Single‐Atom Catalysts for the Electrocatalytic Reduction of Nitrogen to Ammonia under Ambient Conditions

Powered by renewable electricity, the electrochemical reduction of nitrogen to ammonia is proposed as a promising alternative to the energy‐ and capital‐intensive Haber–Bosch process, and has thus attracted much attention from the scientific community. However, this process suffers from low NH₃ yields and Faradaic efficiency. The development of more effective electrocatalysts is of vital importance for the practical applications of this reaction. Of the reported catalysts, single‐atom catalysts (SACs) show the significant advantages of efficient atom utilization and unsaturated coordination configurations, which offer great scope for optimizing their catalytic performance. Herein, progress in state‐of‐the‐art SACs applied in the electrocatalytic N₂ reduction reaction (NRR) is discussed, and the main advantages and challenges for developing more efficient electrocatalysts are also highlighted.     

Vacancy Engineering of Iron‐Doped W18O49 Nanoreactors for Low‐Barrier Electrochemical Nitrogen Reduction

The electrochemical nitrogen reduction reaction (NRR) is a promising energy‐efficient and low‐emission alternative to the traditional Haber–Bosch process. Usually, the competing hydrogen evolution reaction (HER) and the reaction barrier of ambient electrochemical NRR are significant challenges, making a simultaneous high NH₃ formation rate and high Faradic efficiency (FE) difficult. To give effective NRR electrocatalysis and suppressed HER, the surface atomic structure of W₁₈O₄₉, which has exposed active W sites and weak binding for H₂, is doped with Fe. A high NH₃ formation rate of 24.7 μg h^?1 mg_cat^?1 and a high FE of 20.0 % are achieved at an overpotential of only ?0.15 V versus the reversible hydrogen electrode. Ab initio calculations reveal an intercalation‐type doping of Fe atoms in the tunnels of the W₁₈O₄₉ crystal structure, which increases the oxygen vacancies and exposes more W active sites, optimizes the nitrogen adsorption energy, and facilitates the electrocatalytic NRR.     

An Amorphous Noble‐Metal‐Free Electrocatalyst that Enables Nitrogen Fixation under Ambient Conditions

N₂ fixation by the electrocatalytic nitrogen reduction reaction (NRR) under ambient conditions is regarded as a potential approach to achieve NH₃ production, which still heavily relies on the Haber–Bosch process at the cost of huge energy and massive production of CO₂. A noble‐metal‐free Bi₄V₂O₁₁/CeO₂ hybrid with an amorphous phase (BVC‐A) is used as the cathode for electrocatalytic NRR. The amorphous Bi₄V₂O₁₁ contains significant defects, which play a role as active sites. The CeO₂ not only serves as a trigger to induce the amorphous structure, but also establishes band alignment with Bi₄V₂O₁₁ for rapid interfacial charge transfer. Remarkably, BVC‐A shows outstanding electrocatalytic NRR performance with high average yield (NH₃: 23.21 μg h^?1 mg^?1_cat., Faradaic efficiency: 10.16 %) under ambient conditions, which is superior to the Bi₄V₂O₁₁/CeO₂ hybrid with crystalline phase (BVC‐C) counterpart.     

The Crucial Role of Charge Accumulation and Spin Polarization in Activating Carbon‐Based Catalysts for Electrocatalytic Nitrogen Reduction

Cost‐effective carbon‐based catalysts are promising for catalyzing the electrochemical N₂ reduction reaction (NRR). However, the activity origin of carbon‐based catalysts towards NRR remains unclear, and regularities and rules for the rational design of carbon‐based NRR electrocatalysts are still lacking. Based on a combination of theoretical calculations and experimental observations, chalcogen/oxygen group element (O, S, Se, Te) doped carbon materials were systematically evaluated as potential NRR catalysts. Heteroatom‐doping‐induced charge accumulation facilitates N₂ adsorption on carbon atoms and spin polarization boosts the potential‐determining step of the first protonation to form *NNH. Te‐doped and Se‐doped C catalysts exhibited high intrinsic NRR activity that is superior to most metal‐based catalysts. Establishing the correlation between the electronic structure and NRR performance for carbon‐based materials paves the pathway for their NRR application.     

MoS2‐Based Catalysts for N2 Electroreduction to NH3 – An Overview of MoS2 Optimization Strategies

The nitrogen reduction reaction (NRR) has become an ideal alternative to the Haber‐Bosch process, as NRR possesses, among others, the advantage of operating under ambient conditions and saving energy consumption. The key to efficient NRR is to find a suitable electrocatalyst, which helps to break the strong N≡N bond and improves the reaction selectivity. Molybdenum disulfide (MoS₂) as an emerging layered two‐dimensional material has attracted a mass of attention in various fields. In this minireview, we summarize the optimization strategies of MoS₂‐based catalysts which have been developed to improve the weak NRR activity of primitive MoS₂. Some theoretical predictions have also been summarized, which can provide direction for optimizing NRR activity of future MoS₂‐based materials. Finally, an outlook about the optimization of MoS₂‐based catalysts used in electrochemical N₂ fixation are given.     

Vacancy Engineering of Iron-Doped W18O49 Nanoreactors for Low-Barrier Electrochemical Nitrogen Reduction

The electrochemical nitrogen reduction reaction (NRR) is a promising energy-efficient and low-emission alternative to the traditional Haber–Bosch process. Usually, the competing hydrogen evolution reaction (HER) and the reaction barrier of ambient electrochemical NRR are significant challenges, making a simultaneous high NH₃ formation rate and high Faradic efficiency (FE) difficult. To give effective NRR electrocatalysis and suppressed HER, the surface atomic structure of W₁₈O₄₉, which has exposed active W sites and weak binding for H₂, is doped with Fe. A high NH₃ formation rate of 24.7 μg h⁻¹ mg_cat⁻¹ and a high FE of 20.0 % are achieved at an overpotential of only −0.15 V versus the reversible hydrogen electrode. Ab initio calculations reveal an intercalation-type doping of Fe atoms in the tunnels of the W₁₈O₄₉ crystal structure, which increases the oxygen vacancies and exposes more W active sites, optimizes the nitrogen adsorption energy, and facilitates the electrocatalytic NRR.     

Atomically Dispersed Molybdenum Catalysts for Efficient Ambient Nitrogen Fixation

NH₃ synthesis by the electrocatalytic N₂ reduction reaction (NRR) under ambient conditions is an appealing alternative to the currently employed industrial method—the Haber–Bosch process—that requires high temperature and pressure. We report single Mo atoms anchored to nitrogen‐doped porous carbon as a cost‐effective catalyst for the NRR. Benefiting from the optimally high density of active sites and hierarchically porous carbon frameworks, this catalyst achieves a high NH₃ yield rate (34.0±3.6 μg h^?1 mg_cat.^?1) and a high Faradaic efficiency (14.6±1.6 %) in 0.1 m KOH at room temperature. These values are considerably higher compared to previously reported non‐precious‐metal electrocatalysts. Moreover, this catalyst displays no obvious current drop during a 50 000 s NRR, and high activity and durability are achieved in 0.1 m HCl. The findings provide a promising lead for the design of efficient and robust single‐atom non‐precious‐metal catalysts for the electrocatalytic NRR.     

Rational Design of a Carbon‐Boron Frustrated Lewis Pair for Metal‐free Dinitrogen Activation

Molecular nitrogen (N₂) is abundant in the atmosphere and, found in many biomolecules, an essential element of life. The Haber–Bosch process, developed over 100 years ago, requires relatively harsh conditions to activate N₂ on the iron surface and generate ammonia for use as fertilizer or to produce other chemicals, leading to consumption of more than 2 % of the world's annual energy supply. Thus, developing “green” approaches for N₂ activation under mild conditions is particularly important and urgent. Here we demonstrate that a metal‐free N₂ activation could be favorable both thermodynamically and kinetically (with an activation energy as low as 9.1 kcal mol^?1) by using a carbon‐boron formal frustrated Lewis pair, which is supported by high‐level coupled cluster calculations. Mechanistic studies reveal that aromaticity plays a crucial role in stabilizing both the transition state and the product. Our findings highlight the importance of a combination of an N‐heterocyclic carbene with a methyleneborane unit in metal‐free N₂ activation, providing conceptual guidance for experimental realization.     

Defect Engineering Metal‐Free Polymeric Carbon Nitride Electrocatalyst for Effective Nitrogen Fixation under Ambient Conditions

Electrocatalytic nitrogen reduction reaction (NRR) under ambient conditions provides an intriguing picture for the conversion of N₂ into NH₃. However, electrocatalytic NRR mainly relies on metal‐based catalysts, and it remains a grand challenge in enabling effective N₂ activation on metal‐free catalysts. Here we report a defect engineering strategy to realize effective NRR performance (NH₃ yield: 8.09 μg h^?1 mg^?1_cat., Faradaic efficiency: 11.59 %) on metal‐free polymeric carbon nitride (PCN) catalyst. Illustrated by density functional theory calculations, dinitrogen molecule can be chemisorbed on as‐engineered nitrogen vacancies of PCN through constructing a dinuclear end‐on bound structure for spatial electron transfer. Furthermore, the N?N bond length of adsorbed N₂ increases dramatically, which corresponds to “strong activation” system to reduce N₂ into NH₃. This work also highlights the significance of defect engineering for improving electrocatalysts with weak N₂ adsorption and activation ability.     

Carbon‐Based Metal‐Free Catalysts for Electrocatalysis beyond the ORR

Besides their use in fuel cells for energy conversion through the oxygen reduction reaction (ORR), carbon‐based metal‐free catalysts have also been demonstrated to be promising alternatives to noble‐metal/metal oxide catalysts for the oxygen evolution reaction (OER) in metal–air batteries for energy storage and for the splitting of water to produce hydrogen fuels through the hydrogen evolution reaction (HER). This Review focuses on recent progress in the development of carbon‐based metal‐free catalysts for the OER and HER, along with challenges and perspectives in the emerging field of metal‐free electrocatalysis.     

Carbon‐Nanoplated CoS@TiO2 Nanofibrous Membrane: An Interface‐Engineered Heterojunction for High‐Efficiency Electrocatalytic Nitrogen Reduction

Developing noble‐metal‐free electrocatalysts is important to industrially viable ammonia synthesis through the nitrogen reduction reaction (NRR). However, the present transition‐metal electrocatalysts still suffer from low activity and Faradaic efficiency due to poor interfacial reaction kinetics. Herein, an interface‐engineered heterojunction, composed of CoS nanosheets anchored on a TiO₂ nanofibrous membrane, is developed. The TiO₂ nanofibrous membrane can uniformly confine the CoS nanosheets against agglomeration, and contribute substantially to the NRR performance. The intimate coupling between CoS and TiO₂ enables easy charge transfer, resulting in fast reaction kinetics at the heterointerface. The conductivity and structural integrity of the heterojunction are further enhanced by carbon nanoplating. The resulting C@CoS@TiO₂ electrocatalyst achieves a high ammonia yield (8.09×10^?10 mol s^?1 cm^?2) and Faradaic efficiency (28.6 %), as well as long‐term durability.     

Stability and Activity of Non‐Noble‐Metal‐Based Catalysts Toward the Hydrogen Evolution Reaction

A fundamental understanding of the behavior of non‐noble based materials toward the hydrogen evolution reaction is crucial for the successful implementation into practical devices. Through the implementation of a highly sensitive inductively coupled plasma mass spectrometer coupled to a scanning flow cell, the activity and stability of non‐noble electrocatalysts is presented. The studied catalysts comprise a range of compositions, including metal carbides (WC), sulfides (MoS₂), phosphides (Ni₅P₄, Co₂P), and their base metals (W, Ni, Mo, Co); their activity, stability, and degradation behavior was elaborated and compared to the state‐of‐the‐art catalyst platinum. The non‐noble materials are stable at HER potentials but dissolve substantially when no current is flowing. Through pre‐ and post‐characterization of the catalysts, explanations of their stability (thermodynamics and kinetics) are discussed, challenges for the application in real devices are analyzed, and strategies for circumventing dissolution are suggested. The precise correlation of metal dissolution with applied potential/current density allows for narrowing down suitable material choices as replacement for precious group metals as for example, platinum and opens up new ways in finding cost‐efficient, active, and stable new‐generation electrocatalysts.     

Delivery of Highly Active Noble‐Metal Nanoparticles into Microspherical Supports by an Aerosol‐Spray Method

Noble metal nanoparticles (NPs) with 1–5 nm diameter obtained from NaHB₄ reduction possess high catalytic activity. However, they are rarely used directly. This work presents a facile, versatile, and efficient aerosol‐spray approach to deliver noble‐metal NPs into metal oxide supports, while maintaining the size of the NPs and the ability to easily adjust the loading amount. In comparison with the conventional spray approach, the size of the loaded noble‐metal nanoparticles can be significantly decreased. An investigation of the 4‐nitrophenol hydrogenation reaction catalyzed by these materials suggests that the NPs/oxides catalysts have high activity and good endurance. For 1 % Au/CeO₂ and Pd/Al₂O₃ catalysts, the rate constants reach 2.03 and 1.46 min^?1, which is much higher than many other reports with the same noble‐metal loading scale. Besides, the thermal stability of catalysts can be significantly enhanced by modifying the supports. Therefore, this work contributes an efficient method as well as some guidance on how to produce highly active and stable supported noble‐metal catalysts.     

基于非贵金属催化剂常温常压电化学合成氨

氨是一种重要的化工原料和能量载体,“哈伯反应”是工业上合成氨最主要的方法,但是该方法存在着能耗高,大量排放温室气体CO₂以及转化率低等问题。近年来,常温常压下基于多相催化剂的电化学还原N₂反应(NRR)来制备氨因其原料(N₂ + H₂O)易得,不依赖传统化石能源以及条件温和等原因而表现出巨大的应用潜能,并受到了科学家的广泛关注。然而目前NRR仍存在着如催化剂以贵金属材料为主,催化效率低和催化机理未明确等问题亟待解决。本综述主要总结了电催化NRR的最新研究成果,首先介绍了电催化NRR热力学和催化机理,接着重点列举了基于非贵金属催化剂的研究进展,包括过渡金属氧化物、氮化物、硫化物、非金属催化剂及单原子催化剂等,然后讨论了几种NRR电催化剂的改性方法,以及常见的产物氨的定性定量方法,最后,就目前该研究方向中仍待解决的问题进行了总结,并对下一步的研究进行了展望。     

Knitting N‐doped Hierarchical Porous Polymers to Stabilize Ultra‐small Pd Nanoparticles for Solvent‐Free Catalysis

Hierarchical porous polymers with more than one pore size distributions can effectively support noble metal catalysts and circumvent the limitation of the diffusion of the reactants, and thus exhibit both excellent catalytic activity and superior diffusive properties. Herein, we report a simple one‐step Friedel–Crafts reaction to knit a series of benzene heterocycle monomers, such as indane, indoline, indole and tetrahydronaphthalene to obtain hierarchical porous polymers with high surface areas and/or abundant N sites. These polymers can be directly used to immobilize Pd ions, and stabilize Pd nanoparticles during the thermal reduction process to obtain Pd/polymer catalysts. In particular, indoline‐ and indole‐based polymers with high N content up to 7 wt % exhibit outstanding ability to stabilize uniform ultrasmall Pd nanoparticles. The obtained Pd‐polymers exhibit excellent catalytic activity in the solvent‐free oxidation of benzyl alcohol with O₂.     

Ag10Ti28‐Oxo Cluster Containing Single‐Atom Silver Sites: Atomic Structure and Synergistic Electronic Properties

Supported single‐atom catalysts have been emerging as promising materials in a variety of energy catalysis applications. However, studying the role of metal–support interactions at the molecular level remains a major challenge, primarily due to the lack of precise atomic structures. In this work, by replacing the frequently used TiO₂ support with its molecular analogue, titanium‐oxo cluster (TOC), we successfully produced a new kind of Ti‐O material doped with single silver sites. The as‐obtained Ag₁₀Ti₂₈ cluster, containing four exposed and six embedded Ag sites, is the largest noble‐metal‐doped Ti‐O cluster reported to date. Density functional theory (DFT) calculations show that the Ag₁₀Ti₂₈ core exhibits properties distinct from those of metallic Ag‐based materials. This Ti‐O material doped with single Ag sites presents a high ?_d and moderate CO binding capacity comparable to that of metallic Cu‐based catalysts, suggesting that it might display different catalytic performance from the common Ag‐based catalysts, for example, for CO₂ reduction. These results prove that the synergism of active surface metal atoms and the Ti‐O cluster support result in unique physical properties, which might open a new direction for single‐atom‐included catalysts.     

Electrocatalysis of N2 to NH3 by HKUST‐1 with High NH3 Yield

Electrolytic ammonia synthesis from nitrogen at ambient conditions is appearing as a promising alternative to the Haber‐Bosch process which is consuming high energy and emitting CO₂. Here, a typical MOF material, HKUST‐1 (Cu?BTC, BTC=benzene‐1,3,5‐tricarboxylate), was selected as an electrocatalyst for the reaction of converting N₂ to NH₃ under ambient conditions. At ?0.75 V vs. reversible hydrogen electrode, it achieves excellent catalytic performance in the electrochemical synthesis of ammonia with high NH₃ yield (46.63 μg h^?1 mg^?1 _cat. or 4.66 μg h^?1 cm^?2) and good Faraday efficiency (2.45%). It is indicated that the good performance of the HKUST‐1 catalyst may originate from the formation of Cu(I). In addition, the catalyst also has good selectivity for N₂ to NH₃.     

Boosting ORR Catalytic Activity by Integrating Pyridine‐N Dopants,a High Degree of Graphitization,and Hierarchical Pores into a MOF‐Derived N‐Doped Carbon in a Tandem Synthesis

N‐doped carbon materials represent promising metal‐free electrocatalysts for the oxygen reduction reaction (ORR), the cathode reaction in fuel cells, metal–air batteries, and so on. A challenge for optimizing the ORR catalytic activities of these electrocatalysts is to tune their local structures and chemical compositions in a rational and controlled way that can achieve the synergistic function of each factor. Herein, we report a tandem synthetic strategy that integrates multiple contributing factors into an N‐doped carbon. With an N‐containing MOF (ZIF‐8) as the precursor, carbonization at higher temperatures leads to a higher degree of graphitization. Subsequent NH₃ etching of this highly graphitic carbon enabled the introduction of a higher content of pyridine‐N sites and higher porosity. By optimizing these three factors, the resultant carbon materials displayed ORR activity that was far superior to that of carbon derived from a one‐step pyrolysis. The onset potential of 0.955 V versus a reversible hydrogen electrode (RHE) and the half‐wave potential of 0.835 V versus RHE are among the top ranks of metal‐free ORR catalysts and are comparable to commercial Pt/C (20 wt %) catalysts. Kinetic studies revealed lower H₂O₂ yields, higher electron‐transfer numbers, and lower Tafel slopes for these carbon materials compared with that derived from a one‐step carbonization. These findings verify the effectiveness of this tandem synthetic strategy to enhance the ORR activity of N‐doped carbon materials.     

A Janus Fe‐SnO2 Catalyst that Enables Bifunctional Electrochemical Nitrogen Fixation

Electrochemical N₂ reduction reactions (NRR) and the N₂ oxidation reaction (NOR), using H₂O and N₂, are a sustainable approach to N₂ fixation. To date, owing to the chemical inertness of nitrogen, emerging electrocatalysts for the electrochemical NRR and NOR at room temperature and atmospheric pressure remain largely underexplored. Herein, a new‐type Fe‐SnO₂ was designed as a Janus electrocatalyst for achieving highly efficient NRR and NOR catalysis. A high NH₃ yield of 82.7 μg h^?1 mg_cat.^?1 and a Faraday efficiency (FE) of 20.4 % were obtained for NRR. This catalyst can also serve as an excellent NOR electrocatalyst with a NO₃^? yields of 42.9 μg h^?1 mg_cat.^?1 and a FE of 0.84 %. By means of experiments and DFT calculations, it is revealed that the oxygen vacancy‐anchored single‐atom Fe can effectively adsorb and activate chemical inert N₂ molecules, lowering the energy barrier for the vital breakage of N≡N and resulting in the enhanced N₂ fixation performance.     

Synthesis of Sub‐2 nm Iron‐Doped NiSe2 Nanowires and Their Surface‐Confined Oxidation for Oxygen Evolution Catalysis

Ultrathin nanostructures are attractive for diverse applications owing to their unique properties compared to their bulk materials. Transition‐metal chalcogenides are promising electrocatalysts, yet it remains difficult to make ultrathin structures (sub‐2 nm), and the realization of their chemical doping is even more challenging. Herein we describe a soft‐template mediated colloidal synthesis of Fe‐doped NiSe₂ ultrathin nanowires (UNWs) with diameter down to 1.7 nm. The synergistic interplay between oleylamine and 1‐dodecanethiol is crucial to yield these UNWs. The in situ formed amorphous hydroxide layers that is confined to the surface of the ultrathin scaffolds enable efficient oxygen evolution electrocatalysis. The UNWs exhibit a very low overpotential of 268 mV at 10 mA cm^?2 in 0.1 m KOH, as well as remarkable long‐term stability, representing one of the most efficient noble‐metal‐free catalysts.