NRR selectivity of ∼32%.

Thus far, most reported studies of iron oxide catalysts also focused on Fe₂O₃.¹⁰⁰ High-index facets are usually represented by a set of Miller indices {hkl} with at least one index greater than 1. These facets have a high density of exposed low-coordination atoms and are therefore more open and active than the tightly packed {111} and {100} low-index facets.¹⁰⁸ The energy barrier of HER can be elevated at some specific HIFs, while a more feasible NRR pathway is opened to improve NRR selectivity.¹⁰⁹ Xie et al. combined the facet modulation and nanoarray structure design to improve the NRR performance of α-Fe₂O₃.¹¹⁰ The α-Fe₂O₃ nanorod arrays (NAs) with exposed (104) HIF offered abundant unsaturated Fe atomic dangling bonds as the NRR active sites. Moreover, the nanoarray structure achieved an enhanced electrochemical surface area (ECSA) to fully display the active sites and maintain sufficient mass diffusion. Thus, the selectively exposed (104) surface coupled with the high ECSA of the nanoarray structure achieves the superior electrocatalytic performance of α-Fe₂O₃ NAs. Unfortunately, current studies on HIFs to optimize the electrocatalytic selectivity and activity are popular in noble metal-based materials and their alloys, such as Pt and Au.^87,111,112 The lattice facets control engineering on iron materials still needs to be explored.

Given the fact that adjusting the OV concentration is an effective method for improving the oxide catalyst properties for NRR, various strategies have been applied for this target, such as annealing and heteroatom doping. High-temperature annealing is a feasible way to increase the concentration of OVs. Cui et al. reported an o-Fe₂O₃–Ar/CNT catalyst that was annealed under argon (Ar) utilizing commercial hematite nanopowder as raw material.⁴⁴ It was found that the NRR performance of o-Fe₂O₃–Ar with the highest OV concentration (12.26%) was better than that of o-Fe₂O₃-air and Fe₂O₃-untreated. After 16 hours of the chronoamperometry test, the NH₃ production rate and FE decreased significantly, suggesting that the stability of the structure needs to be improved. However, high-temperature annealing is an energy-intensive process, which is typically carried out in a reducing atmosphere. Thus, the introduction of OVs by doping with low valence metals is highly desirable to be developed as a relatively environmentally friendly synthesis method. Yu et al. developed zinc-doped Fe₂O₃ nanoparticles via a wet chemical route at only 160 °C.¹¹³ Zinc has a similar ion radius to that of Fe³⁺, which is able to induce OVs in Fe₂O₃ nanoparticles. In neutral media, it offers a maximum FE of 10.4% and an NH₃ formation rate of 15.1 μg h⁻¹ mg_cat⁻¹ at −0.5 V vs. RHE. Furthermore, Zhang et al. were the first to propose a task-specific ionic liquid strategy for OV-rich α-Fe₂O₃ nanocubes (denoted as Fe₂O₃-IL), achieving an NH₃ production rate of 32.13 μg h⁻¹ mg_cat⁻¹ with a FE of 6.63% at 0.3 V vs. RHE.¹¹⁴ The ionic liquid n-octylammonium formate (OAF) simultaneously worked as the reaction medium and structure-directing template for growing α-Fe₂O₃ nanocubes while the reductant formic anion of ionic liquid drove rich OVs in situ in α-Fe₂O₃. This innovative approach allows the OV concentration to be adjusted under mild conditions. Additionally, the low surface tensions of OAF compared with water resulted in a higher nucleation rate of Fe₂O₃, than the growth rate. Consequently, Fe₂O₃-IL had a much smaller particle size of 23.2 ± 1.8 nm than that of Fe₂O₃ synthesized in a water medium. As a result, a larger specific surface area and more active sites were delivered in the catalytic process.

Most of the catalysts we mentioned above were prepared with the additive of surfactants through solution methods. However, the surfactants had a passivation effect on the catalyst surface, which decreased the activity of the catalyst. Therefore, developing a strategy to construct a clean surface could be critical for achieving high activity.¹¹⁵ Herein, Wang et al. reported porous Fe₂O₃ nanorods grown on carbon cloth (p-Fe₂O₃/CC).¹¹⁶ The EDX element mapping image suggested that Fe and O elements with a ratio of about 2/3 are uniformly distributed throughout the nanorods. With the assistance of Na₂SO₄ as a structure-directing agent, the self-supported structure of p-Fe₂O₃/CC with a large surface area provided abundant active sites and accessible channels for promoting mass/charge transfer during the NRR process. Furthermore, Wang et al. successfully synthesized clean-surface Fe₂O₃ nanoparticles without any surfactants or structure-directing agents.¹¹⁵ Fe(NO₃)₃ was dispersed in glycerine for prior hydrothermal treatment, followed by high-temperature calcination. This clean-surface catalyst exhibited better performance with a high yield of NH₃ 22 μg h⁻¹ mg_cat⁻¹ and a FE of 3.5% at −0.5 V vs. RHE. The as-synthesized catalyst showed good catalytic and structural stability even without a binder.

The conventional preparation of iron oxide materials mostly requires high-temperature annealing, which is not convenient and highly energy intensive. Preparation strategies at room temperature need more exploration. In addition, iron oxide suffers from poor electrical conductivity, which is against the high catalytic efficiency. Good electron donors can be introduced to modulate the electronic structure of the catalyst. Many of the current iron oxide electrocatalysts for NRR struggle with low ammonia yields or FEs. Further theoretical studies are recommended to explore their catalytic intrinsic principles and thus guide the catalyst improvement.

3.2 Iron chalcogenides

3.2.1 Iron sulfide. S is one of the most important elements in the structure and composition of natural nitrogenase, which is primarily in the form of metal–S units. Metal sulfides have proven to have great potential for clean electrocatalytic nitrogen fixation.^2,117–120 As mentioned above, Fe is the basic entity in FeMo, FeFe, and FeV nitrogenases, providing some insight into the design of NRR catalysts. The typical Fe₄S₄ clusters, which are found in these nitrogenases as electron transfer carriers for reducing N₂, further highlight the significance of iron sulfide complexes for ammonia synthesis.¹²¹ Suitable two-dimensional (2D) iron sulfide has a unique layered structure with extraordinary electrochemical properties and strong anisotropy, which can be used as a platform for hosting active sites.⁵² In addition, pyrite (FeS₂) is the most common sulfide mineral on earth, being conducive to economically viable large-scale industrial production.¹²² Zhao et al. reported an impressive catalyst of NRR based on DES (polyethylene glycol-200/thiourea)-derived Fe₃S₄ nanosheets via a one-step solvothermal synthesis (Fig. 4A).¹²³ The crumpled and mesoporous structure might be formed due to the shape control of surfactant, which is favorable for fast ion/molecule transports. The Fe₃S₄ nanosheets presented a novel NH₃ yield rate of 75.4 μg h⁻¹ mg_cat⁻¹ in 0.1 M HCl at −0.4 V, much better than those of CoS₂ and NiS₂ prepared by the same method, highlighting the importance of iron element for the NRR. Xiong et al. reported a self-supportive iron sulfide (FeS_x) NRR electrocatalyst.¹²⁴ The H₂S-plasma treatment sulfurized the Fe surface to afford the self-supportive FeS_x layer on the Fe foam (FeS_x/Fe), which exhibited a high NH₃ yield rate of 4.13 × 10⁻¹⁰ mol s⁻¹ cm⁻² and a FE of 17.6%. The FeS_x/Fe electrocatalyst demonstrated excellent stability, with a negligible decrease in current density after 20 h of electrolysis.

Fig. 4 (A) Schematic of the formation of Fe₃S₄ in the PEG based DES. Reproduced with permission: copyright 2018, Royal Society of Chemistry¹²³ (B) chronoamperometry curve of Vs-FePS₃ in N₂-saturated electrolyte with the addition of SCN⁻ ion. Inset: FEs with and without the SCN⁻ ion. (C) XPS spectra of S 2p of Vs-FePS₃ before and after NRR. Reproduced with permission: copyright 2021, Elsevier.⁵² (D) Schematic diagram of coordination configuration in FeS₂. (E) Free energy profile for NRR in Fe atoms as the active sites. Reproduced with permission: copyright 2020, American Chemical Society.¹²⁶ (F) Nyquist plots and (G) ammonia yield and FE of FeS₂@GO, GO and FeS₂ at −0.3 V in N₂-saturated 0.1 M HCl solution. (H) Six-time recycling test of FeS₂@GO. Reproduced with permission: copyright 2021, American Chemical Society.¹²⁸

Inducing surface defects is an available strategy to increase the active sites of NRR via heteroatom doping. In a recent work, a surface-defective FeS₂ catalyst was obtained by sulfurization of stainless-steel foil at 500 °C.¹²⁵ The fitting of the XPS spectrum sulfur S 2p peaks and the HRTEM image proved the existence of abundant SVs. Meanwhile, such defects were considered to be the active sites for the stable and efficient adsorption of dinitrogen molecules. It was strongly advised that the generation of an active hydrogen atom was primarily caused by the exposed (210) facet of iron pyrite. The stainless-steel-based catalyst possessed a considerably higher FE of 14.6% in contrast to the pure iron-based catalyst in 0.1 M Li₂SO₄ at −0.2 V. Because of the existence of Cr species, the SS-based electrode with anti-corrosion properties presented a superior catalytic stability under acidic conditions. Hou et al. reported a porous exfoliated FePS₃ nanosheet with rich SVs (Vs-FePS3 NSs), which was formed through electrochemical exfoliation and hydrogenation.⁵² Vs-FePS₃ NSs showed significant activity in the electrocatalytic NRR with a high NH₃ yield rate of 3.88 μg h⁻¹ cm⁻² at −0.25 V and a FE of 12.36% at −0.20 V versus RHE due to their rich SV and unique 2D morphology. To identify the actual active sites of Vs-FePS₃ NSs for NRR, SCN⁻ ion was used to act as a poisoning reagent for metal-centered active sites. A significant decrease appeared in the current density and FE (from 12.36% to 5.31%), indicating that the active Fe site was effectively shielded (Fig. 4B). The shift of S 2p main peaks to higher binding energies in high-resolution XPS spectra suggested that the SVs boosted the hydrogenation of N₂ (Fig. 4C).

The suitable ligand configuration of the active sites plays a significant role in enhancing the catalytic performance. It is found that a metal sulfur cluster with four permanent ligands in a twisted tetrahedral ligation shell is an effective active center of NRR. This coordination structure is well demonstrated in FeS₂, where the NRR process proceeds preferably on Fe atoms (Fig. 4D). In a recent study, a FeS₂ NRR catalyst was synthesized via the one-step hydrothermal method.¹²⁶ The catalyst exhibited an outstanding NRR performance, achieving a high NH₃ yield rate of 37.2 μg h⁻¹ mg_cat⁻¹ and a FE of 11.2% at −0.5 V versus RHE. The current density and original structure of FeS₂ had no significant change after 12 h of electrolysis, indicating excellent durability and stability. Fe atoms were proven to be the active sites instead of S atoms by poisoning experiments and free energy profile for NRR in Fe sites and S sites. The reaction mechanism was investigated by DFT calculations that showed that the associative distal pathway was preferred and the conversion from *NH₂ to *NH₃ was regarded as the potential-determining step (PDS) (Fig. 4E).

The heterostructure of iron sulfide-based catalysts has also been developed to enhance their catalytic performance. The interface within heterostructures can usually regulate the electronic structure and expose extra active sites simultaneously. Chen and coworkers designed a hybrid catalyst of crystalline Fe₂O₃ with amorphous FeS (Fe₂O₃/FeS) via a DES-regulated fabrication, followed by an annealing process.¹²⁷ The hybrid catalyst exhibited abundant channels and a high specific surface area of up to 103.94 m² g⁻¹, providing more exposed electrochemical active sites for N₂ fixation and accelerating mass convection. Interface engineering between crystalline Fe₂O₃ and amorphous FeS optimized electron distribution and surface structure, being conducive to the absorption and activation of the N₂ molecules. In another work, Gao et al. created a graphene oxide-wrapped pyrite nanoparticle (FeS₂@GO) via Fe(NO)₃, NH₃·H₂O, and graphene oxide as a precursor.¹²⁸ The coupling effect between the catalyst and the substrate accelerated the electron transport property, FeS₂@GO exhibited the smallest charge transfer resistance, highest NH₃ production, and FE, much better than those of pure FeS₂ and GO (Fig. 4F and G). Meanwhile, the NH₃ yield rate and FE remained almost constant during the six-time recycling tests (Fig. 4H). Importantly, the low-coordinated Fe atoms are activated as highly active sites, where the free energy change ΔG (*N₂) of N₂ binding was reduced to −0.57 V, leading to stronger binding of N₂ and longer N≡N bond of absorbed *N₂ with respect to pristine FeS₂ (100).

Further progress in iron sulfide can be made by modulating the electronic structures of electrocatalysts by doping, tailoring surface vacancies, defect engineering, and/or heterointerface engineering. Meanwhile, refining and creating more active sites are also proposed to optimize the adsorption and activation of the nitrogen species.

3.2.2 Iron selenide/iron telluride. Other chalcogenides with similar properties to sulfur have also been studied for developing iron-based catalysts. In contrast to metal oxides and sulfides, metal tellurides with metallic properties are emerging as a promising class of electrocatalysts, due to their good electrical conductivity for favorable electrochemical kinetics. Guo and coworkers supported FeTe₂ nanoparticles on reduced graphene oxide to form a hybrid (FeTe₂/RGO) (Fig. 5A), which performed a novel NRR activity with ammonia yield of 39.2 μg h⁻¹ mg_cat⁻¹ and a FE of 18.1%.¹²⁹ The 3-fold coordinated Fe (Fe_3c) sites acted as the NRR active centers of FeTe₂, which could absorb the N₂ molecule and inject −0.12e to *N₂ to elongate the N≡N triple bond to 1.136 Å (Fig. 5B). According to the Gibbs free energy diagrams, the mixed pathway is the most favored reaction route. In addition, the FeTe₂ effectively suppressed HER, via a highly positive adsorption-free energy of *H of 0.71 eV compared to the negative ΔG_*N2 of −0.27 eV (Fig. 5C). 2D ferromagnetic materials with large spin moments are newly discovered as high-performance NRR electrocatalysts. In theory, the Fe₃GeTe₂ monolayer was explored as an NRR electrocatalyst through DFT calculations.¹³⁰ The highly centralized spin-polarization on the exposed Fe₄ center in defective Fe₃GeTe₂ with TeVs contributed to strong back-donation and effective activation of the N≡N triple bond (Fig. 5D). Importantly, an enzymatic pathway is preferred, and all six hydrogenation steps are exothermic and thus spontaneous, during the NRR process (N₂ + 6H⁺ + 6e⁻ = 2NH₃) (Fig. 5E). HER can also be suppressed by the rapid termination of N₂ molecules on TeVs.

Fig. 5 (A) TEM image of FeTe₂/RGO. (B) Optimized structures of *N₂ over Fe_3c sites. (C) Free energy profiles of three NRR pathways on Fe_3c site at U = 0. Reproduced with permission: copyright 2021, Elsevier.¹²⁹ (D) Side views of the defective Fe₃GeTe₂ monolayer. (color code: red, Fe; bule, Ge; gray, Te) The blue arrows stand for local magnetic moments of Fe atoms up aligned. (E) Possible reaction pathways for NRR. Reproduced with permission: copyright 2021, Royal Society of Chemistry.¹³⁰ (F) Electron location function of FeMoSe (left), FeMoS (right), in which the Fe, Mo, Se and S atoms are denoted by red, green, yellow and cyan balls, respectively. (G) Schematic illustration for N₂ binding to FeMoSe and FeMoS. NH₃ yields and FEs of FeMoSe, FeMoS (H) and other comparative samples (I).¹³²

2D layered double hydroxide (LDH) with evenly distributed ferric ions exhibited N₂ fixation activity, but the intrinsic resistivity and poor charge transfer properties astricted its further application. A NiFe-LDH-derived NiFe-selenide (Ni_0.75Fe_0.25Se₂) was designed to solve this weakness, the introduction of selenium conspicuously accelerated the p–d coupled electron transfer during nitrogen reduction.¹³¹ Through theoretical analysis, the selenization of iron caused lattice distortion, which may arise from the uneven occupation of t⁴_2g and e²_g orbitals in Fe²⁺. More N₂ molecule adsorption sites were introduced by lattice distortion regulation. A bio-inspired joint electron pool was realized in FeMo(Se, Te) by replacing sulfur with selenium or tellurium.¹³² Notably, Se and Te had a lower electron-negativity as compared with S, allowing a more expansive electron cloud to form the joint electron pool, which originated from the delocalized electronic structure (Fig. 5F). The density of state (DOS) profiles of the top valence band near the Fermi level in FeMoSe is mainly composed of Fe d, Mo d, and Se p valence orbitals, illustrating a higher electron density and charge transport ability than those of FeMoS and MoSe₂. Theoretical models for the joint electron pool displayed stronger nitrogen adsorption, which is critical for the subsequent hydrogenation steps. As these, FeMoSe had a greater ability to inject electrons into N₂ (Fig. 5G), exhibiting an NH₃ yield rate of 20.14 μg h⁻¹ mg_cat⁻¹ and a FE of 13.65%, which were far better than those of FeMoS and other comparative samples (Fig. 5H and I).

Briefly, the novel metal chalcogenides may be promising electrocatalysts with high efficiency for ammonia synthesis from gaseous dinitrogen. There is a large space for scientists to apply multi-element doping and heterostructure engineering similar to that used in iron sulfides to further promote NRR electrocatalysis. More in-depth research about iron selenide/iron telluride is expected to promote this electrocatalytic NRR field.

3.3 Bimetallic Fe-containing catalysts

3.3.1 Bimetallic Fe–Mo-based catalysts. Based on the structure of Mo,Fe-nitrogenase from natural conditions, Mo–Fe bimetallic catalysts have attracted extensive attention.^{101,133–135} Theoretical calculations and experimental works proofed that synergy and electron coupling effect between Fe and Mo facilitate the charge transfer from metal orbitals to nitrogen species, and thus weaken the inert nitrogen triple bond.^{56,118,136,137} Singstock et al. explored how Fe₂Mo₆S₈ broke NRR scaling relationships using grand-canonical density functional theory (GC-DFT).¹³⁶ According to theoretical predictions, Mo sites exhibited the intrinsic characteristics of strong N₂ adsorption and large NH₃ dissociation barriers, even poisoning due to the over-binding of *NH₃. Fe active sites with sharp bands of free-atom-like d-states conspicuously overlapped with the nitrogen p-states of *NNH, instead of those of *N₂ or *NH₃ (Fig. 6A–E), stabilizing the key *NNH intermediates and ensuring smooth release of NH₃. Experiments well supported the above idea, and further revealed the principle of HER inhibition effect.¹³⁷ During the catalysis process, Fe/Mo sites effectively absorbed and activated N₂ molecules. Then, the ligand effect where Fe donated electrons to Mo₆S₈ strengthened this HER-suppressed effect. This work also unraveled the multisite functions: Fe/Mo sites for N₂ adsorption and activation, and S sites for *H intermediate stabilization. Moreover, synergistic multisites of Fe, Mo, and S in the Fe₂Mo₆S₈ optimized the reactive energy of the intermediates and concurrently depressed hydrogen evolution, to achieve higher selectivity and yield of ammonia (Fig. 6F and G). Reasonable design of Mo–Fe bimetallic catalysts can be realized by heteroatom doping, interface engineering, single atom dispersion, and metal–organic framework precursors design; the related progress will be discussed in this section.

Fig. 6 Structure and projected density of states (pDOS) for the clean Fe₂Mo₆S₈ surface (A), the Fe₂Mo₆S₈ surface with *N₂ (B), *NNH (C), *NH₃ (D), and *NNH (E) with the Fe surface atom fixed at its position on the clean surface. Reproduced with permission: copyright 2022, American Chemical Society.¹³⁶ (F) Illustration of the proposed binding patterns for N₂ absorption and subsequential conversion to NH₃. (G) Comparison of FE and NH₃ yield for Mo₆S₈ and M₂Mo₆S₈ (M = Fe, Mn, and Cu) electrocatalysts. Reproduced with permission: copyright 2021, American Chemical Society.¹³⁷ (H–J) Charge density difference of MoS₂–Fe, MoS₂, MoS₂–V (from left to right). Electron excess and deficiency are denoted by red and green isosurfaces, respectively. Isosurface was set as 0.003 Å⁻³. (K) Free-energy profiles of the NRR of MoS₂–Fe, MoS₂, and MoS₂–V. Reproduced with permission: copyright 2022, reproduced with permission: copyright 2021, Springer.¹⁴⁰

3.3.1.1. Heteroatom doping. Single Fe-based or Mo-based catalysts can be mutually doped to achieve better N₂ fixation performance by modifying their intrinsic nature. MoS₂ was recently indicated to be a potential material for NRR, as its sufficient Mo (at IV valence states) active sites are able to accept the electrons from N₂.^6,56,79 Nevertheless, its further application is limited by the strong adsorption capacity of Mo to N₂, which impedes the desorption of it and release of NH₃, leading to strong HER competition.^138,139 The doping of Fe was proved to efficaciously modulate the electron density state of the catalyst. The roles of Fe doping for N₂ adsorption, N₂ activation, and NH₃ desorption were further revealed by Sun and coworkers, through a combination of experiments and theoretical calculations.¹⁴⁰ The Fe-doped MoS₂ electrocatalyst demonstrated an NH₃ yield of 20.11 μg h⁻¹ mg_cat⁻¹ at −0.35 V vs. RHE, which was significantly higher than that of its MoS₂ and MoS₂–V counterparts. When N₂ molecules were adsorbed on the catalyst, the charge of MoS₂–V was localized between V and N₂ molecules, leading to the strong adsorption ability of N₂, while the electrons of MoS₂–Fe were more concentrated on Mo, which may favor the ammonia release (Fig. 6H–J). According to the free energy profiles, MoS₂–Fe had the lowest rate-determining step (RDS) energy barrier, indicating a more thermodynamically favorable pathway on MoS₂–Fe (Fig. 6K). Their work even provided enlightenment on how to achieve the optimal balance between the three key stages (nitrogen adsorption, hydrogenation, and ammonia desorption) of NRR for superior electrocatalysis by introducing proper doping materials to tune the surface electronic structure.

Progress has also been made in bionic design from FeMoS-nitrogenases.¹⁴¹ In recent work, Du and coworkers designed a Mo(IV)-doped FeS₂ (FeS₂–Mo) via the hydrothermal reaction of MoCl₅, Fe(NO₃)_2, and thioacetamide.⁵⁶ Energy Dispersive Spectrometer (EDS) mapping indicated that Mo, Fe, and S elements were distributed uniformly throughout the nanosheets. Electrochemical measurements suggested that pure FeS₂ presented HER negative and a relatively low yield rate for NH₃. However, the NRR activity was significantly promoted after Mo doping and increased gradually with the Mo content, proving that Mo could act as active sites. The Mo-doped-modified catalyst exhibited a high FE of 14.41% and an NH₃ yield rate of 26.15 μg h⁻¹ mg_cat⁻¹. Afterwards, the optimal pathways for NRR in FeS₂ and FeS₂–Mo were proposed by DFT calculations. Moreover, the doping of Mo opened up an associative distal pathway in FeS₂–Mo, instead of the associative alternating pathway in FeS₂. The reaction energy barrier of the critical dinitrogen hydrogenation step (NN* + H → NNH*) was reduced from 1.24 eV to 0.62 eV. Therefore, the FeS₂ substrate was regarded to suppress the competitive HER, while the doping of Mo favored the adsorption and activation of nitrogen. Similarly, Su et al. achieved the suppression of HER by designing atomically dispersed Fe-decorated MoS₂ (Fe–MoS₂).¹⁴² The edge S atoms in MoS₂ were generally considered active sites for HER, while the Mo atoms acted as active centers for NRR.¹³⁵ Therefore, better selectivity for NRR could be accomplished by Fe modification of the S sites without shielding the Mo sites to achieve high FE (18.8% at −0.3 V versus RHE). DFT calculations proved that the energy barriers of HER in Fe–MoS₂ (0.15 eV) were obviously magnified with respect to pure MoS₂ (0.03 eV). It is worth mentioning that our group predicted and achieved Fe–O–Mo subunits for highly active NRR in Mo-doped Fe₂O₃ (Fig. 7A).¹⁴³ The introduction of Mo destroyed the ordered crystal structure and enriched lattice defects, creating more active sites. It also tuned the electronic structures of Fe₂O₃ to enhance the electron injection into N₂, resulting in an elongated and weakened nitrogen bond (Fig. 7C and D). The as-synthesized material showed much higher ammonia yields (21.3 μg h⁻¹ mg_cat⁻¹) and FE (11.2%) than pure Fe₂O₃, proving the significant role of Mo doping (Fig. 7B). An optimized reaction pathway with a lower energy barrier was confirmed on Mo-doped Fe₂O₃(110) in comparison with pristine α-Fe₂O₃(110) (Fig. 7E). Unfortunately, this work could not achieve the doping amount control. It needs further effort to reveal the phase transition mechanism of the catalytic material during the long-term electrocatalytic process.

Fig. 7 (A) Diagrammatic representation of the formation of Mo-doped Fe₂O₃ porous nanospheres. (B) Comparison of NH₃ production rates and FEs at −0.6 V for Mo-doped Fe₂O₃ and Fe₂O₃. The charge density difference of N₂-adsorbed α-Fe₂O₃(110) (C) and Mo-doped α-Fe₂O₃(110) (D) with an isosurface value of 0.006 e Å⁻³. (E) Free energy diagram for the NRR process on α-Fe₂O₃(110) and Mo-doped α-Fe₂O₃(110), respectively. Reproduced with permission: copyright 2022, American Chemical Society.¹⁴³ (F) Schematic illustration for the formation routes of Fe–MoS₂/CC. (G) NH₃ yield and FE with different mass ratios of Fe–MoS₂. Loading mass: 2.15 mg cm⁻². Reproduced with permission: copyright 2019, Royal Society of Chemistry.¹⁵²

Doping heteroatoms is effective to promote the electrocatalytic activity of basic materials. Further endeavors should be devoted to disclosing the position of the doping exogenous atom, the coordination and valent state, and the corresponding function during the whole catalytic process.

3.3.1.2. Interface engineering. Interface engineering also provides an effective scheme for conjugate iron and molybdenum compounds. A single material may have too strong or too weak adsorption capacity for nitrogen species, while the hybrid structure can effectively modulate the energy barrier in the reaction stages.^144,145 Abundant active sites and high specific surface area can also be obtained by rational interface engineering.^146,147 Moreover, two or more components could induce electronic coupling and generate a fast electron transport channel to speed up charge transfer.^148–151

A FeS₂ and MoO₂ nanocomposite anchored on a three-dimensional (3D) graphene aerogel (MoO₂/FeS₂/GA) was developed by Liu and coworkers.¹⁴⁶ FeS₂ and MoO₂ nanoparticles were embedded as active sites to absorb and activate N₂ in an interconnected 3D graphene-based mesoporous aerogel, which provided good conductivity and large accessible specific surface areas. Synergistic effects of such a ternary microstructure exhibited superior electrochemical performance over either single structure, endowed the hybrid catalyst with an impressively high FE of 37.44% and NH₃ yield of 40.18 μg h⁻¹ mg_cat⁻¹ at −0.25 V in 0.1 M HCl. Zhao et al. reported Fe nanodot-decorated MoS₂ nanosheets on carbon cloth (Fe–MoS₂/CC) via a chemical reduction strategy (Fig. 7F).¹⁵² The strong electronic interaction of Fe and MoS₂ was confirmed by the shift of XPS binding energies of Mo and S and the red shifts in Raman spectra. Furthermore, the effect of Fe loading on the NRR performance was also explored via electrochemical measurements. The best NH₃ yield reached 12.5 μg h⁻¹ cm⁻² with a FE of 10.8% at a Fe/MoS₂ mass ratio of 0.077 (Fig. 7G). When the Fe content got too large, the recession in NRR performance was speculated to be attributed to the degraded structures. In another work, FeS-dotted MoS₂ nanosheets covered carbon fiber cloth (CFC) (FeS@MoS₂/CFC) was prepared through a facile single-step hydrothermal preparation.⁵⁴ FeS and MoS₂ uniformly supported the highly conductive CFC and successfully simulated the composition of the active site in FeMoS-cofactor. Notably, it exhibited considerable NRR catalytic activity over a wide pH range.

3.3.1.3. MOF precursors design. Metal–organic frameworks (MOFs) are widely used to design composite electrocatalysts due to their porous structure with high specific surface area for accommodating active sites and facilitating charge/mass transfer.^153–158 Moreover, the desirable nanostructure of the catalysts could be controlled by the morphological structure inheritance of MOFs during thermal annealing processes. The organic ligands in MOFs can be pyrolyzed into porous carbons to improve the conductivity and stability of the derivatives and serve as a support for the active metal sites.^159–162 A handful of excellent reviews have focused on the derived nanomaterials from typical MOFs precursors, including zeolite imidazolate frameworks (ZIFs), Prussian blue analogs (PBAs), as electrocatalysts in energy-related fields, such as hydrogen evolution reaction, oxygen evolution reaction, and oxygen reduction reaction.^{157,163–169} These will further provide guidance for advanced materials exploration for NRR electrocatalysis.

Chen et al. reported a one-step pyrolysis–phosphating process from MoFe-MOF precursors to fabricate phosphorus-doped carbon microspheres (MoFe-PC), as a cost-effective and highly active catalyst for N₂ fixation under ambient conditions.¹⁷⁰ Electrochemical measurements confirmed that P-doped MoFe-PC (34.23 μg h⁻¹ mg_cat⁻¹, FE: 16.83%) showed more excellent activity than MoFe–C (24.73 μg h⁻¹ mg_cat⁻¹, FE: 12.47%). On the one hand, the introduction of P to form a P–C connection increased the conductivity of the hybrids by optimizing the electronic structure. On the other hand, the synergistic interaction between bimetallic oxides and P-doped carbon ensured abundant active sites for the adsorption and activation of nitrogen. Chen and coworkers applied a nanostructure tailoring strategy to successfully synthesize unique hollow Ni_αFe_1−α–MoS₂ nanocubes through vulcanization treatment of Ni–Fe PBA by ammonium thiomolybdate.¹⁷¹ The as-prepared material showed high stability during the two-hour NRR process at various applied potentials (Fig. 8A). The charge redistribution of the catalyst in an optimal atomic ratio of the NiFe (α = 0.3) as compared to the pure MoS₂ was presented by the calculated electron density difference (EDD) mappings (Fig. 8B–C). Based on the electron transfer pathways of Ni_αFe_1−α–MoS₂ catalysts, the spin-polarized density of states, and the corresponding partial density of states, it was suggested that the coupling of S 2p and Mo 3d orbitals generated the Ni_0.3Fe_0.7 active sites, which could easily donate electrons to the absorbed N₂ molecules. Furthermore, a possible pathway of the NRR on the NiFe–MoS₂ NCs catalyst was proposed by electrochemical-FTIR spectroscopy analysis and DFT calculations (Fig. 8D and E). Another Fe-containing MOF precursor, famous as MIL, has also been demonstrated to synthesize Fe₂O₃@MoS₂ composites for NRR electrocatalysis and achieved a remarkable NH₃ yield rate of 112.15 μg h⁻¹ mg_cat⁻¹ and a FE of 8.62%.¹⁵⁰

Fig. 8 (A) Time-dependent current density (j) of Ni–Fe–MoS₂ at various potentials. Electron density difference of the optimized structures of MoS₂ (B) and Ni_0.3–Fe_0.7@MoS₂ (C) (electron excess and deficiency are denoted by red and blue, respectively). (D) In situ electrochemical-FTIR spectra of the NRR process on the Ni–Fe–MoS₂ NCs catalyst. (E) Proposed electrocatalytic mechanism on Ni–Fe@MoS₂. Reproduced with permission: copyright 2020, Royal Society of Chemistry.¹⁷¹ (F) Schematic diagram of the preparation routes of Fe_1.89Mo_4.11O₇/FeS₂@C and FeMoO₄/FeS₂@C composites. (G) Comparison of NH₃ production rate and FE of FeS₂@C, FeMoO₄@FeS₂@C, and Fe_1.89Mo_4.11O₇@FeS₂@C (1.6 wt%) at the optimum voltage, respectively. Reproduced with permission: copyright 2020, Royal Society of Chemistry.⁵⁵

Polyoxometalate-based metal–organic frameworks (POMOFs) combine the advantages of both MOFs and POMs to provide specific target structures derived from organic ligands and serve as a source of metals such as Co, Fe, Ni, Mo.^169,172,173 Lately, Ma et al. made an exploration using PMo₁₂@MOF-100(Fe)@PVP as a precursor to synthesize Fe_1.89Mo_4.11O₇/FeS₂@C for efficient electrocatalysis of nitrogen reduction (Fig. 8F).⁵⁵ Experiments and characterization analysis concluded that the ratio of Fe and Mo could be controlled by accurately regulating the sulfuration time and temperature. Besides, a certain amount of PVP was convinced to be converted into a defected graphitization configuration, according to the intensity ratio of the D band and G band in Raman spectroscopy. Most of the FeS₂ was successfully encapsulated in the graphitic carbon layers leading to obtaining good conductivity. Fe_1.89Mo_4.11O₇/FeS₂@C exhibited an extremely high NH₃ yield of 105.3 μg h⁻¹ mg_cat⁻¹ and a FE of 54.7% at −0.4 V (Fig. 8G), as well as long-period electrolytic stability. The above works open new schemes for the rational design and fabrication of bimetallic MoFe-composites derived from MOFs for NRR. It can be further envisioned that the MOFs and even COFs (COF = covalent organic framework) containing Fe and Mo simultaneously may be used as electrocatalysts and through coordination modulation to regulate the electrocatalytic NRR performance. Moreover, MOFs and COFs may also be utilized as templates or precursors to structurally direct functional nanomaterials with special nano-morphologies, multi-component, and even heterostructures for high-performance NRR electrocatalysis. Thus, Fe,Mo-MOF/COF should be paid much attention to for the materials synthesis and structure design.

3.3.1.4. Ferric molybdate materials. Ferric molybdate materials are a kind of NRR catalysts that are widely available, easy to prepare, and cost-efficient. Wu et al. reported a one-step solvothermal method to synthesize FeMoO₄ nanorods (NRs) from FeCl₂ and Na₂MoO₄.¹⁷⁴ Thanks to synergy interaction between Mo and Fe, the as-prepared FeMoO₄ NRs exhibited superior NH₃ yield (17.51 μg h⁻¹ mg_cat⁻¹) and FE (10.53%) than that of Fe₂O₃ or MoO₃, in 0.1 M Na₂SO₄. The current density remained almost unchanged in the long-term stability test for 12 hours under different applied potentials, indicating superb electrochemical stability. Notably, no N element was included in the raw materials, which helped to prove the source of the product ammonia. The catalytic performance and potential mechanism of FeMoO₄ nanorods were also investigated by Chu and workers.¹⁷⁵ The bimetallic nature of FeMoO₄ could bring about accelerated electron transport and improved reaction kinetics, and thus it demonstrated a smaller charge transfer resistance than Fe₂O₃ and MoO₃. Besides, the considerable elongation of N–N bond (1.128 Å) and decrease of Mo–N distance (2.014 Å) from Mullikan charge analysis suggested effective *N₂ polarization and activation on 3-fold coordinated Mo (Mo_3c) (Fig. 9A). Xian et al. synthesized Fe₂(MoO₄)₃ nanoparticles (FMO-NPs) through a wet chemical process (Fig. 9B).¹⁷⁶ No by-product N₂H₄ was detected during electrolysis, and an impressive NH₃ yield (18.16 μg h⁻¹ mg_cat⁻¹) and FE (9.1%) were obtained. Ulteriorly, distal and alternating pathways at the Fe₂(MoO₄)₃ (110) surface were studied through material calculations (Fig. 9C). The catalyst surface showed strong adsorption of nitrogen molecules, as a deep downhill of −1.22 eV at this step. Then, the first hydrogenation step (*N₂ → *NNH) was confirmed to be the PDS. At the following hydrogenation of the N atom, the bond lengths of N≡N and N–Fe showed regular elongation and curtailment, respectively. Importantly, the suitable desorption energy barriers of the distal pathway ensured the smooth release of NH₃ and reaction completion.

Fig. 9 (A) Optimized structures for N₂ adsorption on various types of Fe and Mo atoms. Reproduced with permission: copyright 2020, American Chemical Society.¹⁷⁵ (B) Schematic illustration of the synthesis for FMO-NPs. (C) Variation of free energy for the distal and alternating pathways at the Fe₂(MoO₄)₃ (110) surface. Reproduced with permission: copyright 2020, American Chemical Society.¹⁷⁶ (D) The p-DOS and schematic illustrations of 4d orbitals of Mo atoms on Au/Fe₂(MoO₄)₃, 2p-orbitals of the N₂ gas molecule, and their interaction within -Au/Fe₂(MoO₄)₃. (E) Synthetic route of Au/Fe₂(MoO₄)₃. (F) TEM image of Au/Fe₂(MoO₄)₃. Reproduced with permission: copyright 2021, Wiley-VCH.¹⁷⁸

The deposition of noble metal nanoparticles is universal to improve the electrocatalytic activity. The above-mentioned iron molybdate materials demonstrate satisfying NRR activities, meanwhile, Au has generally low activity of HER.¹⁷⁷ Therefore, it is a reasonable scheme to introduce Au to optimize the atomic structure of Fe₂(MoO₄)₃ and enhance the performances of NRR. Yao et al. supported Au on the Fe₂(MoO₄)₃ substrate to construct an Au/Fe₂(MoO₄)₃ hybrid via a facile room-temperature reduction approach, achieving an excellent FE of 18.79% at −0.4 V (Fig. 9E and F).¹⁷⁸ Compared with Fe₂(MoO₄)₃ and Au/Fe₂(MoO₄)₃via the analysis of temperature-programmed desorption of N₂, the later one showed stronger binding strength of N₂, which originated from the Au regulating promotion. To get a profound understanding of the activation of N≡N, the projected density of states plots of Mo atom and N₂ were calculated. The d-σ and d-π* orbitals hybridization facilitated the electron transfer from Mo 4d orbitals to N₂ 2p orbitals (Fig. 9D), thus effectively weakening the inert N≡N triple bond. As a result, the reaction energy barrier of RDS of the distal pathway at Mo active centers was reduced from the former 0.98 to 0.6 eV after Au loading. Unfortunately, these catalytic ferric molybdate materials with large particle sizes suffer from insufficient active sites. In the future, material scientists may develop novel synthetic routes to decrease the size to expose more sites. The electrocatalytic performance can be further enhanced by simultaneous heteroatom doping and heterostructure engineering.

3.3.2 Other Fe-containing bimetallic catalysts. Bimetallic catalysts other than Fe–Mo materials have also been widely studied for heterogeneous surface catalysis due to their flexible compositions.¹⁷⁹ Metal alloys with multiple different active sites have the potential to perform better than pure metals.¹⁸⁰ Consequently, the development of alloy-based heterogeneous electrocatalysts may show NRR activity with several degrees of enhancement compared to pure iron-based electrocatalysts.¹⁸¹ Xiao et al. combined DFT calculations and a computational hydrogen electrode (CHE) model to confirm the efficient catalytic properties of various bimetallic alloys, including CuFe.¹⁸² Das et al. systematically modelled iron-rich bimetallic Fe-TM alloy-based structural catalysts by incorporating a first row of TM atoms into the Fe (110) surface. Most Fe-TM alloys were suggested to exhibit reliable stability and Fe–Co material had reasonable *NNH formation with sufficient *N₂ activation to be more selective for ammonia production.¹⁸³

Due to the coupling effect, the multicomponent nanostructures generally demonstrate more advanced NRR performance than their individual counterparts.^32,184 Introducing the second metal compound to a Fe-based material will be a compelling strategy to attain further improved performance. For example, doping Co elements into Fe-based materials can be an effective way to rationally regulate the electronic structure of catalysts due to the small atomic size and electronegativity of Co atoms, and the d–d coupling reduces the free energy barrier for N≡N triple bond dissociation. This idea was confirmed experimentally by Chen and coworkers by designing the Co-doped Fe₃S₄ nanoflowers (Fig. 10A). The introduced Co behaved as a new active center to promote N₂ absorption and reduce the energy barrier, achieving a good catalytic performance (Fig. 10B).¹⁸⁵

Fig. 10 (A) Schematic of Co-doped Fe₃S₄ nanoflowers. (B) NH₃ yields and FEs of Co-doped Fe₃S₄ nanoflowers at a series of applied potentials. Reproduced with permission: copyright 2020, American Chemical Society.¹⁸⁵ (C) TEM image of Au–Fe₃O₄ NPs. (D) N₂ temperature-programmed desorption curves of different catalysts. (E) The geometric structure of various intermediate states during NRR, in which the hydrogen, oxygen, nitrogen, iron, and gold atoms are denoted by white, red, blue, slate blue, and yellow balls, respectively. (F) Free energy diagram of the NRR pathways on Au–Fe₃O₄ (101), Fe₃O₄ (101), and Au (111). Reproduced with permission: copyright 2020, Wiley-VCH¹⁸⁴ (G) Illustration of synthesis and electronic interactions of dendritic Fe₂O₃/Cu electrocatalyst. (H) XPS valence band spectra of Fe₂O₃/Cu, Cu, and Fe₂O₃. Reproduced with permission: copyright 2019, Elsevier.¹⁸⁶

Zhang et al. prepared heterogeneous Au–Fe₃O₄ nanoparticles (NPs) (Fig. 10C) via a one-pot wet-chemical reaction of HAuCl₄ and diironnonacarbonyl (Fe₂(CO)₉).¹⁸⁴ Such Au–Fe₃O₄ NPs exhibited good performance with a high ammonia yield of 21.42 μg h⁻¹ mg_cat⁻¹ and a favorable FE of 10.54%. The N₂ temperature-programmed desorption (TPD) (Fig. 10D), and surface valence band spectra together confirmed that Au–Fe₃O₄ NPs had a strong synergistic effect for adsorbing reaction species and suitable interface for charge and proton transfer. The theoretical calculations showed that Fe performed as an active site to fix N₂ into *N₂H while the introduction of Au boosted the adsorption of NRR intermediates and provided an energetic-favorable pathway for NRR (Fig. 10E and F). Huang et al. developed a Fe₂O₃/Cu composite catalyst by electro-deposition in the presence of FeCl₂ and CuCl₂ as electrolytes (Fig. 10G), which reached its highest yield of 15.66 μg h⁻¹ mg_cat⁻¹ with a FE of 24.4% at −0.1 V vs. RHE.¹⁸⁶ The composite improved the electron transfer on Fe₂O₃, as copper is a good conductor of electrons. As d-band theory suggests, the closer the d-band center approaches the Fermi level, the stronger the adsorption is. The electronic interactions between Fe₂O₃ and Cu elevated the d-state center (Fig. 10H), enabling strong adsorption and back-bonding of N₂ molecules, leading to high catalytic activity.

3.4 Iron boride

Transition metal borides, also known as MBenes, are an emergent class of catalysts that are still in their infancy in the electrochemical NRR field.^187–189 MBenes as NRR electrocatalysts have the advantage of dual active edges from both the exposed metal end and the exposed boride end.¹⁹⁰ In the previous section, we have elaborated on how the electrons and orbitals of iron activate nitrogen bonds. Similarly, boron has three valence electrons in the 2s²2p¹ configuration with an electron-deficient feature and an empty orbital to accept lone pair electrons from N₂. B-to-N π-back bonding could be formed between the sp³ hybrid orbital of the boron atom and 2p orbital of N₂, which is conducive to N₂ adsorption on the B-active ends (Fig. 11A).^79,191–195

Fig. 11 (A) Schematic of π-backdonation between boron and N₂. Reproduced with permission: copyright 2019, American Chemical Society.¹⁹² Top and side views of FeB (B) and FeB₂ (C). Fe and B are represented as purple and pink balls, respectively. Models and bonding network for (D) FeB₆ (α) and (E) FeB₆ (β). The length of Fe–B_5c (short) and Fe–B_6c (long) bonds are labeled in units of Å. Reproduced with permission: copyright 2019, Royal Society of Chemistry.⁵⁸ (F) Illustration of the phase transformation from a-FeB₂ to c-FeB₂. (G) Charge density difference: the electron accumulation and depletion are represented as red and blue regions, respectively. (H) Free energy profiles of *N₂ and *N₂H adsorption on c-FeB₂ and a-FeB₂. Reproduced with permission: copyright 2021, Elsevier.⁵⁷

In a theoretical study, four iron-borides were constructed and explored as potential NRR catalysts, including FeB, FeB₂, FeB₆(α), and FeB₆(β) under the framework of DFT (Fig. 11B–E).⁵⁸ The authors proposed that Fe–B bonding could remarkably affect Fe–N interaction by examining the free energy differences in reaction pathways on the Fe-sites of FeB₂ and FeB, respectively. Additionally, the NH₃ release issue had been solved by establishing the hypercoordinated FeB₆(α) and FeB₆(β). Among these, FeB₆(β) offered the lowest ΔG_max of 0.68 eV, owing to the low oxidized Fe in the β-phase. Thus, N₂ could acquire electrons as a newly oxidizing reagent and be activated on the lightly oxidized Fe sites. In the evaluation of selectivity, the calculated ΔG(H*) for the four catalysts is less than the ΔG_max of NRR, but H* adsorption is weak in contrast to N₂ adsorption. The heterostructure construction was also applied to design high-performance iron boride-based electrocatalysts. In the current work, the process of NRR was studied on iron atoms supported by B_n-doped graphyne (GY).¹⁹⁶ A distal pathway was successfully identified with a minimum limiting potential of −0.53 V using DFT calculations. Additionally, the FeB@GY surface could effectively promote electron transfer efficiency and significantly improve the stability of intermediate adsorption during NRR. The substitution of B and N atoms for the C atoms in the doped GY structure is suitable for improving surface activity. From the results of the Bader charge analysis and charge density difference, it was detected that the reaction intermediates could not only accept electrons but also extract electrons from FeB@GY. The analysis of the partial density of states indicated a strong interaction between Fe-3d and N-2p in addition to B-2p, which was caused by the existence of the overlapped orbitals.

Chu's group pioneered the experimental synthesis of amorphous FeB₂ porous nanosheets (α-FeB₂ PNSs) through a facile reflux method.⁵⁷ Amorphization engineering produced vast unsaturated coordination sites and surface-exposed defects, which served as active sites to promote the electrocatalytic reaction energetics and kinetics. In 0.5 M LiClO₄ electrolyte, α-FeB₂ PNSs showed a maximum NH₃ yield rate of 39.8 μg h⁻¹ mg_cat⁻¹ at −0.3 V and a FE of 16.7% at −0.2 V. Furthermore, the chrono-potentiometric response showed no obvious decays for 20 h of consecutive electrolysis, with a 92.8% retention of the initial NH₃ yield, indicating excellent long-term stability. In this work, crystalline FeB₂ PNSs (c-FeB₂ PNSs) were also prepared by annealing at 600 °C for 3 h (Fig. 11F). When comparing the NRR performance, c-FeB₂ showed smaller ECSA and weaker N₂ absorption than a-FeB₂. Meanwhile, a-FeB₂ had strong charge transfer between *N₂ and active substrates and more injected electrons to activate the N₂ molecule to a greater extent, as observed in Fig. 11G. A lower G_*N2H (0.45 eV) on a-FeB₂ was provided to stabilize *N₂H and reduce the initial reduction barrier (Fig. 11H). Currently, most research on TMBs (TM = transition metal) especially iron borides as NRR electrocatalysts is focused on the theoretical calculations.^58,197–200 There is a deficiency of reports about experimental fruits on iron borides for NRR electrocatalysis, probably due to the harsh preparation conditions.

3.5 Fe-based single-atom catalysts

Up to now, single-atom catalysts (SACs), for which isolated atoms are anchored on various substrates to generate stable configuration, have been extensively investigated for nitrogen reduction reactions.^34,201–204 Compared with traditional catalyst materials, decreasing metal nanoparticles into single atoms can significantly improve the catalytic activity and selectivity towards NRR.²⁰⁵ The SACs may maximize atom utilization and effectively reduce economic costs in large-scale industrial production.^206–208 Meanwhile, the high dispersibility of SACs ensures their larger specific surface area and more exposed active sites, leading to stronger catalytic activity.^202,209 Moreover, the selection of substrate plays an important role in improving anchoring stability and modulating the electronic structure of SACs.^207,210 The interactions between substrates and loaded metal atoms give rise to tunable electronic structures, which favors the adsorption and activation pathway of the reactants at the catalytic sites to lower the energy barrier of NRR, in conjunction with unsaturated coordination environments.^211,212 However, some problems still exist when single atoms are applied as catalysts such as low stabilities for long cycle catalysis, low active metal atom loadings, difficulties in efficiently dispersing and isolating individual atoms, and the complex preparation process.^213,214 As a result, developing suitable substrates to achieve stable and efficient loading of single atoms is an urgent need.

Nitrogen-doped carbon is one of the most encouraging candidates due to its enhanced electronic conduction, optimized charge distribution, and electronic structure of active sites, which is caused by the interfacial interactions between iron atoms and the surrounding nitrogen/carbon atoms.²¹⁰ As revealed by DFT calculations, atomically dispersed Fe–N₃ site on nitrogen-doped graphene (Fe–N₃/graphene) can substantially activate the inert N≡N bond of N₂ with high spin-polarization and a localized magnetic moment.²¹⁵ Yang et al. anchored Fe single-atoms on a nitrogen-doped carbon substrate (Fe SAC/N–C) to realize the coordination structure of Fe–N₃.²¹⁰ The HAADF-STEM images indicated that the majority of Fe objects were downsizing to single atoms with a range of 0.1 to 0.2 nm, and there were no obvious Fe nanoparticles or clusters. The catalyst achieved an enhanced NH₃ yield rate of 53.12 μg h⁻¹ mg_cat⁻¹ and a high FE of 39.6% in 0.1 M KOH with ignored decay in seven consecutive electrolysis cycles. Computational studies provided direct evidence of the robust binding capability of Fe–N₃ catalytic active sites to N₂ molecules to achieve the high selectivity of Fe SAC/N–C for NH₃ formation. The calculated Gibbs free energy diagrams illustrated that the alternating pathway was a more convincing mechanism on highly dispersed Fe sites compared with the distal pathway, as its lower energy barrier of ΔG = 0.915 eV (Fig. 12A).

Fig. 12 (A) Gibbs free energy profiles of NRR on Fe–N₃ and Fe–N₄ through alternating and distal pathways. The inset shows the optimized structure of Fe–N₃ coordination and the corresponding adsorption of the NRR intermediates. Reproduced with permission: copyright 2020, American Chemical Society.²¹⁰ (B) SEM images of carbon substrate. Reproduced with permission: copyright 2019, American Chemical Society.²¹⁶ (C) Schematic illustration of the synthesis routes of Fe–N/C-CPs. (D) HAADF-STEM image and magnified image of Fe–N/C. The Fe-SA centers are marked by red circles. Reproduced with permission: copyright 2021, American Chemical Society.²¹³ (E) NH₃ yield rates and FEs at each given potential of FeSA–N–C. (F) Free energy profiles for NRR on the FeSA–N–C catalyst at U = 0 V. The inset represents the corresponding adsorption models (H, C, N, O, and Fe atoms are represented with gray, orange, blue, red and green spheres, respectively). Reproduced with permission: copyright 2019, Springer.²¹⁸ (G) NH₃ yields and FEs during six-recycling tests at −0.45 V vs. RHE. (H) Charge density difference and the N–N bond length after N₂ adsorption (I) The density of states for C 2p, B 2p, N 2p and Fe 3d for Fe–B/NC. (J) Gibbs free energy profiles of the H₂ evolution on the as-prepared sample. Reproduced with permission: copyright 2022, Elsevier.²²²

Fe–N₄ configurations are also developed for electrocatalytic ammonia synthesis. He et al. reported an atomically dispersed iron phthalocyanine (FePc) molecules on porous carbon with Fe–N₄ coordination structure, which achieved a dramatically high NH₃ yield rate of 137.95 μg h⁻¹ mg_cat⁻¹ at a low potential of −0.3 V (vs. RHE).²¹⁶ The SEM image showed the comb-like structure of 3D carbon substrate with massive nanopores, which contributed to the high loading ability of 7.43 wt% (the loading of FePc) (Fig. 12B). The current density decreased significantly in poisoning treatment by SCN⁻ suggests that the Fe center of FePc is the dominant active site for NH₃ formation. Liu et al. explored the realization of high Fe–N₄ loading capacity.²¹³ Fe-TPP, C₅H₈N₂, and N,N-diethylformylamide (DEF) etc. were milled in a zirconia-ball grinding mill to form the Fe-TPP⊂rho-ZIF precursor, and Fe–N/C was obtained by pyrolyzing in a tube heating furnace at 950 °C for 3 h under an N₂ atmosphere (Fig. 12C). The HAADF-STEM image indicated that the Fe atoms were uniformly distributed on the carbon substrate, in which the Fe single-atom loading reached 3.5 wt% (Fig. 12D). The sublimation of Zn in the pyrolysis process of the precursor was favorable to expose the catalytic sites and increase the specific surface area,²¹⁷ which reached up to 1088.96 m² g⁻¹. Wang et al. reported a Fe_SA–N–C catalyst with an extremely high FE of over 56.55% (Fig. 12E).²¹⁸ Single Fe atoms were uniformly loaded throughout the N-doped carbon support, and no Fe nanoparticles or aggregations were obviously detected. The adsorption behavior of N₂ toward Fe active sites was illustrated by molecular dynamics simulations, which suggested a lower energy barrier of 2.38 kJ mol⁻¹ at approximately 0.54 nm for efficient N₂ access. As depicted in Fig. 12F, the alternating pathway with lower energy uphill of 0.28 eV was more favorable than the distal pathway. Additionally, the high energy barriers (2.91 eV) for *H adsorption resulted, in particular, sluggish HER kinetics. Based on the above characteristics, the desired selectivity and FE were achieved by enhancing nitrogen reduction and suppressing hydrogen evolution.

Fe SACs that anchored on strain-engineered graphene substrates were modeled and systematically examined by first-principles calculations, suggesting that curvature strongly influences the catalytic activity and selectivity of SACs for NRR.²¹⁹ Bu et al. studied the NRR pathways for a dual-atom Fe₂ species on graphitic carbon nitride (g-C₃N₄) by changing external conditions.²²⁰ It was found that applying an appropriate range of tensile to the graphitic carbon nitride (g-C₃N₄) substrate could reduce the overpotential values, which arose from a break in the intrinsic scaling relationship of the adsorption energy between the *N₂ and *HNN adsorbates. More influences of structural distortion on the NRR catalytic properties of Fe, N codoped carbon (Fe–N–C) were explored.²²¹ The NRR intermediate species *NNH could be strongly bound at the FeN₄ and FeN₃ active sites embedded in strained graphene layers, contributing to the enhanced NRR activity. In contrast to *NNH, *H showed a greater adsorption energy barrier, which indicated the less favored competitive HER and higher selectivity of NRR. Their computational work provides evidence for improving the catalytic activity and selectivity of Fe–N–C materials towards NRR by tuning the degree of compressive strain in their graphene layers. Although the theoretical studies of strain engineering on Fe-SACs have achieved promising results, the predictions need to be further confirmed by experimental works.

The doping of multi-element atoms into the substrate is an effective and well-explored strategy for adjusting the Fe active site configuration to enhance NRR. Wang et al. developed boron-induced electron-rich SACs (Fe–B/NC) by dispersing single Fe sites on boron, nitrogen-co-doped carbon.²²² The desired selectivity and stability were proved by no detectable N₂H₄ and six recycling tests (Fig. 12G). The introduction of B atoms with low electronegativity (χ = 2.0) broke the symmetric electron distribution of the Fe–N₄ coordination, engendering electron donor (B)/acceptor (N) pairs along the B–Fe–N bridge to enhance electron transfer. After being adsorbed on B–Fe sites, the triple bond in N₂ was elongated by 0.115 Å with a comparison of free N₂, due to back-donation from B and Fe to N₂, which was confirmed by the Bader charge after N₂ adsorption analysis (Fig. 12H). Moreover, the coupling with B 2p could delocalize the electrons in the Fe 3d orbital (Fig. 12I), increasing the electron conductivity for N₂ adsorption/activation, thereby enhancing the NRR performance. In addition, the catalyst exhibited a hindered effect on HER relative to the boron-free sample, owing to a large ΔG_H* (Fig. 12J). Another coordination mode for anchoring Fe SA has also been explored, typically as Fe-(O–C₂)₄ single-atom sites on lignocellulose-derived carbon with impressive catalytic activity (32.1 μg h⁻¹ mg_cat⁻¹ equal to 5350 μg h⁻¹ mg_Fe⁻¹), by Zhao and coworkers.⁴⁵

In addition to nitrogen-doped carbon, other substrates also can be applied to support Fe single atoms as SACs for NRR. Porous materials with large surface areas or vacancies are ideal supports that can accommodate ample active sites, realizing the high metal loading. Chen et al. prepared S-coordinated Fe SACs on mesoporous TiO₂ (Fe₁S_x@TiO₂), which were constructed by a lattice-confined strategy.⁶⁰ FeS₂O₂ sites were confirmed as active centers by theoretical calculations. No peak was observed for a Fe–Fe bond in the Fe-K edge EXAFS spectra of Fe₁S_x@TiO₂, indicating that Fe atoms are distributed individually (Fig. 13A). Abundant OVs and Ti–S coordination bonds were detected by Ti K-edge EXAFS spectra, resulting in high dispersion stability. The finite element analysis showed that confinement of opened and ordered mesopores could promote mass transfer and provide a large active surface area. As a result, the catalyst showed a good NH₃ production rate of 18.3 μg h⁻¹ mg_cat⁻¹ with 17.3% FE, and the origin of NH₃ was verified via an isotopic-labeling experiment (Fig. 13B). Moreover, the relationship between Fe loading capability and NRR performance was also investigated, when the ratio of Fe : Ti is 6 : 100, NRR activity reached maximum without Fe aggregation. First-principles calculations revealed the existence of an enzymatic mechanism with a low overpotential of 0.21 V and side-on adsorption of N₂ in MoS₂-supported Fe₂ clusters (Fe₂/MoS₂), which is conducive to effective N≡N bond activation.²²³ Li et al. creatively designed a single Fe atom supported on atomically thin molybdenum disulfide (SACs-MoS₂–Fe) to form high curvatures with interfacial protrusion-like shape (Fig. 13C).²²⁴ Triggering interfacial polarization by curvature-rich surface and interface demonstrates potential universality in wide-scope electrocatalysis, including the promising and energy-effective breaking of N≡N bond. Fe single atoms worked as external adatoms conjugating with MoS₂ to create 3D protrusions, instead of interior dopants substituting in MoS₂ skeletons, which was distinct from extensively reported metal–N_x and defect-trapped-SACs. The unique geometry stimulated electron injection into N≡N efficiently enabled N₂ polarization and reduction. When the atomic ratio of Fe and MoS₂ reached 2.0, SACs-MoS₂–Fe₂ demonstrated the best catalytic performance of ammonia yield (97.5–6 μg h⁻¹ cm⁻²) and FE (31.6–2%) at −0.2 V.

Fig. 13 (A) k³-weighted Fourier-transform EXAFS spectra of the Fe R-space for FeS₂ and Fe foil, Fe₁S_x@TiO₂ and Fe–TiO₂, the inset is the first peak of Fe₁S_x@TiO₂ and Fe–TiO₂. (B) ¹H-NMR spectra of ¹⁴NH₄⁺ and ¹⁵NH₄⁺ produced in the NRR process. Reproduced with permission: copyright 2022, Wiley-VCH.⁶⁰ (C) 3D topographic potential distribution images of MoS₂ and pure MoS₂. Reproduced with permission: copyright 2020, Elsevier.²²⁴ (D) In situ FTIR spectra obtained on the surfaces of FeMo/NC, Fe/NC, and Mo/NC SACs, respectively (from left to right). (E) Proposed NRR pathways on FeMo/NC, Fe/NC, and Mo/NC surfaces. Reproduced with permission: copyright 2022, Springer.²³¹ (F) NH₃ yields and FEs on various applied potentials. (G) Fe and Co K edge EXAFS fitting curves of Fe/Co–O–C-1.0 at R space. Inset is the predicted bimetallic Fe–Co coordination configuration in Fe/Co–O–C-1.0. (C, O, Fe, and Co are denoted by brown, red, orange, and blue, respectively). Reproduced with permission: copyright 2022, Springer.²³³ (H) Differential charge densities of absorbed N₂ on Pd and PdFe₁, where electron accumulation and depletion are denoted by yellow and cyan, respectively. (I) 3D topographic atom images of PdFe₁. Reproduced with permission: copyright 2022, Wiley-VCH.⁵⁹

Diatomic catalysts (DACs) derived from single-atom catalysts have been widely explored for highly efficient electrocatalysis.^225–228 According to related investigations, the catalytic activity of graphene-based Mn, Ni, Co, and Fe DACs exceeds that of the corresponding SACs.^229,230 Zhang and coworkers reported FeMo/NC DSACs as a “dual-site” material by anchoring isolated Fe and Mo atoms on hierarchical N-doped carbon nanotubes.²³¹ This catalyst demonstrated an NH₃ yield of 26.8 μg h⁻¹ mg_cat⁻¹ and a FE of 11.8, which is 1.6 and 2.5 times larger than those of Mo/NC and Fe/NC. Time-dependent in situ Fourier transform infrared spectroscopy (in situ FTIR) measurements were conducted for dual-site FeMo/NC DSACs and single-site Fe/NC and Mo/NC to investigate the synergy mechanism of Mo and Fe single atoms (Fig. 13D). Three positive-going bands of N–N stretching, –NH₂ wagging, and H–N–H bending were appeared at the in situ FTIR spectra of FeMo/NC, suggesting the formation of *N₂H_y intermediates during the NRR process. No positive-going N=N stretching was observed on the FeMo/NC surface, which implied that the *N₂H_x intermediates (0 ≤ x ≤ 2) had no obstacle to separate into a single bond N–N of *N₂H_y intermediates (1 ≤ y ≤ 4). The NRR over FeMo/NC may follow an associative mechanism (Fig. 13E), due to the existence of the three main intermediates, as well as no N₂H₄ was detected. Ma et al. revealed the catalytic principle of Fe/Mo–N–C from the point of view of the spin moment of the active centers through theoretical calculations.²³² The neighboring Fe could modulate the spin state of the Mo center in MoN₄ to switch from a high-spin state to a medium-spin state, effectively relieving the strong overlap between the Mo 4d orbital and the N_xH_y intermediates, facilitating the desorption of NH₃, and ultimately leading to a lower overpotential. Recent work reported that an extraordinary NRR performance of 579.2 ± 27.8 μg h⁻¹ mg_cat⁻¹ NH₃ yield rate and 79.0 ± 3.8% FE could be achieved on an atomically dispersed, bimetallic Fe–Co active site catalyst (Fe/Co–O–C-r) (Fig. 13F).²³³ The observation and fitting of EXAFS spectra of Fe/Co–O–C-1.0 suggested that the atomically dispersed Fe, Co atoms are anchored on carbonized BC (CBC) by a bimetallic Fe–Co arrangement anchoring to graphitic carbon through six O-bridging bonds, for which, [(O–C₂)₃Fe–Co(O–C₂)₃] is the likely bimetallic unit (Fig. 13G). Calculations pointed out that an energy-favorable transformation from [(O–C₂)₃Fe–Co(O–C₂)₃] to [(O–C₂)₃Fe–Co(O–C)C₂] can occur at the end of an NRR cycle on desorption of *NH₃. [(O–C₂)₃Fe–Co(O–C)C₂] was further confirmed to be the actual active site of Fe/Co–O–C-r and resulted in remarkable NRR performance.

Traditional SACs have the inherent disadvantage that single metal atoms tend to agglomerate due to their large surface free energy. Avoiding the aggregation of individual atoms at the expense of low metal loading limits the improvement of NRR performance and practical applications. A new variety of SACs, single-atom alloys (SAAs), in which, the isolated metal atoms are anchored on the substrate metal has been proposed with integrated merits.^234–236 Specifically, the extremely strong metal–support interactions endow the high thermodynamic stability to break limitations in metal loading. Moreover, the alloying effect of SAAs tunes the electronic structure of atomic metal sites to enable an advantageous balance between the appropriate binding of intermediates and the effective dissociation of reactants. Li et al. first developed a PdFe₁ single-atom alloy metallene, which demonstrated an NH₃ yield of 111.9 μg h⁻¹ mg_cat⁻¹ and a FE of 37.8% at −0.2 V vs. RHE.⁵⁹ Preliminary theoretical computations indicated the enhancement of N₂ affinity on PdFe₁ and the Lewis acid–base interaction facilitated the following N₂ polarization and cleavage (Fig. 13H). In the free energy diagrams, the energy barrier of the rate-limiting step was reduced to 0.72 eV, due to the efficient N₂ activation on Fe sites. Encouraged by the above theoretical results, the preconceived PdFe₁ (Fe: 3.91 at%) was prepared via a one-step wet chemistry strategy. The low-peak-intensity spots in 3D topographic atom images suggested that Fe atoms successfully substituted the partial Pd atoms on the PdFe₁ substrate (Fig. 13I). Thus, this stably distributed the SAA catalyst and presented a steady current density during the 100 h of continuous electrolysis in chronoamperometry, and no apparent recession in NRR yields and FEs within ten cycling tests were observed.

4. Conclusions and outlook

Significant and encouraging advancements in ambient electrochemical NRR for gaseous nitrogen molecules to NH₃ have been made in the past five years and even the decade. However, efficient nitrogen reduction under ambient conditions faces challenges for both fundamental research and practical applications. These substantial barriers include, for example, the inertness and low water solubility of N₂ molecules, weak N₂ adsorption on catalyst surfaces, high overpotentials, low catalytic lifetimes, and the involvement of multi-proton and multi-electron pathways leading to low selectivity. To negotiate these obstacles and achieve efficient NRR with a high production rate, high FE, and high energy utilization efficiency, functional Fe-based materials have been paid much attention to and explored for this promising field, possibly inspired by both the Fe-containing industrial catalyst and biological enzyme. In this review, a brief catalytic mechanism for the electrocatalytic NRR and a comprehensive overview of the research fruits of Fe-based electrocatalytic materials for NRR are presented. Table 1 summarized and compared the electrocatalytic performances of the representative Fe-based electrocatalysts, including iron oxide, sulfides, bimetallic Fe-containing materials, single-atom, and even dual-atomic materials. Unfortunately, the activity, selectivity, and longevity of the Fe-based materials and others are not satisfactory. Therefore, to optimize the electrocatalytic performance and further promote significant development, we propose the following perspectives and further development directions (Fig. 14).

Table 1 NRR performance of the Fe-based catalysts reported since 2018

Catalyst	Electrolyte	NH3 yield	Quantification methods	FE (%)	Isotope labeling	Ref.
o-Fe2O3–Ar/CNT	0.1 M KOH	0.46 μg h^−1 cm^−2	Nessler colorimetry	6.0	No	44
Fe/Fe3O4-300	0.1 M phosphate buffer solution	0.19 μg h^−1 cm^−2	Indophenol blue colorimetry	8.29	No	100
Fe–Fe3O4 nanoparticles	0.1 M Na2SO4	24.6 μg h^−1 mgcat^−1	Indophenol blue colorimetry	53.2	No	50
Fe3O4-300 nanosheet	0.1 M Na2SO4	12.09 μg h^−1 mgcat^−1	Indophenol blue colorimetry	34.38	Yes	49
Fe–Fe3O4 nanoparticles	0.1 M Na2SO4	22 μg h^−1 mgcat^−1	Indophenol blue colorimetry	3.5	No	115
p-Fe2O3/CC	0.1 M Na2SO4	6.78 μg h^−1 cm^−2	Indophenol blue colorimetry	7.69	No	116
Zn doped Fe2O3	0.1 M Na2SO4	15.1 μg h^−1 mgcat^−1	Indophenol blue colorimetry	10.4	Yes	113
α-Fe2O3 nanocubes	0.1 M KOH	32.13 μg h^−1 mgcat^−1	Indophenol blue colorimetry	6.63	Yes	114
Au–Fe3O4 nanoparticles	0.1 M KOH	21.42 μg h^−1 mgcat^−1	Nessler colorimetry	10.54	Yes	184
Fe2O3/Cu	0.1 M KOH	15.66 μg h^−1 mgcat^−1	Indophenol blue colorimetry	24.4	No	186
Fe3S4 nanosheets	0.1 M HCl	75.4 μg h^−1 mgcat^−1	Indophenol blue colorimetry	6.45	Yes	123
FeSx/Fe	0.1 M KOH	4.13 × 10^−10 mol s^−1 cm^−2	Ion chromatography, indophenol blue colorimetry, Nessler colorimetry	17.6	Yes	124
Vs-FePS3 NSs	0.1 M HCl	3.88 μg h^−1 cm^−2	Ion chromatography, indophenol blue colorimetry	12.36	No	52
FeS2	0.1 M Na2SO4	37.2 μg h^−1 mgcat^−1	Indophenol blue colorimetry	11.2	No	126
FeS2	0.1 M Li2SO4	11.5 μg h^−1 mgFe^−1	Nessler colorimetry	14.6	No	125
Fe2O3/FeS	0.1 M KOH	34.31 μg h^−1 mgcat^−1	Indophenol blue colorimetry	18.06	Yes	127
FeS2@GO	0.1 M HCl & 0.1 M Na2SO4	78.6 μg h^−1 mgcat^−1 in 0.1 M HCl, 27.9 μg h^−1 mgcat^−1 in 0.1 M Na2SO4	Indophenol blue colorimetry	4.7 in 0.1 M HCl, 6.8 in 0.1 M Na2SO4	No	128
FeTe2/RGO	0.5 M LiClO4	39.2 μg h^−1 mgcat^−1	Indophenol blue colorimetry	18.1	Yes	129
FeMoSe	0.1 M Na2SO4	20.14 μg h^−1 mgcat^−1	Indophenol blue colorimetry	13.65	Yes	132
FeS2–Mo	0.1 M KOH	26.15 μg h^−1 mgcat^−1	Indophenol blue colorimetry, Nessler colorimetry, ion chromatography	14.41	Yes	56
Fe2Mo6S8	0.5 M Na2SO4 mixed with 0.1 M sodium citrate buffer	70 μg h^−1 mgcat^−1	Indophenol blue colorimetry	12.5	Yes	137
MoS2–Fe	0.1 M Na2SO4	20.11 μg h^−1 mgcat^−1	Indophenol blue colorimetry	15.72	No	140
Fe–MoS2	0.5 M K2SO4	8.63 μg h^−1 mgcat^−1	Indophenol blue colorimetry	18.8	Yes	142
Mo-doped Fe2O3	0.1 M Na2SO4	21.3 μg h^−1 mgcat^−1	Indophenol blue colorimetry	11.2	No	143
MoO2/FeS2/GA	0.1 M HCl	40.18 μg h^−1 mgcat^−1	Indophenol blue colorimetry	37.44	Yes	146
FeS@MoS2/CFC	0.1 M Na2SO4	8.45 μg h^−1 cm^−2	Indophenol blue colorimetry	2.96	No	54
Fe–MoS2/CC	0.1 M KOH	12.5 μg h^−1 cm^−2	Indophenol blue colorimetry	10.8	Yes	152
SACs-MoS2-Fe	0.1 M KCl	97.5 μg h^−1 cm^−2	Indophenol blue colorimetry	31.6	Yes	224
MoFe-PC	0.1 M HCl	34.23 μg h^−1 mgcat^−1	Indophenol blue colorimetry	16.83	No	170
Ni0.3–Fe0.7@MoS2	0.1 M Na2SO4	128.17 μg h^−1 mgcat^−1	Indophenol blue colorimetry	11.34	No	171
Fe1.89Mo4.11O7/FeS2@C	0.5 M K2SO4	105.3 μg h^−1 mgcat^−1	Nessler colorimetry	54.7	Yes	55
FeMoO4 nanorods	0.1 M Na2SO4	17.51 μg h^−1 mgcat^−1	Indophenol blue colorimetry	10.53	No	174
FeMoO4 nanorods	0.5 M LiClO4	45.8 μg h^−1 mgcat^−1	Indophenol blue colorimetry	13.2	Yes	175
Fe2(MoO4)3 nanoparticles	0.1 M Na2SO4	18.16 μg h^−1 mgcat^−1	Indophenol blue colorimetry	9.1	No	176
Au/Fe2(MoO4)3	0.2 M Na2SO4	7.61 μg h^−1 mgcat^−1	^1H-NMR method		Yes	178
α-FeB2 PNS	0.5 M LiClO4	39.8 μg h^−1 mgcat^−1	Indophenol blue colorimetry	16.7	Yes	57
Fe SAC/N–C	0.1 M KOH	53.12 μg h^−1 mgcat^−1	Indophenol blue colorimetry	39.6	Yes	210
Fe–N/C	0.5 M K2SO4	2.27 μg h^−1 mgcat^−1	Indophenol blue colorimetry	7.67	Yes	213
FePc/C	0.1 M Na2SO4	137.95 μg h^−1 mgFePc^−1	Indophenol blue colorimetry	10.05	No	216
		10.25 μg h^−1 mgcat^−1
FeSA–N–C	0.1 M KOH	7.48 μg h^−1 mgcat^−1	Indophenol blue colorimetry	56.55	Yes	218
Fe–B/NC	0.1 M KOH	88.7 μg h^−1 mgcat^−1	Indophenol blue colorimetry	9.2	Yes	222
Fe1Sx@TiO2	0.1 M HCl	18.3 μg h^−1 mgcat^−1	Indophenol blue colorimetry	17.3	Yes	60
FeMo/NC DSACs	0.1 M Na2SO4	26.8 μg h^−1 mgcat^−1	Indophenol blue colorimetry	11.8	No	231
Fe/Co–O–C-r	0.1 M Na2SO4	579 μg h^−1 mgcat^−1	Indophenol blue colorimetry, ^1H-NMR method	79.0	Yes	233
PdFe1	0.5 M LiClO4	111.9 μg h^−1 mgcat^−1	Indophenol blue colorimetry	37.8	Yes	59

---

Fig. 14 Strategies to boost NRR performances and perspectives for future developments of iron-based catalysts. Regulate in-depth mechanism: reproduced with permission: copyright 2022, Wiley-VCH.⁵⁹ Coupling strategy: reproduced with permission: copyright 2020, Royal Society of Chemistry.²³⁷ Copyright 2022, Elsevier.¹⁵⁰ Copyright 2020, Elsevier.²²⁴ Theoretical calculation: reproduced with permission: copyright 2022, American Chemical Society.¹³⁶

4.1 From an in-depth mechanistic reaction pathway

According to the possible associative distal/alternating pathway, and enzymatic pathway, the nitrogen adsorption ability, activation ability, and even the N₂ adsorption mode of the metal site are very crucial. The Fe element with variable valence will be endowed with different electronic configurations.²³⁸ Furthermore, the crystal field and the local coordination environments can be further elaborately designed to modulate the spin states, including high spin, low spin, and even intermedia spin, of the Fe center.⁷¹ It may provide a perspective solution to strengthen the activation ability. From another point of view, the microscopic reaction mechanism indicates that the hydrogenation process is another key factor to complete NRR. However, few researchers pay attention to hydrogen sources, especially for the hydrogen intermediate H* in the pathway and their transfer process. Wang and coworkers recognized that the directly significant suppression of HER may be counterproductive for electrocatalytic NRR because N₂ hydrogenation also needs active H species.²³⁹ It is anticipated that scientific research should focus more on the solution of the intermedia H*-related issues, including its source, its production, its transfer, and subsequent hydrogenation for N₂. Understanding these above two key points may provide meaningful guidance to explore catalytic materials with advanced structures and performances.

4.2 Exploring coupling strategies for materials design

The strategies of enzyme mimicking, elemental doping, heterostructures engineering, and single-atom dispersion on functional supports, etc. presented in this review have all yielded exciting results. These tactics are usually adopted to optimize the binding energy of the reactants and key intermediates through the synergistic effect, thereby enhancing the intrinsic per-site activity. More exposed active sites will promote the electrochemical reaction, which is believed to maximize extrinsic activity. Advanced nanoarchitectures, such as 2D nanosheets, hierarchical structures, and porous structures are advantageous to facilitate charge/mass transportation. Moreover, the compositions and the local coordination environments are also needed to disclose their importance. These mentioned strategies are not single-functioned and not self-governed. A deeper understanding and sensible coupling of these strategies can further improve both the intrinsic and extrinsic catalytic activity of iron-based catalysts, simultaneously.²⁴⁰ According to the first point of perspective, the creation of multi-model active sites may be efficient to improve the catalytic performance. The real function of each site and their distance, interaction, or synergistic effect should be further investigated in depth. Therefore, it is of great significance to develop new synthetic methods to accomplish the above coupling strategies and analyze the design principles that would provide great opportunities for rational exploration of superior NRR electrocatalysts.

4.3 Establishing viable and standard protocols for electrocatalytic NRR evaluation

Up to now, the researchers have used different methods to quantify produced ammonia, including commonly used colorimetry (indophenol blue and Nessler's reagent), ion chromatography, and NMR.²⁴¹ Unfortunately, some of them are not completely reliable. For example, ion chromatography detection is susceptible to interference from other cations (e.g. Li⁺, K⁺, etc.). Therefore, it is recommended to integrate multiple methods under repeatable experiments to produce meaningful data with an error bar. In order to exclude exogenous nitrogen pollutants (NH₃, nitrate/nitrite, and nitrous oxide), the N₂ feed gas, electrolytes, and electrocatalysts need to be purified for NRR tests. It is necessary to conduct isotope (¹⁵N₂) labelling experiments to identify the origin of the ammonia, a key experiment to verify the true electrocatalytic positive. Moreover, the configurations of the electrolytic cell may be further studied to guide its rational design. The flow cell within an enclosed system may be better to ensure a continuous supply of nitrogen with enough solubility to the catalyst surface. It will also realize the cycling of ¹⁵N₂ to reduce both economic cost and contamination. It is also urgently required to develop more selective, precise, real-time on-line methods for NH₃ determination. Therefore, it is a burdensome system engineering to standardize the evaluation protocol, yet, is of great significance for materials comparison, and this NRR field is rapidly growing.

4.4 Combining theoretical calculations and in situ characterizations

The true reaction pathway in electrocatalytic NRR is expected to unravel from fundamental research that integrates experiments and theoretical calculations. In situ/operando measurements by various techniques, such as surface-enhanced infrared absorption spectroscopy, Raman, near ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS), X-ray absorption spectroscopy (XAS) by synchrotron radiation and others, can directly define *N_xH_y intermediate products, revealing the actual catalytic active sites and even understand the deactivation reasons.²⁴² These will help researchers to clarify the elementary reactions during the NRR process and build up more valid structure models for theoretical investigations. Great endeavors will be devoted to investigating the adsorption strength of different intermediates, and the Gibbs free energy change during NRR, helping to reveal the mechanism from a theoretical view. Improving the resolution of these operando techniques and providing a programmable measurement database are needed. Different in situ methods with varying functions are recommended for in-depth investigations. The combination of theoretical calculations and in situ characterizations would bridge the gap between mechanistic thoughts and experimental electrocatalytic performances.²⁴³ It will further underpin the structure–property relationships and guide new directions for electrocatalytic materials design.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are grateful for the financial support from the National Natural Science Foundation of China (No. 22271036), the Fundamental Research Funds for the Central Universities of China (DUT22LK15), and the Open Funds of the State Key Laboratory of Rare Earth Resource Utilization (RERU2022011).

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