Enhanced Electrocatalytic N 2 Reduction via Partial Anion Substitution in Titanium Oxide–Carbon Composites

The Tandem Nitrate and CO2 Reduction for Urea Electrosynthesis: Role of Surface N-Intermediates in CO2 Capture and Activation

Electrochemical reduction of CO2 and nitrate offers a promising avenue to produce valuable chemicals through the using of greenhouse gas and nitrogen-containing wastewater. However, the generally proposed reaction pathway of concurrent CO2 and nitrate reduction for urea synthesis requires the catalysts to be both efficient in both CO2 and nitrate reduction, thus narrowing the selection range of suitable catalysts. Herein, we demonstrate a distinct mechanism in urea synthesis, a tandem NO3 - and CO2 reduction, in which the surface amino species generated by nitrate reduction play the role to capture free CO2 and subsequent initiate its activation. When using the TiO2 electrocatalyst derived from MIL-125-NH2, it intrinsically exhibits low activity in aqueous CO2 reduction, however, in the presence of both nitrate and CO2, this catalyst achieves an excellent urea yield rate of 43.37 mmol ⋅ g-1 ⋅ h-1 and a Faradaic efficiency of 48.88 % at -0.9 V vs. RHE in a flow cell. Even at a low CO2 level of 15 %, the Faradaic efficiency of urea synthesis remains robust at 42.33 %. The tandem reduction procedure was further confirmed by in situ spectroscopies and theoretical calculations. This research provides new insights into the selection and design of electrocatalysts for urea synthesis.

Ionic Liquids Modulating Local Microenvironment of Ni-Fe Binary Single Atom Catalyst for Efficient Electrochemical CO2 Reduction

The Ni and Fe dual-atom catalysts still undergo strikingly attenuation under high current density and high overpotential. To ameliorate the issue, the ionic liquids with different cations or anions are used in this work to regulate the micro-surface of nitrogen-doped carbon supported Ni and Fe dual-atom sites catalyst (NiFe-N-C) by an impregnation method. The experimental data reveals the dual function of ionic liquids, which enhances CO2 adsorption ability and modulates electronic structure, facilitating CO2 anion radical (CO2 •¯) stabilization and decreasing onset potential. The theoretical calculation results prove that the attachment of ionic liquids modulates electronic structure, reduces energy barrier of CO2 •¯ formation, and enhances overall ECR performance. Based on these merits, BMImPF6 modified NiFe-N-C (NiFe-N-C/BMImPF6) achieves the high CO faradaic efficiency of 91.9% with a CO partial current density of -120 mA cm-2 at -1.0 V. When the NiFe-N-C/BMImPF6 is assembled as cathode of Zn-CO2 battery, it delivers the highest power density of 2.61 mW cm-2 at 2.57 mA cm-2 and superior cycling stability. This work will afford a direction to modify the microenvironment of other dual-atom catalysts for high-performance CO2 electroreduction.

Metal-Organic Framework (MOF)-Based Clean Energy Conversion: Recent Advances in Unlocking its Underlying Mechanisms

Carbon neutrality is an important goal for humanity . As an eco-friendly technology, electrocatalytic clean energy conversion technology has emerged in the 21st century. Currently, metal-organic framework (MOF)-based electrocatalysis, including oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), hydrogen oxidation reaction (HOR), carbon dioxide reduction reaction (CO2RR), nitrogen reduction reaction (NRR), are the mainstream energy catalytic reactions, which are driven by electrocatalysis. In this paper, the current advanced characterizations for the analyses of MOF-based electrocatalytic energy reactions have been described in details, such as density function theory (DFT), machine learning, operando/in situ characterization, which provide in-depth analyses of the reaction mechanisms related to the above reactions reported in the past years. The practical applications that have been developed for some of the responses that are of application values, such as fuel cells, metal-air batteries, and water splitting have also been demonstrated. This paper aims to maximize the potential of MOF-based electrocatalysts in the field of energy catalysis, and to shed light on the development of current intense energy situations.

Revealing the Adsorption Behavior of Nitrogen Reduction Reaction on Strained Ti2 CO2 by a Spin-Polarized d-band Center Model

Electrocatalytic reduction of dinitrogen to ammonia has attracted significant research interest. Herein, it reports the boosting performance of electrocatalytic nitrogen reduction on Ti2 CO2 MXene with an oxygen vacancy through biaxial tensile strain engineering. Specifically, tensile strain modified electronic structures and formation energy of oxygen vacancy are evaluated. The exposed Ti atoms with additional electron states near the Fermi level serve as active site for intermediate adsorption, leading to superior catalytic performance (Ulimit = -0.44 V) under 2.5% biaxial tensile strain through a distal mechanism. However, the two sides of the "Sabatier optimum" in volcano plot are not limited by two different electronic steps, but are induced by the diverse adsorption behaviors of intermediates. Crucially, the "Sabatier optimum" results from the different response speeds of the adsorption energy for *N2 and *NNH to strains. Moreover, the authors observe conventional d-band adsorption for *N2 and *NNH, non-linear adsorption for *NNH2 , and abnormal d-band adsorption for *N, *NH, *NH2 , and *NH3 , which can be explained by the competition between attractive orbital hybridization and repulsive orbital orthogonalization with the spin-polarized d-band model, which further clarifies the contributions of 3σ → dz2 and dxz /dyz → 2π* to the overall population of bonding and anti-bonding states.

Recent Advances in Metal-Organic Framework-Based Nanomaterials for Electrocatalytic Nitrogen Reduction

The production of ammonia under moderate conditions is of environmental and sustainable importance. The electrochemical nitrogen reduction reaction (E-NRR) method has been intensively investigated in the recent decades. Nowadays, the further development of E-NRR is largely hindered by the lack of competent electrocatalysts. Metal-organic frameworks (MOFs) are considered as the next-generation catalysts for E-NRR, featuring their tailorable structures, abundant active sites and favorable porosity. To present a comprehensive review on both the fundamental and advanced development in MOFs catalyst-based E-NRR field, this paper first introduces the basic principles of E-NRR, including the reaction mechanism, major apparatus components, performance criteria, and ammonia detection protocols. Next, the synthesis and characterization methods for MOFs and their derivatives are discussed. In addition, a reaction mechanism study via density functional theory calculations is also presented. After that, the recent advancement of MOF-based catalysts in the E-NRR field as well as the modification approaches on MOFs for E-NRR optimization is elaborated. Finally, the current challenges and outlook of MOF catalyst-based E-NRR field are emphasized.

Nanoengineering Metal-Organic Frameworks and Derivatives for Electrosynthesis of Ammonia

Electrocatalytic synthesis under mild conditions has become increasingly important as one of the practical alternatives for industrial applications, especially for the green ammonia (NH3) industry. A properly engineered electrocatalyst plays a vital role in the realization of superior catalytic performance. Among various types of promising nanomaterials, metal-organic frameworks (MOFs) are competitive candidates for developing efficient electrocatalytic NH3 synthesis from simple nitrogen-containing molecules or ions, such as N2 and NO3-. In this review, recent advances in the development of electrocatalysts derived from MOFs for the electrosynthesis of NH3 are collected, categorized, and discussed, including their application in the N2 reduction reaction (NRR) and the NO3- reduction reaction (NO3RR). Firstly, the fundamental principles are illustrated, such as plausible mechanisms of NH3 generation from N2 and NO3-, the apparatus of corresponding electrocatalysis, parameters for evaluation of reaction efficiency, and detection methods of yielding NH3. Then, the electrocatalysts for NRR processes are discussed in detail, including pristine MOFs, MOF-hybrids, MOF-derived N-doped porous carbons, single atomic catalysts from pyrolysis of MOFs, and other MOF-related materials. Subsequently, MOF-related NO3RR processes are also listed and discussed. Finally, the existing challenges and prospects for the rational design and fabrication of electrocatalysts from MOFs for electrochemical NH3 synthesis are presented, such as the evolution of investigation methods with artificial intelligence, innovation in synthetic methods of MOF-related catalysts, advancement of characterization techniques, and extended electrocatalytic reactions.

High-Performance Electrocatalytic Reduction of Nitrite to Ammonia under Ambient Conditions on a FeP@TiO2 Nanoribbon Array

Electrochemical nitrite (NO2-) reduction is recognized as a promising strategy for synthesizing valuable ammonia (NH3) and degrading NO2- pollutants in wastewater. The six-electron process for the NO2- reduction reaction is complex and necessitates a highly selective and stable electrocatalyst for efficient conversion of NO2- to NH3. Herein, a FeP nanoparticle-decorated TiO2 nanoribbon array on a titanium plate (FeP@TiO2/TP) is proposed as an efficient catalyst for NH3 production under ambient conditions. In 0.1 M NaOH with 0.1 M NO2-, such a FeP@TiO2/TP affords a large NH3 yield of 346.6 μmol h-1 cm-2 and a high Faradaic efficiency of 97.1%. Additionally, it demonstrates excellent stability and durability during long-term cycling tests and electrolysis experiments.

First-principles screening of transition metal doped anatase TiO2(101) surfaces for the electrocatalytic nitrogen reduction

The electrocatalytic nitrogen reduction reaction (eNRR) has been attracting intensive scientific attention as a potential alternative to the industrial Haber-Bosch process for ammonia production. Although many materials have been investigated, optimal catalysts for the reaction remain to be found. In this work, we performed the theoretical screening of 3d-5d transition metal doped anatase TiO₂ for the eNRR. The most favorable doping site of each transition metal on the (101) surface was located. We found that the doping of transition metals promotes the formation of oxygen vacancies which are beneficial for the reaction. The scaling relations between the energies of the key intermediates were investigated. Using a machine learning algorithm (SVM), we identified two adsorption modes for the end-on adsorbed *HNN, which exhibited different scaling relations with *NH₂. From a two-step process, we screened out several candidates, among which Au and Ta were proposed to be the most efficient dopants. Electronic structure analysis reveals that they can efficiently lower the energy of the intermediates. These results should be helpful for the design of more efficient TiO₂-based catalysts for the eNRR.

Metal-Organic Framework Materials for Production and Distribution of Ammonia

The efficient production of ammonia (NH₃) from dinitrogen (N₂) and water (H₂O) using renewable energy is an important step on the roadmap to the ammonia economy. The productivity of this conversion hinges on the design and development of new active catalysts. In the wide scope of materials that have been examined as catalysts for the photo- and electro-driven reduction of N₂ to NH₃, functional metal-organic framework (MOF) catalysts exhibit unique properties and appealing features. By elucidating their structural and spectroscopic properties and linking this to the observed activity of MOF-based catalysts, valuable information can be gathered to inspire new generations of advanced catalysts to produce green NH₃. NH₃ is also a surrogate for the hydrogen (H₂) economy, and the potential application of MOFs for the practical and effective capture, safe storage, and transport of NH₃ is also discussed. This Perspective analyzes the contribution that MOFs can make toward the ammonia economy.

Unifying the Nitrogen Reduction Activity of Anatase and Rutile TiO2 Surfaces

TiO₂ is a model transition metal oxide that has been applied frequently in both photocatalytic and electrocatalytic nitrogen reduction reactions (NRR). However, the phase which is more NRR active still remains a puzzle. This work presents a theoretical study on the NRR activity of the (001), (100), (101), and (110) surfaces of both anatase and rutile TiO₂ . We found that perfect surfaces are not active for NRR, while the oxygen vacancy can promote the reaction by providing excess electrons and low-coordinated Ti atoms that enhance the binding of the key intermediate (HNN*). The NRR activity of the eight facets can be unified into a single scaling line. The anatase TiO₂ (101) and rutile TiO₂ (101) surfaces were found to be the most and the second most active surfaces with a limiting potential of -0.91 V and -0.95 V respectively, suggesting that the TiO₂ NRR activity is not very phase-sensitive. For photocatalytic NRR, the results suggest that the anatase TiO₂ (101) surface is still the most active facet. We further found that the binding strength of key intermediates scale well with the formation energy of oxygen vacancy, which is determined by the oxygen coordination number and the degree of relaxation of the surface after the creation of oxygen vacancy. This work provides a comprehensive understanding of the activity of TiO₂ surfaces. The results should be helpful for the design of more efficient TiO₂ -based NRR catalysts.

Electrochemical Generation of Catalytically Active Edge Sites in C2 N-Type Carbon Materials for Artificial Nitrogen Fixation

The electrochemical nitrogen reduction reaction (NRR) to ammonia (NH₃ ) is a potentially carbon-neutral and decentralized supplement to the established Haber-Bosch process. Catalytic activation of the highly stable dinitrogen molecules remains a great challenge. Especially metal-free nitrogen-doped carbon catalysts do not often reach the desired selectivity and ammonia production rates due to their low concentration of NRR active sites and possible instability of heteroatoms under electrochemical potential, which can even contribute to false positive results. In this context, the electrochemical activation of nitrogen-doped carbon electrocatalysts is an attractive, but not yet established method to create NRR catalytic sites. Herein, a metal-free C₂ N material (HAT-700) is electrochemically etched prior to application in NRR to form active edge-sites originating from the removal of terminal nitrile groups. Resulting activated metal-free HAT-700-A shows remarkable catalytic activity in electrochemical nitrogen fixation with a maximum Faradaic efficiency of 11.4% and NH₃ yield of 5.86 µg mg^-1 _cat h^-1 . Experimental results and theoretical calculations are combined, and it is proposed that carbon radicals formed during activation together with adjacent pyridinic nitrogen atoms play a crucial role in nitrogen adsorption and activation. The results demonstrate the possibility to create catalytically active sites on purpose by etching labile functional groups prior to NRR.

High-Efficiency Ammonia Electrosynthesis on Anatase TiO2-x Nanobelt Arrays with Oxygen Vacancies by Selective Reduction of Nitrite

Electrocatalytic reduction of nitrite to NH₃ provides a new route for the treatment of nitrite in wastewater, as well as an attractive alternative to NH₃ synthesis. Here, we report that an oxygen vacancy-rich TiO_2-x nanoarray with different crystal structures self-supported on the Ti plate can be prepared by hydrothermal synthesis and by subsequently annealing it in an Ar/H₂ atmosphere. Anatase TiO_2-x (A-TiO_2-x) can be a superb catalyst for the efficient conversion of NO₂^- to NH₃; a high NH₃ yield of 12,230.1 ± 406.9 μg h^-1 cm^-2 along with a Faradaic efficiency of 91.1 ± 5.5% can be achieved in a 0.1 M NaOH solution containing 0.1 M NaNO₂ at -0.8 V, which also exhibits preferable durability with almost no decay of catalytic performances after cycling tests and long-term electrolysis. Furthermore, a Zn-NO₂^- battery with such A-TiO_2-x as a cathode delivers a power density of 2.38 mW cm^-2 as well as a NH₃ yield of 885 μg h^-1 cm^-2.

Lattice-Confined Single-Atom Fe1 Sx on Mesoporous TiO2 for Boosting Ambient Electrocatalytic N2 Reduction Reaction

Mimicking natural nitrogenase to create highly efficient single-atom catalysts (SACs) for ambient N₂ fixation is highly desired, but still challenging. Herein, S-coordinated Fe SACs on mesoporous TiO₂ have been constructed by a lattice-confined strategy. The extended X-ray absorption fine structure and X-ray photoelectron spectroscopy spectra demonstrate that Fe atoms are anchored in TiO₂ lattice via the FeS₂ O₂ coordination configuration. Theoretical calculations reveal that FeS₂ O₂ sites are the active centers for electrocatalytic nitrogen reduction reaction (NRR). Moreover, the finite element analysis shows that confinement of opened and ordered mesopores can facilitate the mass transport and offer an enlarged active surface area for NRR. As a result, this catalyst delivers a favorable NH₃ yield rate of 18.3 μg h^-1  mg_cat. ^-1 with a high Faradaic efficiency of 17.3 % at -0.20 V versus a reversible hydrogen electrode. Most importantly, this lattice-confined strategy is universal and can also be applied to Ni₁ S_x @TiO₂ , Co₁ S_x @TiO₂ , Mo₁ S_x @TiO₂ , and Cu₁ S_x @TiO₂ SACs. Our study provides new hints for the design and biomimetic synthesis of highly efficient NRR electrocatalysts.

Modulating the Oxidation State of Titanium via Dual Anions Substitution for Efficient N2 Electroreduction

The electrocatalytic nitrogen reduction reaction (NRR) is a promising approach for renewable ammonia synthesis but remains significantly challenging due to the low yield and poor selectivity. Herein, a facile N and S dual anions substitution strategy is developed to tune the Ti oxidation states of TiO₂ nanohybrid catalyst (NS-TiO₂ /C), in which anatase TiO₂ nanoplates with dense Ti³⁺ active sites are uniformly dispersed on porous carbon derived from 2D Ti₃ C₂ T_x nanosheets. The catalyst NS-TiO₂ /C exhibits a superior ambient NRR efficiency with an NH₃ yield rate of 19.97 µg h^-1 mg^-1_cat and Faradaic efficiency of 25.49% and is coupled with a remarkable 50 h long-term stability at -0.25 V versus RHE. Both experimental and theoretical results reveal that the N and S dual-substitution effectively regulate the Ti oxidation state and electronical properties of the NS-TiO₂ /C via simultaneously forming interstitial and substitutional TiS and TiN bonds in the anatase TiO₂ lattice, inducing oxygen vacancies and dense Ti³⁺ active species as well as better electronic conductivity, which substantially facilitates N₂ chemisorption and activation, and reduces the energy barrier of the rate-determining step, thereby essentially boosting NRR efficiency. This work provides a valuable approach to the rational design of advanced materials by modulating oxidation states for efficient electrocatalysis.

Ambient Ammonia Synthesis via Electrochemical Reduction of Nitrate Enabled by NiCo2 O4 Nanowire Array

NiCo₂ O₄ nanowire array on carbon cloth (NiCo₂ O₄ /CC) is proposed as a highly active electrocatalyst for ambient nitrate (NO₃ ^- ) reduction to ammonia (NH₃ ). In 0.1 m NaOH solution with 0.1 m NaNO₃ , such NiCo₂ O₄ /CC achieves a high Faradic efficiency of 99.0% and a large NH₃ yield up to 973.2 µmol h^-1  cm^-2 . The superior catalytic activity of NiCo₂ O₄ comes from its half-metal feature and optimized adsorption energy due to the existence of Ni in the crystal structure. A Zn-NO₃ ^- battery with NiCo₂ O₄ /CC cathode also shows a record-high battery performance.

Insight into Hydrogenation Selectivity of the Electrocatalytic Nitrate-to-Ammonia Reduction Reaction via Enhancing the Proton Transport

The electrocatalytic nitrate-to-ammonia reduction reaction route (NARR) is one of the emerging routes toward green ammonia synthesis, and its conversion efficiency is controlled mainly by the hydrogenation selectivity. This study proposed a likely NARR route feasible and effective even in a neutral condition. Its high catalytic selectivity and efficiency were achieved by a switch of the sulfate solution to the phosphate buffer solution (PBS), while conditions of NO₃ ^- concentration, pH, and applied potential were maintained unchanged. Specifically, the faradaic efficiencies toward NH₃ (FE NH 3 ) in Na₂ SO₄ were as low as 9.8, 19.8, and 11.4 % versus remarkably jumping to 82.8, 90.5, and 89.5 % in PBS under -0.75, -1.0, and -1.25 V, respectively. The corresponding faradaic efficiencies toward NO₂ ^- (FE NO 2 - ), 77.0, 69.2, and 73.7 % in Na₂ SO₄ , significantly dropped to10.8, 7.4, and 4.4 % in PBS, evidencing an unexpected selectivity reversal of the nitrate reduction from NO₂ ^- to NH₃ . This insight was further revealed by the visualization of the pH gradient near the electrode surface during NARR and confirmed by density functional theory calculations; PBS notably facilitated the proton transport and active mitigation over the proton transfer barrier. The use of PBS resulted in a maximal partial current density toward NH₃ (J NH 3 ) and NH₃ formation rate (r NH 3 ) up to 133.5 mA cm^-2 and 1.74×10^-7  mol s^-1  cm^-2 in 1.0 m KNO₃ at -1.25 V.

Highly Active and Selective Electroreduction of N2 by the Catalysis of Ga Single Atoms Stabilized on Amorphous TiO2 Nanofibers

The electroreduction of N₂ under ambient conditions has emerged as one of the most promising technologies in chemistry, since it is a greener way to make NH₃ than the traditional Haber-Bosch process. However, it is greatly challenged with a low NH₃ yield and faradaic efficiency (FE) because of the lack of highly active and selective catalysts. Inherently, transition (d-block) metals suffer from inferior selectivity due to fierce competition from H₂ evolution, while post-transition (p-block) metals exhibit poor activity due to insufficient "π back-donation" behavior. Considering their distinct yet complementary electronic structures, here we propose a strategy to tackle the activity and selectivity challenge through the atomic dispersion of p-block metal on an all-amorphous transition-metal matrix. To address the activity issue, lotus-root-like amorphous TiO₂ nanofibers are synthesized which, different from vacancy-engineered TiO₂ nanocrystals reported previously, possess abundant intrinsic oxygen vacancies (V_O) together with under-coordinated dangling bonds in nature, resulting in significantly enhanced N₂ activation and electron transport capacity. To address the selectivity issue, well-isolated single atoms (SAs) of Ga are successfully synthesized through the confinement effect of V_O, resulting in Ga-V_O reactive sites with the maximum availability. It is revealed by density functional theory calculations that Ga SAs are favorable for the selective adsorption of N₂ at the catalyst surface, while V_O can facilitate N₂ activation and reduction subsequently. Benefiting from this coupled activity/selectivity design, high NH₃ yield (24.47 μg h^-1 mg^-1) and FE (48.64%) are achieved at an extremely low overpotential of -0.1 V vs RHE.

Design and synthesis of TiO2/C nanosheets with a directional cascade carrier transfer

Directional charge transfer in TiO2 nanosheets is achieved by design of TiO2 lattice-Ti vacancy-interlayered sp2 carbon at the interface.

Recent advances in MoS2-based materials for electrocatalysis

The increasing energy demand and related environmental issues have drawn great attention of the world, thus necessitating the development of sustainable technologies to preserve the ecosystems for future generations. Electrocatalysts...

Graphdiyne-Induced Iron Vacancy for Efficient Nitrogen Conversion

An iron vacancy-rich ferroferric oxide/graphdiyne heterostructure (IVR-FO/GDY) is rationally designed and fabricated for high-efficiency electrocatalytic nitrogen fixation to ammonia (ENFA). Experimental and theoretical results show that the GDY-induced iron vacancies in IVR-FO/GDY promote the electrocatalysis, and activate the local O sites to transfer electrons towards GDY to boost ENFA, resulting in promising electrocatalytic performances with a highest ammonia yield (YNH3 ) of 134.02 µg h-1 mgcat.-1 and Faradaic efficiency (FE) of up to 60.88%, as well as the high long-term stability in neutral electrolytes. The cationic vacancy activation strategy proposed in this work has strong general and universal guiding significance to the design of new efficient electrocatalysts for various electrochemical energy conversion reactions. Such defect engineering may be used efficiently in electrocatalysis, leading to the development and progress of energy industry.

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.

Emerging Materials and Methods toward Ammonia-Based Energy Storage and Conversion

Efficient storage and conversion of renewable energies is of critical importance to the sustainable growth of human society. With its distinguishing features of high hydrogen content, high energy density, facile storage/transportation, and zero-carbon emission, ammonia has been recently considered as a promising energy carrier for long-term and large-scale energy storage. Under this scenario, the synthesis, storage, and utilization of ammonia are key components for the implementation of ammonia-mediated energy system. Being different from fossil fuels, renewable energies normally have intermittent and variable nature, and thus pose demands on the improvement of existing technologies and simultaneously the development of alternative methods and materials for ammonia synthesis and storage. The energy release from ammonia in an efficient manner, on the other hand, is vital to achieve a sustainable energy supply and complete the nitrogen circle. Herein, recent advances in the thermal-, electro-, plasma-, and photocatalytic ammonia synthesis, ammonia storage or separation, ammonia thermal/electrochemical decomposition and conversion are summarized with the emphasis on the latest developments of new methods and materials (catalysts, electrodes, and sorbents) for these processes. The challenges and potential solutions are discussed.

Shedding Light on the Role of Chemical Bond in Catalysis of Nitrogen Fixation

Ammonia (NH₃ ) and nitrates are essential for human society because of their widespread utilization for producing medicines, fibers, fertilizers, etc. In recent years, the development on nitrogen fixation under mild reaction conditions has attracted much attention. However, the very low conversion efficiency and ambiguous catalytic mechanism remain the major hurdles for the research of nitrogen fixation. This review aims to clarify the role of chemical bond in catalytic nitrogen fixation by summarizing and analyzing the recent development of nitrogen fixation research. In detail, the atomic-scale mechanism of nitrogen fixation reaction, the various methods to improve the nitrogen fixation performance, and the computational investigation of nitrogen fixation are discussed, all from a chemical bond perspective. It is hoped that this review could trigger more profound pondering and deeper exploration in the field of catalytic nitrogen fixation.

Synergism of Interfaces and Defects: Cu/Oxygen Vacancy-Rich Cu-Mn3O4 Heterostructured Ultrathin Nanosheet Arrays for Selective Nitrate Electroreduction to Ammonia

Achieving high efficiency in nitrate (NO₃^-) to ammonia (NH₃) electrocatalysis requires the exploration of advanced electrocatalysts with a well-designed composition and architecture. In this work, a facile one-step hydrothermal approach was developed for the construction of novel Cu/oxygen vacancy-rich Cu-Mn₃O₄ heterostructured ultrathin nanosheet arrays on Cu foam (Cu/Cu-Mn₃O₄ NSAs/CF). Two-dimensional ultrathin nanosheet arrays could increase the exposure of catalytically active centers, and the heterogeneous nanointerface and oxygen vacancies synergistically improve the nitrate-to-ammonia activity over the active centers. Due to the desirable compositional and structural advantages, the Cu/Cu-Mn₃O₄ NSAs/CF demonstrated excellent performance for the electrocatalytic nitrate reduction to ammonia with high ammonia selectivity (87.6%) and Faradic efficiency (92.4%).

Structural Regulation and Light Hydrocarbon Adsorption/Separation of Three Zirconium-Organic Frameworks Based on Different V-Shaped Ligands

On the basis of different V-shaped ligands, three zirconium-organic frameworks (JLU-Liu45, Zr-SDBA, and Zr-OBBA) have been successfully constructed. By regulating spatial configuration and functional groups of organic ligands, these as-synthesized Zr-MOFs (MOF = metal-organic framework) display distinct structures and different light hydrocarbon adsorption/separation capabilities. JLU-Liu45, with a double-walled interpenetrated 3D primitive cubic (pcu) framework, exhibits good gas-adsorption capacity but not prominent selective separation ability. Through regulating sizes and torsion angles of the organic ligands, Zr-SDBA possesses a 2D square lattice (sql) network, while Zr-OBBA displays a non-interpenetrated 3D pcu framework. Furthermore, by regulating functional groups on the ligands, Zr-SDBA shows prominent C₂H₂ uptake (101.2 cm³·g^-1) and the best C₂H₂/CH₄ selectivity (230.5, 1:1) among the three Zr-MOFs, and Zr-OBBA shows a significant C₃H₈/CH₄ selectivity (105.6, 1:1). This work demonstrates the feasibility of structural regulation for MOF materials in the light hydrocarbon adsorption/separation field.

Reduction of N2 to NH3 by TiO2-supported Ni cluster catalysts: a DFT study

Electrochemical techniques for ammonia synthesis are considered as an encouraging energy conversion technology to efficiently meet the challenge of nitrogen cycle balance. Herein, we find that TiO2(101)-supported Ni4 and Ni13 clusters can serve as efficient catalysts for electrocatalytic N2 reduction based on theoretical calculations. Electronic property calculations reveal the formation of electron-deficient Ni clusters on the TiO2 surface, which provides multiple active sites for N2 adsorption and activation. Theoretical calculation identifies the strongest activated configuration of N2* on the catalysts and confirms the potential-limiting step in the nitrogen reduction reaction (NRR). On Ni4-TiO2(101), N2* → NNH* is the potential-limiting step with a very small free energy increase (ΔG) of 0.24 eV (the corresponding overpotential is 0.33 V), while on Ni13-TiO2(101) the potential-limiting step occurs at NH* → NH2* with ΔG of 0.49 eV (the corresponding overpotential is 0.58 V). Moreover, the Nin-TiO2(101) catalyst, especially Ni13-TiO2(101), involves in a highly selective NRR even at the corresponding NRR overpotential. This work will enlighten material design to construct metal oxide supported transition metal clusters for the highly efficient NRR and NH3 synthesis.

Intrinsic Electron Localization of Metastable MoS2 Boosts Electrocatalytic Nitrogen Reduction to Ammonia

The advancement of efficient electrocatalysts toward the nitrogen reduction reaction (NRR) is critical in sustainable ammonia synthesis under ambient pressure and temperature. Manipulating the electronic configuration of electrocatalysts is particularly vital to form metal-nitrogen (MN) bonds during the NRR through regulating the active electronic states of sites. Here, in sharp contrast to stable 2H MoS₂ without metal chains, MoMo bonding in metastable polymorphs of MoS₂ bulk (zigzag chain in the 1T' phase and diamond chain in the 1T″' phase) is discovered to significantly increase intrinsic electron localization around the metal chains. This can enhance the charge transfer from the adsorbed nitrogen molecule to the metal chains, allowing for boosted NRR kinetics. The electrochemical experiments show that the NH₃ yield rate and the faradaic efficiency of the metastable 1T″' MoS₂ rich with abundant Mo-Mo bonds are about 9 and 12 times above average than those of 2H MoS₂ , correspondingly. Theoretical simulations reveal the high local electron density surrounding the MoMo chains and sites can promote π back-donation, which is beneficial for increasing nitrogen adsorption, strengthening the MN bonds, and reducing the cleavage barrier of the triple NN bond.

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.

"Giant" Nitrogen Uptake in Ionic Liquids Confined in Carbon Pores

Ionic liquids are well known for their high gas absorption capacity. It is shown that this is not a solvent constant, but can be enhanced by another factor of 10 by pore confinement, here of the ionic liquid (IL) 1-ethyl-3-methylimidazolium acetate (EmimOAc) in the pores of carbon materials. A matrix of four different carbon compounds with micro- and mesopores as well as with and without nitrogen doping is utilized to investigate the influence of the carbons structure on the nitrogen uptake in the pore-confined EmimOAc. In general, the absorption is most improved for IL in micropores and in nitrogen-doped carbon. This effect is so large that it is already seen in TGA and DSC experiments. Due to the low vapor pressure of the IL, standard volumetric sorption experiments can be used to quantify details of this effect. It is reasoned that it is the change of the molecular arrangement of the ions in the restricted space of the pores that creates additional free volume to host molecular nitrogen.

Amorphous ReS2 decorated TiO2 nanowire arrays for highly-efficient nitrogen reduction

Herein, we constructed amorphous ReS₂ nanosheets anchored on TiO₂ nanowires by delicately forming Ti-O-Re bonds, which exhibited considerable average ammonia yield and faradaic efficiency as high as 5.3 μg h^-1 cm^-2_cat. and 49.8% at an applied potential of -0.2 V (vs. RHE) in 0.1 M Na₂SO₄. The high NRR performance could be attributed to the amorphous feature of the ReS₂ nanosheets, the rich oxygen vacancies in the TiO₂ nanowires and their semiconducting feature, which not only facilitate the sufficient exposure of active sites but also efficiently boost the faradaic efficiency due to the good regulation of surface proton or electron accessibility.

Facet-controlled palladium nanocrystalline for enhanced nitrate reduction towards ammonia

Electrochemical nitrate reduction reaction (NO₃^-RR) is considered an appealing way for producing ammonia (NH₃) under ambient conditions and solving environmental problems caused by nitrate, whereas the lack of adequate catalysts hampers the development of NO₃^-RR. Here, we firstly demonstrate that the Pd nanocrystalline with a well-desired facet can act as a highly efficient NO₃^-RR electrocatalyst for ambient ammonia synthesis. Pd (1 1 1) exhibits excellent activity and selectivity in reducing NO₃^- to NH₄⁺ with a Faradaic efficiency of 79.91% and an NH₄⁺ production of 0.5485 mmol h^-1 cm^-2 (2.74 mmol h^-1 mg^-1) in 0.1 M Na₂SO₄ (containing 0.1 M NO₃^-), which is 1.4 times higher than Pd (1 0 0) and 1.9 times higher than Pd (1 1 0), respectively. Density functional theory (DFT) calculation reveals that the superior NO₃^-RR activity of Pd (1 1 1) originates from its optimized activity of NO₃^- adsorption, smaller free energy change of the rate-limiting step (*NH₃ to NH₃), and poorer hydrogen evolution reaction activity (HER, competitive reaction). This work not only highlights the potentials of Pd-based nanocatalysts for NO₃^-RR but also provides new insight for the applications in NO₃^-RR of other facet-orientation nanomaterials.

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.

Multistep Functional Embellishment for p-ZnTe as a Cathode to Boost the Faraday Efficiency of Nitrogen Conversion

An electrochemical N₂ reduction reaction (N₂RR) is a selective sustainable approach to obtain NH₃ at mild conditions and has been proposed as an alternative to the full-blown Haber-Bosch process. However, achieving high yields of NH₃ and high faraday efficiency (FE) at a low overpotential remains a big challenge but has high expectations for the electrocatalytic N₂RR. Herein, a novel p-ZnTe cathode multistep embellished with NiO_x and ZnO thin films was prepared for boosting faraday efficiency to 9.89% for N₂RR at -0.2 V vs reversible hydrogen electrode (RHE), about 12 times of p-ZnTe@ZnO. All components within the NiO_x@p-ZnTe/ZnO electrode work cooperatively. A N source was determined through a ¹⁵N isotopic-labeling experiment. Using steady-state photoluminescence, electrochemical impedance spectroscopy, and control experiments, a possible model of charge transformation is built. In particular, a NiO_x layer has an important impact on increasing interfacial contact between a bare fluorine-doped tin oxide (FTO) glass and p-ZnTe and further reinforcing interfacial electron transfer. This work provides a practical application and a feasible strategy to develop highly efficient catalysts for N₂ reduction and also affords a guideline for the fabrication of a flat electrode.

A large scaled-up monocrystalline 3R MoS2 electrocatalyst for efficient nitrogen reduction reactions

Large-scale 3R MoS₂ was shown to be an efficient electrocatalyst for the NRR, and the NRR performance can be enhanced via improving the crystallinity of MoS₂ due to decreased resistance.