Ammonia Synthesis from Electrocatalytic N2 Reduction under Ambient Conditions by Fe2 O3 Nanorods

FeP-Fe3O4 nanospheres for electrocatalytic N2 reduction to NH3 under ambient conditions

The electrocatalytic nitrogen reduction reaction (eNRR) under ambient conditions is deemed a promising alternative for NH3 synthesis. In this paper, an FeP-Fe3O4 nanocomposite electrocatalyst was prepared by phosphating annealing using Fe2O3 as a precursor, and the resulting FeP-Fe3O4 exhibited excellent N2-to-NH3-producing activity over a wide potential window. The highest faradaic efficiency of FeP-Fe3O4 is 11.02% at -0.1 V vs. reversible hydrogen electrode (RHE), and the maximum NH3 yield reaches 12.73 μg h-1 mgcat-1, comparable to or exceeding the reported values in this field. Furthermore, the FeP-Fe3O4 nanocomposite electrocatalyst presents high electrochemical stability, selectivity, and durability.

Oxygen vacancy modulation in interfacial engineering Fe3O4 over carbon nanofiber boosting ambient electrocatalytic N2 reduction

The oxygen vacancy modulation of interface-engineered Fe3O4 nanograins over carbon nanofiber (Fe@CNF) was achieved to improve electrocatalytic nitrogen reduction reaction (NRR) activity and stability via facile electrospinning and tuning thermal procedure. The optimal catalyst calcined at 800 ℃ (Fe@CNF-800) was endowed with abundant nanograin boundaries and optimized oxygen vacancy (Vo) concentration of iron oxides, thereby affording 37.1 μg h-1 mgcat.-1 (-0.2 V vs. reversible hydrogen electrode (RHE)) NH3 yield and rational Faraday efficiency (10.2%), with 13.6 times atomic activity enhancement compared to of that commercial Fe3O4. The interfacial effect of assembled nanograins in particles correlated with the formation of Vo and more intrinsic active sites, which is conducive to the trapping and activation of nitrogen (N2). The in-situ X-ray photoelectron spectroscopy (XPS) measurement revealed the real consumption of adsorbed oxygen when introducing N2 by the trapping effect of Vo. Density-Functional-Theory (DFT) calculation validates the promotive hydrogenation effect and elimination of hydrogen intermediate (H*) interacted with N2 transferring toward oxygen of the support. The optimal catalyst shows a lasting NRR activity at least 90 h, outperforming most reported Fe-based NRR catalysts.

Nanosheet arrays of iron oxide for enhanced ammonia synthesis via electrochemical nitrogen reduction for prospective algal membrane bioreactors

The earth's nitrogen cycle relies on the effective conversion of nitrogen (N2) to ammonia (NH3). As a result, the research and development of catalysts that are earth-abundant, inexpensive, and highly efficient but do not need precious metals is of the utmost significance. In this investigation, we present a controlled synthesis technique to the fabrication of an iron oxide (Fe2O3) nanosheet array by annealing at temperatures ranging from 350 to 550 °C. This array will be used for the electrochemical reduction of atmospheric N2 to NH3 in electrolytes. The Fe2O3 nanosheet array that was produced as a result displays outstanding electrochemical performance as well as remarkable stability. When compared to a hydrogen electrode working under normal temperature and pressure conditions, the Fe2O3 nanosheet array produces an impressive NH3 production rate of 18.04 g per hour per mg of catalytically active material in 0.1 M KOH electrolyte, exhibiting an enhanced Faradaic efficiency (FE) of 13.5% at -0.35 V. This is accomplished by exhibiting an enhanced Faradaic efficiency (FE) of 0.1 M KOH electrolyte. The results of experiments and electrochemical studies reveal that the existence of cation defects in the Fe2O3 nanosheets plays an essential part in the enhancement of the electrocatalytic activity that takes place during nitrogen reduction reactions (NRR). This study not only contributes to the expanding family of transition-metal-based catalysts with increased electrocatalytic activity for NRR, but it also represents a substantial breakthrough in the design of catalysts that are based on transition metals, so it's a win-win. In addition, the use of Fe2O3 nanosheets as electrocatalysts has a lot of potential in algal membrane bioreactors because it makes nitrogen fixation easier, it encourages algae growth, and it makes nitrogen cycling more resource-efficient.

Recent advances in nanoflowers: compositional and structural diversification for potential applications

In recent years, nanoscience and nanotechnology have emerged as promising fields in materials science. Spectroscopic techniques like scanning tunneling microscopy and atomic force microscopy have revolutionized the characterization, manipulation, and size control of nanomaterials, enabling the creation of diverse materials such as fullerenes, graphene, nanotubes, nanofibers, nanorods, nanowires, nanoparticles, nanocones, and nanosheets. Among these nanomaterials, there has been considerable interest in flower-shaped hierarchical 3D nanostructures, known as nanoflowers. These structures offer advantages like a higher surface-to-volume ratio compared to spherical nanoparticles, cost-effectiveness, and environmentally friendly preparation methods. Researchers have explored various applications of 3D nanostructures with unique morphologies derived from different nanoflowers. The nanoflowers are classified as organic, inorganic and hybrid, and the hybrids are a combination thereof, and most research studies of the nanoflowers have been focused on biomedical applications. Intriguingly, among them, inorganic nanoflowers have been studied extensively in various areas, such as electro, photo, and chemical catalysis, sensors, supercapacitors, and batteries, owing to their high catalytic efficiency and optical characteristics, which arise from their composition, crystal structure, and local surface plasmon resonance (LSPR). Despite the significant interest in inorganic nanoflowers, comprehensive reviews on this topic have been scarce until now. This is the first review focusing on inorganic nanoflowers for applications in electro, photo, and chemical catalysts, sensors, supercapacitors, and batteries. Since the early 2000s, more than 350 papers have been published on this topic with many ongoing research projects. This review categorizes the reported inorganic nanoflowers into four groups based on their composition and structure: metal, metal oxide, alloy, and other nanoflowers, including silica, metal-metal oxide, core-shell, doped, coated, nitride, sulfide, phosphide, selenide, and telluride nanoflowers. The review thoroughly discusses the preparation methods, conditions for morphology and size control, mechanisms, characteristics, and potential applications of these nanoflowers, aiming to facilitate future research and promote highly effective and synergistic applications in various fields.

Charge transfer and vacancy engineering of Fe2O3 nanoparticle catalysts for highly selective N2 reduction towards NH3 synthesis

The development of electrocatalysts for N₂ reduction reaction (NRR) is significant for scalable and renewable NH₃ synthesis, but calls for a technology innovation to overcome the specific problems of low efficiency and poor selectivity. Herein, we prepare a core-shell nanostructure by coating polypyrrole (PPy) onto sulfur-doped iron oxide nanoparticles (denoted as S-Fe₂O₃@PPy) as the highly selective and durable electrocatalysts for NRR under ambient conditions. Sulfur doping and PPy coating remarkably improve the charge transfer efficiency of S-Fe₂O₃@PPy, and the interactions between PPy and Fe₂O₃ nanoparticles produce abundant oxygen vacancies as active sites for NRR. This catalyst achieves an NH₃ production rate of 22.1 μg h^-1 mg_cat^-1 and a very-high Faradic efficiency of 24.6%, surpassing other Fe₂O₃ based NRR catalysts. Density functional theory calculations show that the S-coordinated iron site can successfully activate the N₂ molecule and optimize the energy barrier during the reduction process, resulting in a small theoretical limiting potential.

Hydrophobicity Tailoring of Ferric Covalent Organic Framework/MXene Nanosheets for High-Efficiency Nitrogen Electroreduction to Ammonia

Electrocatalytic nitrogen reduction reaction (NRR) represents a promising sustainable approach for NH3 synthesis. However, the poor NRR performance of electrocatalysts is a great challenge at this stage, mainly owing to their low activity and the competitive hydrogen evolution reaction (HER). Herein, 2D ferric covalent organic framework/MXene (COF-Fe/MXene) nanosheets with controllable hydrophobic behaviors are successfully prepared via a multiple-in-one synthetic strategy. The boosting hydrophobicity of COF-Fe/MXene can effectively repel water molecules to inhibit the HER for enhanced NRR performances. By virtue of the ultrathin nanostructure, well-defined single Fe sites, nitrogen enrichment effect, and high hydrophobicity, the 1H,1H,2H,2H-perfluorodecanethiol modified COF-Fe/MXene hybrid shows a NH3 yield of 41.8 µg h-1 mgcat. -1 and a Faradaic efficiency of 43.1% at -0.5 V versus RHE in a 0.1 m Na2 SO4 water solution, which are vastly superior to the known Fe-based catalysts and even to the noble metal catalysts. This work provides a universal strategy to design and synthesis of non-precious metal electrocatalysts for high-efficiency N2 reduction to NH3 .

Ambient Electrosynthesis of Urea with Nitrate and Carbon Dioxide over Iron-Based Dual-Sites

The development of efficient electrocatalysts to generate key *NH₂ and *CO intermediates is crucial for ambient urea electrosynthesis with nitrate (NO₃ ^- ) and carbon dioxide (CO₂ ). Here we report a liquid-phase laser irradiation method to fabricate symbiotic graphitic carbon encapsulated amorphous iron and iron oxide nanoparticles on carbon nanotubes (Fe(a)@C-Fe₃ O₄ /CNTs). Fe(a)@C-Fe₃ O₄ /CNTs exhibits superior electrocatalytic activity toward urea synthesis using NO₃ ^- and CO₂ , affording a urea yield of 1341.3±112.6 μg h^-1  mg_cat ^-1 and a faradic efficiency of 16.5±6.1 % at ambient conditions. Both experimental and theoretical results indicate that the formed Fe(a)@C and Fe₃ O₄ on CNTs provide dual active sites for the adsorption and activation of NO₃ ^- and CO₂ , thus generating key *NH₂ and *CO intermediates with lower energy barriers for urea formation. This work would be helpful for design and development of high-efficiency dual-site electrocatalysts for ambient urea synthesis.

Boosting Electrocatalytic Ammonia Synthesis of Bio-Inspired Porous Mo-Doped Hematite via Nitrogen Activation

Electrochemical N₂ reduction reaction (NRR) emerges as a highly attractive alternative to the Haber-Bosch process for producing ammonia (NH₃) under ambient circumstances. Currently, this technology still faces tremendous challenges due to the low ammonia production rate and low Faradaic efficiency, urgently prompting researchers to explore highly efficient electrocatalysts. Inspired by the Fe-Mo cofactor in nitrogenase, we report Mo-doped hematite (Fe₂O₃) porous nanospheres containing Fe-O-Mo subunits for enhanced activity and selectivity in the electrochemical reduction from N₂ to NH₃. Mo-doping induces the morphology change from a solid sphere to a porous sphere and enriches lattice defects, creating more active sites. It also regulates the electronic structures of Fe₂O₃ to accelerate charge transfer and enhance the intrinsic activity. As a consequence, Mo-doped Fe₂O₃ achieves effective N₂ fixation with a high ammonia production rate of 21.3 ± 1.1 μg h^-1 mg_cat.^-1 as well as a prominent Faradaic efficiency (FE) of 11.2 ± 0.6%, superior to the undoped Fe₂O₃ and other iron oxide catalysts. Density functional theory (DFT) calculations further unravel that the Mo-doping in Fe₂O₃ (110) narrows the band gap, promotes the N₂ activation on the Mo site with an elongated N≡N bond length of 1.132 Å in the end-on configuration, and optimizes an associative distal pathway with a decreased energy barrier. Our results may pave the way toward enhancing the electrocatalytic NRR performance of iron-based materials by atomic-scale heteroatom doping.

Bifunctional OER/NRR Catalysts Based on a Thin-Layered Co3O4-x/GO Sandwich Structure

Due to ample low-coordinated surface atoms, a distorted lattice endows thin-layered transition metal oxides with a flexible electronic structure, making them the ideal candidates for overall ammonia synthesis. This work proposes a novel and facile method for the controllable synthesis of thin-layered Co₃O₄ catalysts with graphene as a conductive matrix to further enhance the overall N₂ fixation performance. X-ray photoelectron spectroscopy (XPS) and synchrotron radiation X-ray absorption spectroscopy (XAS) demonstrate that the sandwiched Co₃O_4-x/GO catalysts enable exposure of more coordination unsaturated active sites, resulting in numerous oxygen vacancies. With a higher conductivity and distorted crystalline structure, excellent electrochemical NRR activity is realized with a NH₃ production rate of 5.19 mmol g^-1 h^-1 and a Faradaic efficiency of 10.68% at -0.4 V vs reversible hydrogen electrode (RHE). The density functional theory (DFT) calculation demonstrates that introducing oxygen vacancies in thin-layered cobalt oxides could result in an increased density of states (DOS) near the Fermi level, which would accelerate the NRR rate-determining step. Charge transfer could be accelerated through a weak Co 3d-N 2p σ hybrid bond with a lower energy level. No obvious performance decay could be found after six cycles. Furthermore, the sandwiched Co₃O_4-x/GO catalyst exhibits a low overpotential of 280 mV@10 mA cm^-2 and an outstanding durability for the anode OER, even better than those of the benchmark RuO₂. Such an inexpensive sandwiched transition metal oxide catalyst shows great potential in the field of overall N₂ fixation.

Electrochemical Nitrogen Reduction to Ammonia Under Ambient Conditions: Stakes and Challenges

Aqueous electrochemical nitrogen reduction (ENR) to ammonia (NH₃ ) under ambient conditions is considered as an alternative to the energy-intensive Haber-Bosch process for ammonia production. Many metal, non-metal, carbon-based materials along with metal-chalcogenides, metal-nitrides have been explored for their ENR activity. The reported NH₃ production through ENR is still in the micro-gram level and often falls in the range of NH₃ and NO_x contaminations from the surrounding. The quantification of NH₃ at very low concentration possess enormous challenge in this field and thus many reported ENR electrocatalysts suffer from reproducibility issue. This review highlights in detail the challenges associated with ENR in aqueous medium and necessitates standardization of protocols to quantify the low concentration of NH₃ free of false-positives. It concludes the prospects of electrochemical NH₃ production through lithium-mediated N₂ reduction.

Comparison between Fe2O3/C and Fe3C/Fe2O3/Fe/C Electrocatalysts for N2 Reduction in an Alkaline Electrolyte

Cost-effective and nonprecious iron-based catalysts were synthesized, evaluated, and compared for electrocatalytic N₂ reduction reaction (NRR) under alkaline conditions in the potential range from -0.4 to 0.1 V [vs reversible hydrogen electrode (RHE)] at low temperature (≤60 °C) and atmospheric pressure. The tested H-type cell was separated by an anion exchange membrane in 6 M KOH alkaline electrolyte (pH = over 14) in order to minimize hydrogen evolution reaction and to directly form NH₃ gas. The amount of ammonia synthesized was quantified using an indophenol blue method and cross-checked with ¹H nuclear magnetic resonance spectroscopy and ion chromatography using both ¹⁴N₂ and ¹⁵N₂ gases. Because of the synergistic effect between the Fe₃C, Fe₂O₃, and Fe composites in the NRR, both the ammonia formation rate and faradaic efficiency in Fe₃C/Fe₂O₃/Fe/C were approximately fourfold higher than those in Fe₂O₃/C at 60 °C and 0.1 V (vs RHE). These results can provide insights into designing Fe-based electrocatalysts for NRR at atmospheric pressure.

Boosting electrochemical nitrogen reduction to ammonia with high efficiency using a LiNb3O8 electrocatalyst in neutral media

The nitrogen reduction reaction (NRR) has great potential as a method to replace the industrial Haber-Bosch process for ammonia synthesis. Nevertheless, the efficiency of the NRR is mainly dependent on the rational design of highly efficient and active electrocatalysts on account of the high energy of N₂ and HER as a competitive reaction. Herein, a simple solid-phase synthesis method is adopted to design and synthesize a LiNb₃O₈ (LNO) electrocatalyst, which proves that the synergistic effect of electron-rich Nb and Li elements can effectively improve the NRR activity of commercial Nb₂O₅ and Li₂CO₃. The resultant LNO electrocatalyst presents an ammonia yield rate of 7.85 μg h^-1 mg_cat.^-1 with a faradaic efficiency of 82.83% at -0.4 V vs. RHE under ambient conditions, which are much higher than those of commercial Nb₂O₅ (1.67 μg h^-1 mg_cat.^-1, 13.51%) and Li₂CO₃ (1.93 μg h^-1 mg_cat.^-1, 8.41%). Detailed characterizations demonstrate that the obtained LNO electrocatalyst has a larger specific surface area of electrochemical activity and more active sites to promote the activity of the NRR. Moreover, the synergistic effect of Li and Nb elements greatly improves the hydrophobicity of the material, which is more conducive to the occurrence of the NRR. This work highlights the enormous potential of the LNO electrocatalyst with a hydrophobic surface and easy activation of NN for highly efficient ammonia synthesis under ambient conditions.

Efficient Electrocatalytic Nitrogen Reduction to Ammonia on Ultrafine Sn Nanoparticles

Electrocatalytic nitrogen reduction reaction (NRR) at ambient conditions is a promising route for ammonia (NH₃) synthesis but still suffers from low activity and selectivity. Here, ultrafine Sn nanoparticles (NPs) grown on carbon blacks (Sn_SC/C) have been synthesized through a wet-chemical method using sodium citrate dehydrate as a stabilizing agent. Benefiting from the small sizes of Sn NPs, the Sn_SC/C catalyst exhibits excellent electrocatalytic performance for NRR with a high Faradaic efficiency of 22.76% and an NH₃ yield rate of 17.28 μg h^-1 mg^-1 in the 0.1 M Na₂SO₄ electrolyte, outperforming many reported electrocatalysts for NRR under similar conditions. Density functional theory calculation results reveal that the potential-determining step on Sn NPs is the generation of NHNH* through simultaneous hydrogenation of N₂^* by a H* and a H⁺/e^- pair via Langmuir-Hinshelwood plus Eley-Rideal mechanisms.

Emerging two-dimensional nanomaterials for electrochemical nitrogen reduction

Ammonia (NH₃) is essential to serve as the biological building blocks for maintaining organism function, and as the indispensable nitrogenous fertilizers for increasing the yield of nutritious crops. The current Haber-Bosch process for industrial NH₃ production is highly energy- and capital-intensive. In light of this, the electroreduction of nitrogen (N₂) into valuable NH₃, as an alternative, offers a sustainable pathway for the Haber-Bosch transition, because it utilizes renewable electricity and operates under ambient conditions. Identifying highly efficient electrocatalysts remains the priority in the electrochemical nitrogen reduction reaction (NRR), marking superior selectivity, activity, and stability. Two-dimensional (2D) nanomaterials with sufficient exposed active sites, high specific surface area, good conductivity, rich surface defects, and easily tunable electronic properties hold great promise for the adsorption and activation of nitrogen towards sustainable NRR. Therefore, this Review focuses on the fundamental principles and the key metrics being pursued in NRR. Based on the fundamental understanding, the recent efforts devoted to engineering protocols for constructing 2D electrocatalysts towards NRR are presented. Then, the state-of-the-art 2D electrocatalysts for N₂ reduction to NH₃ are summarized, aiming at providing a comprehensive overview of the structure-performance relationships of 2D electrocatalysts towards NRR. Finally, we propose the challenges and future outlook in this prospective area.

Recent Advances and Perspective on Electrochemical Ammonia Synthesis under Ambient Conditions

Ammonia is an essential chemical for agriculture and industry. To date, NH₃ is mainly supplied by the traditional Haber-Bosch process, which is operated under high-temperature and high-pressure in a centralized way. To achieve ammonia production in an environmentally benign way, electrochemical NH₃ synthesis under ambient conditions has become the frontier of energy and chemical conversion schemes, as it can be powered by renewable energy and operates in a decentralized way. The recent progress on developing different strategies for NH₃ production, including 1) classic NH₃ synthesis pathways over nanomaterials; 2) the Mars-van Krevelen (MvK) mechanism over metal nitrides (MN_x ); 3) reducing the nitrate into NH₃ over Cu-based nanomaterial; and 4) metal-N₂ battery release of NH₃ from Li_x M. Moreover, the most recent advances in engineering strategies for developing highly active materials and the design of the reaction systems for NH₃ synthesis are covered.

Nanomaterials for the electrochemical nitrogen reduction reaction under ambient conditions

As an important chemical product and carbon-free energy carrier, ammonia has a wide range of daily applications in several related fields. Although the industrial synthesis method using the Haber-Bosch process could meet production demands, its huge energy consumption and gas emission limit its long-time development. Therefore, the clean and sustainable electrocatalytic N2 reduction reaction (NRR) operating under conditions have attracted great attention in recent years. However, the chemical inertness of N2 molecules makes it difficult for this reaction to proceed. Therefore, rationally designed catalysts need to be introduced to activate N2 molecules. Here, we summarize the recent progress in low-dimensional nanocatalyst development, including the relationship between the structure and NRR performance from both the theoretical and experimental perspectives. Some insights into the development of NRR electrocatalysts from electronic control aspects are provided. In addition, the theoretical mechanisms, reaction pathways and credibility studies of the NRR are discussed. Some challenges and future prospects of the NRR are also pointed out.

Comprehensive Understanding of the Thriving Ambient Electrochemical Nitrogen Reduction Reaction

The electrochemical method of combining N₂ and H₂ O to produce ammonia (i.e., the electrochemical nitrogen reduction reaction [E-NRR]) continues to draw attention as it is both environmentally friendly and well suited for a progressively distributed farm economy. Despite the multitude of recent works on the E-NRR, further progress in this field faces a bottleneck. On the one hand, despite the extensive exploration and trial-and-error evaluation of E-NRR catalysts, no study has stood out to become the stage protagonist. On the other hand, the current level of ammonia production (microgram-scale) is an almost insurmountable obstacle for its qualitative and quantitative determination, hindering the discrimination between true activity and contamination. Herein i) the popular theory and mechanism of the NRR are introduced; ii) a comprehensive summary of the recent progress in the field of the E-NRR and related catalysts is provided; iii) the operational procedures of the E-NRR are addressed, including the acquisition of key metrics, the challenges faced, and the most suitable solutions; iv) the guiding principles and standardized recommendations for the E-NRR are emphasized and future research directions and prospects are provided.

Engineering an Fe2O3/FeS hybrid catalyst from a deep eutectic solvent for highly efficient electrocatalytic N2 fixation

A hybrid catalyst of crystalline Fe2O3 with amorphous FeS was designed by a deep eutectic solvent approach with a subsequent annealing process, offering a high-efficiency electrocatalyst for N2 fixation with an NH3 yield of 34.31 μg h-1 mgcat.-1 and faradaic efficiency of 18.06% at -0.25 V versus a reversible hydrogen electrode.

Emulsion-template synthesis of mesoporous nickel oxide nanoflowers composed of crossed nanosheets for effective nitrogen reduction

A novel emulsion-template synthesis approach was developed for the preparation of nickel oxide nanoflowers (NiO-NFs) composed of crossed mesoporous nanosheets. The interface assembly process was regulated by tuning the dosage of NH3·H2O, resulting in the tunability of thickness and size of mesoporous NiO nanosheets. The as-prepared NiO-NFs were characterized by field emission scanning electron microscopy, transmission electron microscopy, Brunauer-Emmett-Teller analysis, X-ray diffraction, and X-ray photoelectron spectroscopy (XPS). The results indicate that NiO-NFs have a mesopore size of about 9.5-15 nm and a crossed nanosheet thickness of about 12.4-50 nm. XPS results demonstrated that all NiO-NF samples consisted of Ni2+ and Ni3+. Electrochemical nitrogen reduction reaction (NRR) measurements revealed that NiO-NF-3.0 showed an optimal NRR performance of NH3 yield and faradaic efficiency (16.16 μg h-1 mg-1cat. and 9.17% at -0.4 V vs. RHE) in 0.1 M Na2SO4. Interestingly, NiO-NF-3.0 also displayed the highest Ni3+ content, which correlates with the order of electrochemical NRR performance. This can be attributed to the fact that Ni3+ promotes the electropositivity of NiO-NFs, resulting in more facile adsorption of N2 gas than Ni2+, and leading to enhanced electrocatalytic properties. These enhanced NRR performances are comparable or superior to those of reported noble-metal catalysts. This study provides a novel method for the fabrication of low-cost metal oxide nanomaterials that allows the construction of electrochemical NRR catalysts to meet the needs of industrial production. Also, it provides a new approach to improve the electrochemical properties by increasing the content of high-valent metal ions in a metal oxide.

Iron-Doped MoO3 Nanosheets for Boosting Nitrogen Fixation to Ammonia at Ambient Conditions

Nitrogen can be electrochemically reduced to produce ammonia, which supplies an energy-saving and environmental-benign route at room temperature, but high-efficiency catalysts are sought to reduce the reaction barrier. Here, iron-doped α-MoO₃ nanosheets are thus designed and proposed as potential catalysts for fixing N₂ to NH₃. The α-MoO₃ band structure is intentionally modulated by the iron doping, which narrows the band gap of α-MoO₃ and turns the semiconductor into a metal-like catalyst. Oxygen vacancies, generated by substituting Mo⁶⁺ for Fe³⁺ anions, are beneficial for nitrogen adsorption at the active sites. In 0.1 M Na₂SO₄, the Fe-doped MoO₃ catalyst reached a high faradaic efficiency of 13.3% and an excellent NH₃ yield rate of 28.52 μg h^-1 mg_cat^-1 at -0.7 V versus reversible hydrogen electrode, superior to most of the other metal-based catalysts. Theoretical calculations confirmed that the N₂ reduction reaction at the Fe-MoO₃ surface followed the distal reaction path.

Electrochemical N2 fixation to NH3 under ambient conditions: porous LiFe5O8 nanoparticle–reduced graphene oxide as a highly efficient and selective catalyst

In this work, porous LiFe₅O₈–rGO achieves a high NH₃ yield of 36.025 mg h⁻¹ mg_cat.⁻¹ and a high faradaic efficiency of 13.08% at −0.2 V vs. the reversible hydrogen electrode in 0.1 M HCl.

Anchoring Au(111) on a Bismuth Sulfide Nanorod: Boosting the Artificial Electrocatalytic Nitrogen Reduction Reaction under Ambient Conditions

Electrocatalytic nitrogen reduction reaction (NRR), as a green and sustainable method for ammonia synthesis, has become one of the candidates to substitute industrial Haber-Bosch ammonia synthesis in the near future. In this work, gold nanoparticles (Au NPs) were successfully anchored on bismuth sulfide nanorods (Bi₂S₃ NRs), which acted as highly efficient electrocatalytic NRR catalysts. The N-philic nature of Bi and the unique mutual coordination of Au-S-Bi can greatly promote the nitrogen adsorption and form the intermediate product N₂H*, achieving a boosted improvement in the NRR activity through a continuous hydrogenation reaction. Definitely, the as-synthesized Au(111)@Bi₂S₃ nanorod catalyst exhibits an excellent NH₃ generation rate of 45.57 μg h^-1 mg_cat.^-1 with a faradic efficiency of 3.10% at -0.8 V vs reversible hydrogen electrode. High stability and reproducibility are also demonstrated throughout the electrocatalytic NRR process. Density functional theory calculations were performed to further understand the NRR catalytic mechanism on the Au(111)@Bi₂S₃ nanorods catalyst.

Exploration and Investigation of Periodic Elements for Electrocatalytic Nitrogen Reduction

High demand for green ecosystems has urged the human community to reconsider and revamp the traditional way of synthesis of several compounds. Ammonia (NH₃ ) is one such compound whose applications have been extended from fertilizers to explosives and is still being synthesized using the high energy inhaling Haber-Bosch process. Carbon free electrocatalytic nitrogen reduction reaction (NRR) is considered as a potential replacement for the Haber-Bosch method. However, it has few limitations such as low N₂ adsorption, selectivity (competitive HER reactions), low yield rate etc. Since it is at the early stage, tremendous efforts have been devoted in understanding the reaction mechanism and screening of the electrocatalysts and electrolytes. In this review, the electrocatalysts are classified based on the periodic table with heat maps of Faraday efficiency and yield rate of NH₃ in NRR and their electrocatalytic properties toward NRR are discussed. Also, the activity of each element is discussed and short tables and concise graphs are provided to enable the researchers to understand recent progress on each element. At the end, a perspective is provided on countering the current challenges in NRR. This review may act as handbook for basic NRR understandings, recent progress in NRR, and the design and development of advanced electrocatalysts and systems.

A spinel ferrite catalyst for efficient electroreduction of dinitrogen to ammonia

Ambient electrocatalytic N2 reduction reaction (NRR) provides an eco-friendly way for artificial NH3 production, while an efficient NRR process requires active and stable electrocatalysts. In this communication, we exploit the spinel ferrite NiFe2O4 as a promising NRR catalyst. The developed NiFe2O4 nanocubes/reduced graphene oxide (NiFe2O4/RGO) exhibited an appealing NRR performance with an NH3 yield of 32.2 μg h-1 mg-1 and a faradaic efficiency (FE) of 9.8% at -0.5 V (RHE), as well as a high catalytic durability. Mechanistic investigations revealed that the surface Fe atoms serve as key NRR active sites for favorable N2 adsorption and H+ suppression. These findings may facilitate the understanding and exploration of Earth-abundant spinel ferrite catalysts for electrochemical dinitrogen fixation.

Lithium Iron Oxide (LiFeO2) for Electroreduction of Dinitrogen to Ammonia

Electrochemical nitrogen fixation offers a promising route for sustainable NH₃ production, while the rational design of effective and durable electrocatalysts is urgently required for an effective nitrogen reduction reaction (NRR) process. Herein, we explore lithium iron oxide (LiFeO₂) as a potential NRR catalyst. The developed LiFeO₂/reduced graphene oxide (rGO) delivered a combination of both a high NH₃ yield (40.5 μg h^-1 mg^-1) and high Faradaic efficiency (16.4%), exceeding those of nearly all the previously reported Li- and Fe-based catalysts. Theoretical computations showed that Fe and Li atoms on the LiFeO₂ (111) facet synergistically activated N₂ while Fe atoms served as the key active centers. Meanwhile, the undesired HER can be well impeded on both Fe and Li atoms to enable a high NRR selectivity.

Tuning the electronic structure of transition metals embedded in nitrogen-doped graphene for electrocatalytic nitrogen reduction: a first-principles study

As one of the most important subjects in chemistry, nitrogen activation and reduction to yield ammonia is still a big challenge. The lack of deep understanding of the nitrogen reduction reaction (NRR) impedes the development of high-performance catalysts. In the present study, we introduce a second transition metal (M = Mn, Fe, Co, Ni, Cu, Zn, and Mo) into the active site of a single-atom Fe-N-C catalyst to tune the electronic structure and study the activity of the as-designed neighboring bimetal Fe/M-N-C catalyst in the electrochemical NRR under acidic conditions, by performing first-principles calculations. By checking the stability of the catalysts, the adsorption ability for N₂, the Gibbs free energy change for the potential-determining step in the NRR, and the hydrogen evolution reaction (HER) activity, only the Fe/Mn-N-C catalyst is predicted to be a promising candidate for the NRR as it shows significantly improved catalytic activity and strong selectivity against the HER. A mechanistic study reveals the synergistic effects of the bimetal active sites, and the introduced Mn atom generates a strong multi-reference effect on the electronic configuration to create more tunnels to transfer the d-orbital electrons to activate the inert N[triple bond, length as m-dash]N triple bond, inducing the "acceptance-donation" process to facilitate the activation and reduction of N₂. The current results provide an effective strategy to design stable, active, and selective catalysts for the electrochemical NRR.

Quasi-amorphous and Hierarchical Fe2O3 Supraparticles: Active T1-Weighted Magnetic Resonance Imaging in Vivo and Renal Clearance

The exploration of magnetic resonance imaging (MRI) agents possessing excellent performances and high biosafety is of great importance for both fundamental science research and biomedical applications. In this study, we present that monodisperse Fe₂O₃ supraparticles (SPs) can act as T₁-weighted MRI agents, which not only possess a distinct off-on MRI switch in the tumor microenvironment but also are readily excreted from living bodies due to its quasi-amorphous structure and hierarchical topology design. First, the Fe₂O₃ SPs have a surface-to-volume ratio obviously smaller than that of their building blocks by means of self-assembly processes, which, on the one hand, causes a rather low r₁ relaxivity (0.19 mM^-1 s^-1) and, on the other hand, can effectively prevent their aggregation after intravenous injection. Second, the Fe₂O₃ SPs have a dramatic disassembly/degradation-induced active T₁-weighted signal readout (more than 6 times the r₁ value enhancement and about 20 times the r₂/r₁ ratio decrease) in the tumor microenvironment, resulting in a high signal-to-noise ratio for imaging performances. Therefore, they possess excellent in vivo imaging capacity, even with a tumor size as small as 5 mm³. Third, the disassembled/decomposed behaviors of the Fe₂O₃ SPs facilitate their timely clearance/excretion from living bodies. In particular, they exhibit distinct renal clearance behavior without any kidney damage with the right dosage. Fourth, the favorable biodegradability of the as-prepared Fe₂O₃ SPs can further relieve the concerns about the unclear biological effects, particularly on nanomaterials, in general.

The rational design of carbon coated Fe2(MoO4)3 nanosheets for lithium-ion storage with high initial coulombic efficiency and long cycle life

Binary metal oxides are potential anode materials for lithium-ion storage due to their high theoretical specific capacities. However, the practical applications of metal oxides are limited due to their large volume changes and sluggish reaction kinetics. Herein, carbon coated Fe₂(MoO₄)₃ nanosheets are prepared via a simple method, adopting urea as the template and carbon source. The carbon coating on the surface helps to elevate the conductivity of the active material and maintain structural integrity during the lithium storage process. Combining this with a catalytic effect from the generated Fe, leading to the reversible formation of a solid electrolyte interface layer, a high initial coulombic efficiency (>87%) can be obtained. Based on this, the carbon coated Fe₂(MoO₄)₃ nanosheets show excellent rate capability (a reversible discharge capacity of 983 mA h g^-1 at 5 A g^-1) and stable cycling performance (1376 mA h g^-1 after 250 cycles at 0.5 A g^-1 and 864 mA h g^-1 after 500 cycles at 2 A g^-1). This simple in situ carbonization and template method using urea provides a facile way to optimize electrode materials for next-generation energy storage devices.

Atomic Modulation, Structural Design, and Systematic Optimization for Efficient Electrochemical Nitrogen Reduction

Ammonia (NH3) is a pivotal precursor in fertilizer production and a potential energy carrier. Currently, ammonia production worldwide relies on the traditional Haber-Bosch process, which consumes massive energy and has a large carbon footprint. Recently, electrochemical dinitrogen reduction to ammonia under ambient conditions has attracted considerable interest owing to its advantages of flexibility and environmental friendliness. However, the biggest challenge in dinitrogen electroreduction, i.e., the low efficiency and selectivity caused by poor specificity of electrocatalysts/electrolytic systems, still needs to be overcome. Although substantial progress has been made in recent years, acquiring most available electrocatalysts still relies on low efficiency trial-and-error methods. It is thus imperative to establish some critical guiding principles for nitrogen electroreduction toward a rational design and accelerated development of this field. Herein, a basic understanding of dinitrogen electroreduction processes and the inherent relationships between adsorbates and catalysts from fundamental theory are described, followed by an outline of the crucial principles for designing efficient electrocatalysts/electrocatalytic systems derived from a systematic evaluation of the latest significant achievements. Finally, the future research directions and prospects of this field are given, with a special emphasis on the opportunities available by following the guiding principles.

Boron and nitrogen dual-doped carbon nanospheres for efficient electrochemical reduction of N2 to NH3

Boron and nitrogen dual-doped carbon nanospheres (BNC-NSs) show exceptional electrocatalytic activity toward the nitrogen reduction reaction, with an NH3 yield rate of 15.7 μgNH3 h-1 mgcat.-1 and faradaic efficiency of 8.1% at -400 mV vs. RHE in 0.05 M H2SO4 electrolyte. Meanwhile, stable electrocatalyst activity was also achieved for the BNC-NS electrocatalyst.

Electrochemical Synthesis of Ammonia: Recent Efforts and Future Outlook

Ammonia is a key chemical produced in huge quantities worldwide. Its primary industrial production is via the Haber-Bosch method; a process requiring high temperatures and pressures, and consuming large amounts of energy. In the past two decades, several alternatives to the existing process have been proposed, including the electrochemical synthesis. The present paper reviews literature concerning this approach and the experimental research carried out in aqueous, molten salt, or solid electrolyte cells, over the past three years. The electrochemical systems are grouped, described, and discussed according to the operating temperature, which is determined by the electrolyte used, and their performance is valuated. The problems which need to be addressed further in order to scale-up the electrochemical synthesis of ammonia to the industrial level are examined.

Interface and defect engineer of titanium dioxide supported palladium or platinum for tuning the activity and selectivity of electrocatalytic nitrogen reduction reaction

Electrocatalytic N₂ reduction reaction (NRR) under ambient condition is considered as an alternative and environmental-friendly technique to substitute the conventional process of Haber-Bosch for NH₃ production. However, there are still hurdles for researchers to control the balance between N₂ activation and competitive hydrogen evolution reaction (HER) to obtain high selectivity of NRR. Herein, we synthesized Pt/TiO₂ and Pd/TiO₂ hybrids by using laser ablation in liquid (LAL) technology combined with hydrothermal treatment and compared their activity and selectivity of N₂ reduction. The results concluded that Pt/TiO₂ exhibited a higher NH₃ yield rate whereas Pd/TiO₂ achieved a better FE for artificial N₂ fixation, confirming that enhanced activity surely needs more electrons and protons to participate in the reaction, but the limited protons and electrons furnishing could restrain HER activity and improve selectivity of NRR. Comparing with Pt/TiO₂, Pd/TiO₂ hybrids could serve as a superior catalyst for keeping a balance relationship between HER and NRR to realize excellent selectivity and high yield rate simultaneously in an alkaline solution. Overall, this work will provide a significant practice to rational design electrocatalysts for NRR at ambient conditions and Pd-based materials might open an electrocatalyst paradigm to solve the global energy and ecological crisis.

Enabling the electrocatalytic fixation of N2 to NH3 by C-doped TiO2 nanoparticles under ambient conditions

The conventional Haber-Bosch process for industrial NH₃ production from N₂ and H₂ is highly energy-intensive with a large amount of CO₂ emissions and finding a more suitable method for NH₃ synthesis under mild conditions is a very attractive topic. The electrocatalytic N₂ reduction reaction (NRR) offers us an environmentally benign and sustainable route. In this communication, we report that C-doped TiO₂ nanoparticles act as an efficient electrocatalyst for the NRR with excellent selectivity. In 0.1 M Na₂SO₄, it achieves an NH₃ yield of 16.22 μg h^-1 mg_cat. ^-1 and a faradaic efficiency of 1.84% at -0.7 V vs. the reversible hydrogen electrode. Furthermore, this catalyst also shows good stability during electrolysis and recycling tests.

Boosting electrocatalytic N2 reduction to NH3 on β-FeOOH by fluorine doping

A β-FeO(OH,F) nanorod acts as an efficient electrocatalyst for the conversion of N₂ to NH₃ in 0.5 M LiClO₄, achieving a remarkably large NH₃ yield of 42.38 μg h⁻¹ mg_cat.⁻¹ and a high FE of 9.02%.

Oxygen vacancy-engineered Fe2O3 nanocubes via a task-specific ionic liquid for electrocatalytic N2 fixation

A task-specific ionic liquid strategy was proposed for designing oxygen vacancy-rich α-Fe₂O₃ nanocubes toward excellently electrocatalytic N₂ fixation to NH₃.

An Fe2O3 nanoparticle-reduced graphene oxide composite for ambient electrocatalytic N2 reduction to NH3

Fe₂O₃-rGO behaves as an Earth-abundant NRR electrocatalyst for conversion of N₂ to NH₃ in 0.5 M LiClO₄, achieving a large NH₃ yield of 22.13 μg h⁻¹ mg⁻¹_cat and a high faradaic efficiency of 5.89%.