WO3 nanosheets rich in oxygen vacancies for enhanced electrocatalytic N2 reduction to NH3

Tuning Surface Potential Polarization to Enhance N2 Affinity for Ammonia Electrosynthesis

Electrocatalytic N2 reduction reaction (NRR) to synthesize ammonia is a sustainable reaction that is expected to replace Haber Bosch process. Laminated Bi2WO6 has great potential as an NRR electrocatalyst, however, the effective activity requires that the inert substrate is fully activated. Here, for the first time, success is achieved in activating the Bi2WO6 basal planes with NRR activity through Ti doping. The introduction of Ti successfully tunes the surface potential distribution and enhances the N2 adsorption. The subsequently strong hybrid coupling of d(Ti)-p(N) orbitals fills the electronic state of N2 antibonding molecular orbital, which greatly weakens the bonding strength of N≡N bonds. Further, in situ synchrotron radiation-based Fourier transform infrared (SR-FTIR) spectrum and theoretical calculations show that surface potential polarization enhances the adsorption of HNN* by Bi-Ti dual-metal sites, which is beneficial for the subsequent activation hydrogenation process. The Ti-Bi2WO6 nanosheets achieve 11.44% Faradaic efficiency (-0.2 V vs. RHE), a NH3 yield rate of 23.14 µg mg-1 h-1 (15N calibration), and satisfactory stability in 0.1 M HCl environment. The mutual assistance of theory and experiment can help understand and develop of excellent two-dimensional (2D) materials for the NRR.

Bimetallic Au and Pd Nanoparticles Modified WO3 Nanosheets for Enhancing the Sensitivity and Selectivity of Formaldehyde Assessment in Aquatic Products

Formaldehyde, a common illegal additive in aquatic products, poses a threat to people's health and lives. In this study, a novel metal oxide semiconductor gas sensor based on AuPd-modified WO3 nanosheets (NSs) had been developed for the highly efficient detection of formaldehyde. WO3 NS modified with 2.0% AuPd nanoparticles showed a higher response (Ra/Rg = 94.2) to 50 ppm of formaldehyde at 210 °C, which was 36 times more than the pristine WO3 NS. In addition, the AuPd/WO3 gas sensor had a relatively short response/recovery time of 10 s/9 s for 50 ppm of formaldehyde at 210 °C, with good immunity to other interfering gases and good stability for formaldehyde. The excellent gas-sensitive performance was attributed to the chemical sensitization of Au, the electronic sensitization of Pd, and the synergistic effect of bimetallic AuPd, which facilitated the recognition and response of formaldehyde molecules. Additionally, the high sensitivity and broad application prospect of the 2.0% AuPd/WO3 NS composite-based sensor in real sample detection were also confirmed by using the above sensor for the detection of formaldehyde in aquatic products such as squid and shrimp.

WOx nanoparticles coupled with nitrogen-doped porous carbon toward electrocatalytic N2 reduction

The electrocatalytic nitrogen reduction reaction (eNRR) is a sustainable and green alternative to the traditional Haber-Bosch process. However, the chemical inertness of nitrogen gas and the competitive hydrogen evolution reaction significantly limit the catalytic performance of eNRR. Although tungsten oxide-based eNRR catalysts could donate unpaired electrons to the antibonding orbitals of N2 and accept lone electron pairs from N2 to dissociate NN triple bonds, the low electrical conductivity and the influence of the variable valence of W still affect the catalytic activity. Herein, a high-performance eNRR catalyst WOx nanoparticle/nitrogen-doped porous carbon (WOx/NPC) was prepared by a one-step thermal pyrolysis method. The results reveal that WOx gradually changes from the dominant WO2 phase to the WO3 phase. WOx/NPC-700 °C with WO2 NPs anchored on the surfaces of NPC via W-N bonding could deliver a high NH3 yield of 46.8 μg h-1 mg-1 and a high faradaic efficiency (FE) of 10.2%. The edge W atomic site on WOx/NPC is demonstrated to be the active center which could activate a stable NN triple bond with an electron-donating ability. Benefiting from the covalent interaction between the WOx nanoparticles and NPC, WOx/NPC also shows high electrocatalytic stability.

Favoring Product Desorption by a Tailored Electronic Environment of Oxygen Vacancies in SrTiO3 via Cr Doping for Enhanced and Selective Electrocatalytic CO2 to CO Conversion

The electrochemical CO₂ reduction reaction (ECO₂RR) into value-added products is crucial to address the herculean task of CO₂ mitigation. Several efforts are being made to develop active ECO₂RR catalysts, targeting enhanced CO₂ adsorption and activation. A rational design of ECO₂RR catalysts with a facile product desorption step is seldom reported. Herein, ensuing the Sabatier principle, we report a strategy for an enhanced ECO₂RR with a faradaic efficiency of 85% for CO production by targeting the product desorption step. The energy barrier for product desorption was lowered via a tailored electronic environment of oxygen vacancies (O_vac) in Cr-doped SrTiO₃. The substitutional doping of Cr³⁺ for Ti⁴⁺ into the SrTiO₃ lattice favors the generation of more O_vac and modifies the local electronic environment. Density functional theory analysis evinces the spontaneous dissociation of COOH^# intermediates over O_vac and lower CO intermediate binding on O_vac reducing the energy demand for CO release due to Cr doping.

Oxygen vacancies-rich Cu-W18O49 nanorods supported on reduced graphene oxide for electrochemical reduction ofN2to NH3

High-performance nitrogen fixation is severely limited by the efficiency and selectivity of a catalyst of electrochemical nitrogen reduction reaction (NRR) under ambient conditions. Here, the RGO/WOCu (reduced graphene oxide and Cu-doping W₁₈O₄₉) composite catalysts with abundant oxygen vacancies are prepared by the hydrothermal method. The obtained RGO/WOCu achieves an enhanced NRR performance (NH₃ yield rate:11.4 μg h^-1 mg_cat^-1, Faradaic efficiency: 4.4%) at -0.6 V (vs. RHE) in 0.1 mol L^-1 Na₂SO₄ solution. Furthermore, the NRR performance of the RGO/WOCu still keeps at 95% after four cycles, demonstrating its excellent stability. The Cu⁺-doping increases the concentration of oxygen vacancies, which is conducive to the adsorption and activation of N₂. Meanwhile, the introduction of RGO further improves the electrical conductivity and reaction kinetics of the RGO/WOCu due to the high specific surface area and conductivity. This work provides a simple and effective method for efficient electrochemical reduction ofN₂.

Defect engineering of two-dimensional materials for advanced energy conversion and storage

In the global trend towards carbon neutrality, sustainable energy conversion and storage technologies are of vital significance to tackle the energy crisis and climate change. However, traditional electrode materials gradually reach their property limits. Two-dimensional (2D) materials featuring large aspect ratios and tunable surface properties exhibit tremendous potential for improving the performance of energy conversion and storage devices. To rationally control the physical and chemical properties for specific applications, defect engineering of 2D materials has been investigated extensively, and is becoming a versatile strategy to promote the electrode reaction kinetics. Simultaneously, exploring the in-depth mechanisms underlying defect action in electrode reactions is crucial to provide profound insight into structure tailoring and property optimization. In this review, we highlight the cutting-edge advances in defect engineering in 2D materials as well as their considerable effects in energy-related applications. Moreover, the confronting challenges and promising directions are discussed for the development of advanced energy conversion and storage systems.

Formation Pathways of Lath-Shaped WO3 Nanosheets and Elemental W Nanoparticles from Heating of WO3 Nanocrystals Studied via In Situ TEM

WO3 is a versatile material occurring in many polymorphs, and is used in nanostructured form in many applications, including photocatalysis, gas sensing, and energy storage. We investigated the thermal evolution of cubic-phase nanocrystals with a size range of 5-25 nm by means of in situ heating in the transmission electron microscope (TEM), and found distinct pathways for the formation of either 2D WO3 nanosheets or elemental W nanoparticles, depending on the initial concentration of deposited WO3 nanoparticles. These pristine particles were stable up to 600 °C, after which coalescence and fusion of the nanocrystals were observed. Typically, the nanocrystals transformed into faceted nanocrystals of elemental body-centered-cubic W after annealing to 900 °C. However, in areas where the concentration of dropcast WO3 nanoparticles was high, at a temperature of 900 °C, considerably larger lath-shaped nanosheets (extending for hundreds of nanometers in length and up to 100 nm in width) were formed that are concluded to be in monoclinic WO3 or WO2.7 phases. These lath-shaped 2D particles, which often curled up from their sides into folded 2D nanosheets, are most likely formed from the smaller nanoparticles through a solid-vapor-solid growth mechanism. The findings of the in situ experiments were confirmed by ex situ experiments performed in a high-vacuum chamber.

Constructing Z-scheme 1D/2D heterojunction of ZnIn2S4 nanosheets decorated WO3 nanorods to enhance Cr(VI) photocatalytic reduction and rhodamine B degradation

Photocatalysis has been vastly employed as a feasible and efficient strategy for the removal of environmental pollutants. In this study, a well-designed core-shell heterojunction of WO3 decorated with ZnIn2S4 nanosheets were fabricated under mild in-situ conditions, and fabricated processes were systematically investigated with different fabrication durations. The coupling of WO3 and ZnIn2S4 (ZIS) resulted in a Z-scheme mechanism for charge carrier transfer, holding the respective redox capacity. The as-prepared 1D/2D WO3@ZIS heterostructure displayed the highest removal efficiency within 30 min for 25 mg L-1 Cr(VI), 89.3 and 29.7 times higher than pure WO3 and ZnIn2S4. 1D/2D WO3@ZIS remained excellently stable after 5 cycling experiments. Moreover, 40 mg L-1 RhB could be degraded within 50 min. The broad and short photogenerated electron transportation path is guaranteed by the 1D/2D and Z-scheme charge separation mechanism. It efficiently prevented photo-generated charge carriers from recombination, resulting in a longer carrier lifespan and better photocurrent responses than that of pure ones. This photocatalytic system showed promising results and also provides a framework for an efficient system for photocatalysis with potential for environmental application.

Increased Oxygen Vacancies in CeO2 for Improved Electrocatalytic Nitrogen Reduction Performance

Electrochemical nitrogen fixation is a sustainable and economical strategy to produce ammonia. However, fabricating efficient electrocatalysts for nitrogen fixation is still challenging. Theoretical predictions prove that the oxygen vacancy is able to modulate the electronic state of CeO₂ and enhance its electrical conductivity, thus promoting the electrochemical nitrogen reduction reaction (NRR) process. Herein, CeO₂ with high oxygen vacancy concentration was prepared via a two-step pyrolysis strategy of Ce metal-organic frameworks (MOFs, denoted H-CeO₂). Compared to CeO₂ with low oxygen vacancy concentration synthesized via one-step pyrolysis of Ce-MOFs (denoted L-CeO₂), H-CeO₂ exhibits a large NH₃ yield rate (25.64 μg h^-1 mg_cat^-1 at -0.5 V vs reversible hydrogen electrode, RHE) and high faradaic efficiency (FE, 6.3% at -0.4 V vs RHE).

Shearing Sulfur Edges of VS2 Electrocatalyst Enhances its Nitrogen Reduction Performance

Electrochemical N₂ fixation requires effective electrocatalysts to expedite the nitrogen reduction reaction (NRR) kinetics and suppress the concomitant hydrogen evolution reaction (HER). Although transition metal sulfides have been deemed as efficient NRR electrocatalysts, it remains a great challenge to suppress the serious HER to achieve high Faradaic efficiency (FE). Herein, vanadium disulfide (VS₂ ) is deliberately designed by partially shearing its sulfur (S) edges through a simple calcination treatment at 350 °C. The as-prepared VS₂ -350 electrocatalyst exhibits a highest NH₃ yield of 20.29 µg h^-1 mg_cat^-1 with a promising FE of 3.86%, which is significantly higher than the counterpart of untreated VS₂ (V_NH3 : 15.92 µg h^-1 mg_cat^-1 , FE: 1.69%). Experimental and computational results reveal that shearing the S edges can substantially inhibit the HER and expose more V atoms as active sites. Meanwhile, the mechanistic analysis shows that the N₂ activation at V active sites follows an "acceptance-donation" mechanism, while the N₂ conversion to NH₃ follows a hybrid 2 pathway at the VS₂ -350 electrocatalyst. This work provides a simple strategy of designing high-performance NRR electrocatalysts based on a deep understanding of the atomic sites dependent catalytical activity.

Progress in Mo/W-based electrocatalysts for nitrogen reduction to ammonia under ambient conditions

Ammonia (NH₃), possessing high hydrogen content and energy density, has been widely employed for fertilizers and value-added chemicals in green energy carriers and fuels. However, the current NH₃ synthesis largely depends on the traditional Haber-Bosch process, which needs tremendous energy consumption and generates greenhouse gas, resulting in severe energy and environmental issues. The electrochemical strategy of converting N₂ to NH₃ under mild conditions is a potentially promising route to realize an environmentally friendly concept. Among various catalysts, molybdenum/tungsten-based electrocatalysts have been widely used in electrochemical catalytic and energy conversion. This review describes the latest progress of molybdenum/tungsten-based electrocatalysts for the electrochemical nitrogen reduction reaction. The fundamental roles of morphology, doping, defects, heterojunction, and coupling regulation in improving electrocatalytic performance are mainly discussed. Besides, some tailoring strategies for enhancing the conversion efficiency of N₂ to NH₃ over Mo/W-based electrocatalysts are also summarized. Finally, the existing challenges and limitations of N₂ fixation are proposed, as well as possible future perspectives, which will provide a platform for further development of advanced Mo/W-based N₂ reduction systems.

Fe Foam-Supported FeS2-MoS2 Electrocatalyst for N2 Reduction under Ambient Conditions

Highly efficient catalysts with enough selectivity and stability are essential for electrochemical nitrogen reduction reaction (e-NRR) that has been considered as a green and sustainable route for synthesis of NH₃. In this work, a series of three-dimensional (3D) porous iron foam (abbreviated as IF) self-supported FeS₂-MoS₂ bimetallic hybrid materials, denoted as FeS₂-MoS₂@IF_x, x = 100, 200, 300, and 400, were designed and synthesized and then directly used as the electrode for the NRR. Interestingly, the IF serving as a slow-releasing iron source together with polyoxomolybdates (NH₄)₆Mo₇O₂₄·4H₂O as a Mo source were sulfurized in the presence of thiourea to form self-supported FeS₂-MoS₂ on IF (abbreviated as FeS₂-MoS₂@IF₂₀₀) as an efficient electrocatalyst. Further material characterizations of FeS₂-MoS₂@IF₂₀₀ show that flower cluster-like FeS₂-MoS₂ grows on the 3D skeleton of IF, consisting of interconnected and staggered nanosheets with mesoporous structures. The unique 3D porous structure of FeS₂-MoS₂@IF together with synergy and interface interactions of bimetallic sulfides would make FeS₂-MoS₂@IF possess favorable electron transfer tunnels and expose abundant intrinsic active sites in the e-NRR. It is confirmed that synthesized FeS₂-MoS₂@IF₂₀₀ shows a remarkable NH₃ production rate of 7.1 ×10^-10 mol s^-1 cm^-2 at -0.5 V versus the reversible hydrogen electrode (vs RHE) and an optimal faradaic efficiency of 4.6% at -0.3 V (vs RHE) with outstanding electrochemical and structural stability.

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.

Electrochemically synthesized SnO2 with tunable oxygen vacancies for efficient electrocatalytic nitrogen fixation

Electrochemical nitrogen reduction reaction (NRR) driven by a renewable energy source offers a sustainable and environmentally benign route to produce ammonia (NH₃), but it is highly dependent on efficient and specific catalysts to reduce the high reaction barrier and improve the selectivity. Defect engineering is extensively used to regulate the surface properties of materials to improve their catalytic performance. Herein we synthesized SnO₂ with different oxygen vacancy concentrations by a controllable electrochemical method for electrocatalytic nitrogen (N₂) fixation. The prepared SnO₂ was used as an electrocatalyst and exhibited excellent NRR performance with an optimal NH₃ yield rate of 25.27 μg h^-1 mg_cat.^-1 and faradaic efficiency of 11.48% at -0.6 V (vs. the reversible hydrogen electrode) in 0.1 M Na₂SO₄. Oxygen vacancies provide more active sites and greater electron transfer ability on the catalyst surface to facilitate N₂ adsorption and activation. The electrocatalytic NRR performance of SnO₂ was enhanced with the increase in oxygen vacancy concentration. The density functional theory calculations indicate that the oxygen vacancies in SnO₂ promote the electrocatalytic NRR performance by increasing the number of valence electrons of Sn and decreasing the energy barrier of the potential-determining step, thus promoting the activation of the N-N bond to further achieve efficient N₂ fixation.

Synthesis and applications of WO3 nanosheets: the importance of phase, stoichiometry, and aspect ratio

Tungsten trioxide (WO3) is an abundant, versatile oxide that is widely explored for catalysis, sensing, electrochromic devices, and numerous other applications. The exploitation of WO3 in nanosheet form provides potential advantages in many of these fields because the 2D structures have high surface area and preferentially exposed facets. Relative to bulk WO3, nanosheets expose more active sites for surface-sensitive sensing/catalytic reactions, and improve reaction kinetics in cases where ionic diffusion is a limiting factor (e.g. electrochromic or charge storage). Synthesis of high aspect ratio WO3 nanosheets, however, is more challenging than other 2D materials because bulk WO3 is not an intrinsically layered material, making the widely-studied sonication-based exfoliation methods used for other 2D materials not well-suited to WO3. WO3 is also highly complex in terms of how the synthesis method affects the properties of the final material. Depending on the route used and subsequent post-synthesis treatments, a wide variety of different morphologies, phases, exposed facets, and defect structures are created, all of which must be carefully considered for the chosen application. In this review, the recent developments in WO3 nanosheet synthesis and their impact on performance in various applications are summarized and critically analyzed.

Defect-Induced Ce-Doped Bi2WO6 for Efficient Electrocatalytic N2 Reduction

Electrochemical nitrogen reduction reaction (NRR) is a promising method for synthesizing ammonia (NH₃). However, due to the extremely strong N≡N bond and the competing hydrogen evolution reaction (HER), the electrochemical NRR process remains a great challenge in achieving a high NH₃ yielding rate and a high Faradaic efficiency (FE). Recently, either Bi-based or W-based catalysts have been used in N₂ fixation due to lower HER activity. To further promote N₂ activation, we develop a simple protocol to introduce and adjust the crystal defects in the host lattice of Bi₂WO₆ nanoflowers via adjusting the amount of Ce dopant (denoted as xCe-Bi₂WO₆, where x represents the designed mole percentage of Ce). At -0.20 V versus the reversible hydrogen electrode (RHE), 10%Ce-Bi₂WO₆ manifests a high NH₃ yielding rate (22.5 μg h^-1 mg^-1_cat.), a high FE (15.9%), and excellent electrochemical and structure durability. Its performance is better than most previously reported Bi-based and W-based electrocatalysts for NRR in aqueous solutions. According to density functional theory (DFT) calculations, the introduction of crystal defects into Bi₂WO₆ can strengthen the adsorption and activation of N₂, thus leading to a significant increase in NRR activity.

Oxygen Vacancy Engineering of MOF-Derived Zn-Doped Co3O4 Nanopolyhedrons for Enhanced Electrochemical Nitrogen Fixation

Introducing oxygen vacancy (V_o) has been considered as an effective and significant method to accelerate the sluggish electrocatalytic nitrogen reduction reaction (NRR). In this work, a series of bimetallic zeolitic imidazolate frameworks based on ZIF-67 and ZIF-8 with varied ratios of Co/Zn have been applied as precursors to prepare V_o-rich Zn-doped Co₃O₄ nanopolyhedrons (Zn-Co₃O₄) by a low-temperature oxidation strategy. Zn-Co₃O₄ presents an ammonia yield of 22.71 μg h^-1 mg_cat.^-1 with a high faradaic efficiency of 11.9% for NRR under ambient conditions. The remarkable catalytic performances are believed to result from the plentiful V_o as the Lewis acid sites and electron-rich Co sites to promote the adsorption and dissociation of N₂ molecules. Remarkably, Zn-Co₃O₄ also demonstrates a high electrochemical stability. This work presents a guiding method for developing a stable and efficient electrocatalyst for the NRR.

Two-Dimensional Tungsten Oxide/Selenium Nanocomposite Fabricated for Flexible Supercapacitors with Higher Operational Voltage and Their Charge Storage Mechanism

The present work elaborates the high-energy-density, stable, and flexible supercapacitor devices (full-cell configuration with asymmetric setup) based on a two-dimensional tungsten oxide/selenium (2D WO₃/Se) nanocomposite. For this, the 2D WO₃/Se nanocomposite synthesized by a hydrothermal method followed by air annealing was coated on a flexible carbon cloth current collector and combined separately with both 0.1 M H₂SO₄ and 1-butyl-3-methyl imidazolium tetrafluoroborate room temperature ionic liquid (BmimBF₄ RTIL) as electrolyte. Different physicochemical characterization techniques, viz., transmission electron microscopy, scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy, are utilized for phase confirmation and morphology identification of the obtained samples. The electrochemical analysis was used to evaluate charge storage mechanism. The half-cell configuration (three electrode system) in 0.1 M H₂SO₄ shows a specific capacitance of 564 F g^-1 at 6 A g^-1 current density, whereas with ionic liquid as electrolyte, a higher specific capacitance of 1650 F g^-1 was obtained at a higher current of 40 mA and working potential of 4 V. Importantly, the asymmetric flexible supercapacitor device with PVA-H₂SO₄ electrolyte shows a working voltage of 1.7 V. A specific capacitance of 858 mF g^-1 is obtained for the asymmetric electrode system with an energy density of 47 mWh kg^-1 and a power density of 345 mW kg^-1 at a current density of 0.2 A g^-1.

An amorphous WC thin film enabled high-efficiency N2 reduction electrocatalysis under ambient conditions

Ambient electrochemical N2 reduction offers a promising alternative to the energy-intensive Haber-Bosch process towards renewable NH3 synthesis in aqueous media but needs efficient electrocatalysts to enable the N2 reduction reaction (NRR). Herein, we propose that an amorphous WC thin film magnetron sputtered onto a graphite foil behaves as a superb NRR electrocatalyst for ambient NH3 production with excellent selectivity. In 0.5 M LiClO4, it attains a large NH3 yield of 43.37 μg h-1 mg-1cat. and a high faradaic efficiency of 21.65% at -0.10 V vs. reversible hydrogen electrode. Impressively, this catalyst also shows excellent selectivity and strong durability for NH3 formation.

The application and improvement of TiO2 (titanate) based nanomaterials for the photoelectrochemical conversion of CO2 and N2 into useful products

In this review, we describe the photoelectrochemical (PEC) transformation of atmospheric species such as carbon dioxide (CO₂) and nitrogen (N₂) into useful industrial products on TiO₂ and TiO₂ composite photoelectrodes.

Vacancy engineering of WO3-x nanosheets for electrocatalytic NRR process - a first-principles study

It has been proved experimentally that tungsten oxide (WO3) in a defective state has a positive effect on nitrogen reduction reactions (NRRs) owing to the surface modification by adding oxygen vacancies and doping. However, the role of different oxygen vacancies in the electrocatalytic NRR is still behind the scenes. In this study, we have carried out first-principles calculations to grapple with its causes and consequences, focusing on the two-dimensional WO3-x surface on an atomic scale. Our study shows that WO3 does not promote nitrogen reduction simply by a dimension reduction without oxygen vacancy. Two NRR processes, which are found to follow the associative mechanism in various pathways, are initiated at relatively lower potentials at two possible vacancies. It is the polarized electrons after being adsorbed over the dangling oxygen vacancy, which weaken the N[triple bond, length as m-dash]N bond and enables N2 reduction with a rate-limiting potential as large as -1.89 V. In contrast, the desorption of NH3 from the planar vacancy is kinetically challenging at a cost of 1.47 eV, in which case the d orbitals of under-coordinated W match the p orbitals of N and form both bonding state and anti-bonding state. It demonstrates that P-VO-WO3 and N2 can bond firmly but NH3 desorption ought to pay a price. Two alternative schemes are accordingly proposed to balance good nitrogen adsorption and desorption. This noble metal-free system, by regulating vacancy sites, demonstrates its potential as an eco-friendly and fine-grained artificial material for electrochemical nitrogen reduction.

Regulating kinetics and thermodynamics of electrochemical nitrogen reduction with metal single-atom catalysts in a pressurized electrolyser

The present-day industrial ammonia synthesis is overreliance on the Haber–Bosch process, yet consumes more than 1% of the global energy supply along with gigatonne greenhouse-gas emission per year. Electrochemical nitrogen reduction reaction (ENRR) offers a sustainable path to produce ammonia under mild conditions, while its efficiency achieved by far is fairly low, which requires the development of both catalysts and electrolyzers. Here, by the cooperation of densely populated metal single atoms on graphdiyne substrate with the pressurized electrocatalytic system, the kinetics and the thermodynamic driving force of ENRR are effectively regulated, leading to a record-high ammonia yield rate. This work motivates the technological and material coevolution for the ENRR toward its envisioned application.

Using renewable electricity to synthesize ammonia from nitrogen paves a sustainable route to making value-added chemicals but yet requires further advances in electrocatalyst development and device integration. By engineering both electrocatalyst and electrolyzer to simultaneously regulate chemical kinetics and thermodynamic driving forces of the electrocatalytic nitrogen reduction reaction (ENRR), we report herein stereoconfinement-induced densely populated metal single atoms (Rh, Ru, Co) on graphdiyne (GDY) matrix (formulated as M SA/GDY) and realized a boosted ENRR activity in a pressurized reaction system. Remarkably, under the pressurized environment, the hydrogen evolution reaction of M SA/GDY was effectively suppressed and the desired ENRR activity was strongly amplificated. As a result, the pressurized ENRR activity of Rh SA/GDY at 55 atm exhibited a record-high NH₃ formation rate of 74.15 μg h⁻¹⋅cm⁻², a Faraday efficiency of 20.36%, and a NH₃ partial current of 0.35 mA cm⁻² at −0.20 V versus reversible hydrogen electrode, which, respectively, displayed 7.3-, 4.9-, and 9.2-fold enhancements compared with those obtained under ambient conditions. Furthermore, a time-independent ammonia yield rate using purified ¹⁵N₂ confirmed the concrete ammonia electroproduction. Theoretical calculations reveal that the driving force for the formation of end-on N₂* on Rh SA/GDY increased by 9.62 kJ/mol under the pressurized conditions, facilitating the ENRR process. We envisage that the cooperative regulations of catalysts and electrochemical devices open up the possibilities for industrially viable electrochemical ammonia production.

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.

Structural evolution of CrN nanocube electrocatalysts during nitrogen reduction reaction

Metal nitrides have been suggested as prospective catalysts for the electrochemical nitrogen reduction reaction (NRR) in order to obtain ammonia at room temperature under ambient pressure. Herein, we report that templated chromium nitride porous microspheres built up by nanocubes (NCs) are an efficient noble-metal-free electrocatalyst for NRR. The CrN NCs catalyst exhibits both a high stability and NH3 yield of 31.11 μg h-1 mgcat.-1 with a Faradaic efficiency (FE) of 16.6% in 0.1 M HCl electrolyte. Complementary physical characterization techniques demonstrate partial oxidation of the pristine CrN NCs during reaction. Structural characterization by means of scanning transmission electron microscopy (STEM) combining electron energy loss spectrum (EELS) and energy dispersive X-ray spectroscopy (EDX) analysis reveals the NC structure to consist of an O-rich core and N-rich shell after NRR. This gradient distribution of nitrogen within the CrN NCs upon completed NRR is distinct to previously reported metal nitride NRR catalysts, because no significant loss of nitrogen occurs at the catalyst surface.

Glycerine-based synthesis of a highly efficient Fe2O3 electrocatalyst for N2 fixation

The electrochemical nitrogen reduction reaction (NRR) is a promising approach to convert N₂ into high value-added NH₃. However, it is still a challenge to achieve an efficient electrocatalyst for the NRR. Herein, it is demonstrated that the Fe₂O₃ nanoparticles (NPs) generated from a glycerine-based synthesis can be applied as highly efficient catalysts for the NRR. The Fe₂O₃ NPs show good performance with a high NH₃ yield (22 μg mg_cat ^-1 h^-1) and a favorable Faradaic efficiency (FE) (3.5%) at -0.5 V vs. reversible hydrogen electrode (RHE). The facile synthesis strategy and satisfactory electrochemical properties demonstrate the potential application of the as-synthesized Fe₂O₃ NPs for NRR.

Three-dimensional Pd-Ag-S porous nanosponges for electrocatalytic nitrogen reduction to ammonia

Electrochemical nitrogen reduction reaction (NRR) provides a facile and sustainable route to synthesize ammonia. The preparation of efficient and high-performance catalysts is one of the most important issues in large-scale applications of the electrochemical synthesis of ammonia. Herein, we have devised a simple method to fabricate three-dimensional palladium-silver-sulphur porous nanosponges (Pd-Ag-S PNSs) under room temperature. The porous network can provide more active sites and accessible channels for the reaction species. The incorporation of sulfur reduces the energy barrier of NRR and promotes the nitrogen hydrogenation to ammonia. Intrinsically, the Pd-Ag-S PNSs demonstrates a superior NRR performance with an NH3 yield of 9.73 μg h-1 mg-1cat. and a faradaic efficiency of 18.41% at -0.2 V, superior to those of the undoped Pd-Ag PNSs. The design of the three-dimensional metallic nanosponges with the doping of nonmetallic elements is a highly valuable strategy for NRR and other electrocatalytic reactions.

Ambient Ammonia Electrosynthesis: Current Status, Challenges, and Perspectives

Ammonia (NH₃ ) electrosynthesis from atmospheric nitrogen (N₂ ) and water is emerging as a promising alternative to the energy-intensive Haber-Bosch process; however, such a process is difficult to perform due to the inherent inertness of N₂ molecules together with low solubility in aqueous solutions. Although many active electrocatalysts have been used to electrocatalyze the N₂ reduction reaction (NRR), unsatisfactory NH₃ yields and lower Faraday efficiency are still far from practical industrial production, and thus, considerable research efforts are being devoted to address these problems. Nevertheless, most reports still mainly focus on the preparation of electrocatalysts and largely ignore a summary of optimization-modification strategies for the NRR. In this review, a general introduction to the NRR mechanism is presented to provide a reasonable guide for the design of highly active catalysts. Then, four categories of NRR electrocatalysts, according to chemical compositions, are surveyed, as well as several strategies for promoting the catalytic activity and efficiency. Later, strategies for developing efficient N₂ fixation systems are discussed. Finally, current challenges and future perspectives in the context of the NRR are highlighted. This review sheds some light on the development of highly efficient catalytic systems for NH₃ synthesis and stimulates research interests in the unexplored, but promising, research field of the NRR.

Urchin-like Al-Doped Co3O4 Nanospheres Rich in Surface Oxygen Vacancies Enable Efficient Ammonia Electrosynthesis

Developing cost-efficient electrocatalysts for ambient N₂-to-NH₃ conversion and revealing the reaction mechanism are appealing yet challenging tasks. Some transition metal oxides have been recently used to catalyze the nitrogen reduction reaction (NRR), but their further applications are greatly impeded because of their questionable conductivity, poor dispersion, limited active sites, and so forth. Herein, three-dimensional Ni foam-supported urchin-like Al-doped Co₃O₄ nanospheres rich in surface oxygen vacancies (Al-Co₃O₄/NF) were prepared via a hydrothermal process and subsequent annealing treatment. It is shown that introducing Al atoms into Co₃O₄ effectively tunes the electronic properties of the catalyst, and the increased surface oxygen vacancies induced by Al doping facilitate the activation of nitrogen. What is more, this urchin-like nanostructure, demonstrating an ability to limit the coalescence of gas bubbles, enables the rapid removal of small gas bubbles and better exposure of active sites to N₂, thus yielding an impressive ammonia electrosynthesis activity (NH₃ yield rate: 6.48 × 10^-11 mol s^-1 cm^-2; Faradaic efficiency: 6.25%) in 0.1 M KOH. Electrochemical-based in situ Fourier transform infrared spectroscopy was employed to study the mechanism of NRR, indicating an associative alternating pathway.

WO3 and Ionic Liquids: A Synergic Pair for Pollutant Gas Sensing and Desulfurization

This review deals with the notable results obtained by the synergy between ionic liquids (ILs) and WO3 in the field of pollutant gas sensing and sulfur removal pretreatment of fuels. Starting from the known characteristics of tungsten trioxide as catalytic material, many authors have proposed the use of ionic liquids in order to both direct WO3 production towards controllable nanostructures (nanorods, nanospheres, etc.) and to modify the metal oxide structure (incorporating ILs) in order to increase the gas adsorption ability and, thus, the catalytic efficiency. Moreover, ionic liquids are able to highly disperse WO3 in composites, thus enhancing the contact surface and the catalytic ability of WO3 in both hydrodesulfurization (HDS) and oxidative desulfurization (ODS) of liquid fuels. In particular, the use of ILs in composite synthesis can direct the hydrogenation process (HDS) towards sulfur compounds rather than towards olefins, thus preserving the octane number of the fuel while highly reducing the sulfur content and, thus, the possibility of air pollution with sulfur oxides. A similar performance enhancement was obtained in ODS, where the high dispersion of WO3 (due to the use of ILs during the synthesis) allows for noteworthy results at very low temperatures (50 °C).

Highly boosted gas diffusion for enhanced electrocatalytic reduction of N2 to NH3 on 3D hollow Co-MoS2 nanostructures

Transition metal chalcogenide MoS₂ catalysts are highly selective for the electrochemical reduction of dinitrogen (N₂) to ammonia (NH₃) in aqueous electrolytes. However, due to the low solubility of N₂ in water, limited N₂ diffusion and mass transport have heavily restricted the yield and the faradaic efficiency (FE). Here, we have demonstrated a highly efficacious assembled gas diffusion cathode with hollow Co-MoS₂/N@C nanostructures to significantly improve the electrochemical reduction of N₂ to NH₃. Our results revealed that the synthesized Co-MoS₂ heterojunctions with abundant graphitic N groups exhibited a superb NH₃ yield of 129.93 μg h^-1 mg_cat^-1 and a high faradaic efficiency of 11.21% at -0.4 V vs. the reversible hydrogen electrode (RHE), as well as excellent selectivity and stability. The strategy described in this study offers new inspiration to design high-performance electrocatalyst assemblies for the sustainable environmental and energy applications.

Layered α-MoO3 nanoplates for gas sensing applications

A versatile chemical route to produce rectangular layered α-MoO₃ nanoplates with enhanced NO₂ gas sensing response.

Bifunctional PtCu electrocatalysts for the N2 reduction reaction under ambient conditions and methanol oxidation

PtCu nanoalloys were employed as bifunctional electrocatalysts in both the N₂ reduction and methanol oxidation, in which the electrocatalytic activity and stability is composition dependent and highly improved compared to their counterpart.