High-Efficiency N2 Electroreduction Enabled by Se-Vacancy-Rich WSe2-x in Water-in-Salt Electrolytes

In Situ Characterization Techniques for Electrochemical Nitrogen Reduction Reaction

The electrochemical reduction of nitrogen to produce ammonia is pivotal in modern society due to its environmental friendliness and the substantial influence that ammonia has on food, chemicals, and energy. However, the current electrochemical nitrogen reduction reaction (NRR) mechanism is still imperfect, which seriously impedes the development of NRR. In situ characterization techniques offer insight into the alterations taking place at the electrode/electrolyte interface throughout the NRR process, thereby helping us to explore the NRR mechanism in-depth and ultimately promote the development of efficient catalytic systems for NRR. Herein, we introduce the popular theories and mechanisms of the electrochemical NRR and provide an extensive overview on the application of various in situ characterization approaches for on-site detection of reaction intermediates and catalyst transformations during electrocatalytic NRR processes, including different optical techniques, X-ray-based techniques, electron microscopy, and scanning probe microscopy. Finally, some major challenges and future directions of these in situ techniques are proposed.

Hierarchical NiCo2Se4 Arrays Composed of Atomically Thin Nanosheets: Simultaneous Improvements in Thermodynamics and Kinetics for Electrocatalytic Water Splitting in Neutral Media

The inefficiency of electrocatalysts for water splitting in neutral media stems from a comprehensive impact of poor intrinsic activity, a limited number of active sites, and inadequate mass transport. Herein, hierarchical ultrathin NiCo2Se4 nanosheets are synthesized by the selenization of NiCo2O4 porous nanoneedles. Theoretical and experimental investigations reveal that the intrinsic hydrogen evolution reaction (HER) activity primarily originate from the NiCo2Se4, whereas the high oxygen evolution reaction (OER) performance is related to the NiCoOOH due to the structural reconstruction. The abundant Se and O vacancies introduced by atomically thin nanostructure modulate the electronic structure of NiCo2Se4 and NiCoOOH, thereby improving the intrinsic HER and OER activities, respectively. COMSOL simulation demonstrate the edges of extended nanosheets from the main body significantly promote the charge aggregation, boosting the reduction and oxidation current during HER/OER process. This charge aggregation effect notably exceeds the tip effect for the nanoneedle, highlighting the unique advantage of the hierarchical nanosheet structure. Benefiting from abundant vacancies and unique nanostructure, the hierarchical ultrathin nanosheet simultaneously improve the thermodynamics and kinetics of the electrocatalyst. The optimized samples display an overpotential of 92 mV for HER and 214 mV for OER at 100 mA cm-2, significantly surpassing the performance of currently reported HER/OER catalysts in neutral media.

Solar-Powered Gram-Scale Ammonia Production from Nitrate

The photoelectrochemical (PEC) method has the potential to be an attractive route for converting and storing solar energy as chemical bonds. In this study, a maximum NH3 production yield of 1.01 g L-1 with a solar-to-ammonia conversion efficiency of 8.17% through the photovoltaic electrocatalytic (PV-EC) nitrate (NO3 -) reduction reaction (NO3 -RR) is achieved, using silicon heterojunction solar cell technology. Additionally, the effect of tuning the operation potential of the PV-EC system and its influence on product selectivity are systematically investigated. By using this unique external resistance tuning approach in the PV-EC system, ammonia production through nitrate reduction performance from 96 to 360 mg L-1 is enhanced, a four-fold increase. Furthermore, the NH3 is extracted as NH4Cl powder using acid stripping, which is essential for storing chemical energy. This work demonstrates the possibility of tuning product selectivity in PV-EC systems, with prospects toward pilot scale on value-added product synthesis.

Coordination Structure Modulation in Group-VIB Metal Doped Ag3PO4 Augments Active Site Density for Electrocatalytic Conversion of N2 to NH3

Doping is considered a promising material engineering strategy in electrochemical nitrogen reduction reaction (NRR), provided the role of the active site is rightly identified. This work concerns the doping of group VIB metal in Ag3PO4 to enhance the active site density, accompanied by d-p orbital mixing at the active site/N2 interface. Doping induces compressive strain in the Ag3PO4 lattice and inherently accompanies vacancy generation, the latter is quantified with positron annihilation lifetime studies (PALS). This eventually alters the metal d-electronic states relative to Fermi level and manipulate the active sites for NRR resulting into side-on N2 adsorption at the interface. The charge density deployment reveals Mo as the most efficient dopant, attaining a minimum NRR overpotential, as confirmed by the detailed kinetic study with the rotating ring disk electrode (RRDE) technique. In fact, the Pt ring of RRDE fails to detect N2H4, which is formed as a stable intermediate on the electrode surface, as identified from in-situ attenuated total reflectance-infrared (ATR-IR) spectroscopy. This advocates the complete conversion of N2 to NH3 on Mo/Ag3PO4-10 and the so-formed oxygen vacancies formed during doping act as proton scavengers suppressing hydrogen evolution reaction resulting into a Faradaic efficiency of 54.8% for NRR.

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.

Photocatalytic Dinitrogen Reduction to Ammonia over Biomimetic FeMoSx Nanosheets

Reduction of atmospheric dinitrogen (N2) to ammonia (NH3) using water and sunlight in the absence of sacrificial reducing reagents at room temperature is very challenging and is considered an eco-friendly approach to meet the rapidly increasing demand for nitrogen storage, fertilizers, and a sustainable society. Currently, ammonia production via the energy-intensive Haber-Bosch process causes ∼350 million tons of carbon dioxide (CO2) emission per year. Interestingly, natural N2 fixation by the nitrogenase enzyme occurs under ambient conditions. Unfortunately, N2 fixation on biomimetic catalysts has rarely been studied. To mimic biological nitrogen fixation, herein, we synthesized the novel iron molybdenum sulfide (FeMoSx) micro-/nanosheets via a simple hydrothermal approach for the first time. Further, we successfully demonstrated the photochemical conversion of N2 to NH3 over a biomimetic FeMoSx photocatalyst. The estimated yield is around 99.79 ± 6.0 μmol/h/g photocatalyst with a quantum efficiency of ∼0.028% at 532 nm visible-light wavelength. Besides, we also systematically studied the influence of key factors to further improve NH3 yields. Overall, this study paves a new pathway to fabricate carbon-free, photochemical N2 fixation materials for future applications.

The Influence of Ions on the Electrochemical Stability of Aqueous Electrolytes

The electrochemical stability window of water is known to vary with the type and concentration of dissolved salts. However, the underlying influence of ions on the thermodynamic stability of aqueous solutions has not been fully understood. Here, we investigated the electrolytic behaviors of aqueous electrolytes as a function of different ions. Our findings indicate that ions with high ionic potentials, i.e., charge density, promote the formation of their respective hydration structures, enhancing electrolytic reactions via an inductive effect, particularly for small cations. Conversely, ions with lower ionic potentials increase the proportion of free water molecules-those not engaged in hydration shells or hydrogen-bonding networks-leading to greater electrolytic stability. Furthermore, we observe that the chemical environment created by bulky ions with lower ionic potentials impedes electrolytic reactions by frustrating the solvation of protons and hydroxide ions, the products of oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. We found that the solvation of protons plays a more substantial role than that of hydroxide, which explains a greater shift for OER than for HER, a puzzle that cannot be rationalized by the notion of varying O-H bond strengths of water. These insights will help the design of aqueous systems.

NH3 Electrosynthesis from N2 Molecules: Progresses, Challenges, and Future Perspectives

Green ammonia (NH3), made by using renewable electricity to split nearly limitless nitrogen (N2) molecules, is a vital platform molecule and an ideal fuel to drive the sustainable development of human society without carbon dioxide emission. The NH3 electrosynthesis field currently faces the dilemma of low yield rate and efficiency; however, decoupling the overlapping issues of this area and providing guidelines for its development directions are not trivial because it involves complex reaction process and multidisciplinary entries (for example, electrochemistry, catalysis, interfaces, processes, etc.). In this Perspective, we introduce a classification scheme for NH3 electrosynthesis based on the reaction process, namely, direct (N2 reduction reaction) and indirect electrosynthesis (Li-mediated/plasma-enabled NH3 electrosynthesis). This categorization allows us to finely decouple the complicated reaction pathways and identify the specific rate-determining steps/bottleneck issues for each synthesis approach such as N2 activation, H2 evolution side reaction, solid-electrolyte interphase engineering, plasma process, etc. We then present a detailed overview of the latest progresses on solving these core issues in terms of the whole electrochemical system covering the electrocatalysts, electrodes, electrolytes, electrolyzers, etc. Finally, we discuss the research focuses and the promising strategies for the development of NH3 electrosynthesis in the future with a multiscale perspective of atomistic mechanisms, nanoscale electrocatalysts, microscale electrodes/interfaces, and macroscale electrolyzers/processes. It is expected that this Perspective will provide the readers with an in-depth understanding of the bottleneck issues and insightful guidance on designing the efficient NH3 electrosynthesis systems.

Thermodynamic, Kinetic, and Transport Contributions to Hydrogen Evolution Activity and Electrolyte-Stability Windows for Water-in-Salt Electrolytes

Concentrated water-in-salt electrolytes (WiSEs) are used in aqueous batteries and to control electrochemical reactions for fuel production. The hydrogen evolution reaction is a parasitic reaction at the negative electrode that limits cell voltage in WiSE batteries and leads to self-discharge, and affects selectivity for electrosynthesis. Mitigating and modulating these processes is hampered by a limited fundamental understanding of HER kinetics in WiSEs. Here, we quantitatively assess how thermodynamics, kinetics, and interface layers control the apparent HER activities in 20 m LiTFSI. When the LiTFSI concentration is increased from 1 to 20 m, an increase in proton activity causes a positive shift in the HER equilibrium potential of 71 mV. The exchange current density, io, derived from the HER branch for 20 m LiTFSI in 98% purity (0.56 ± 0.05 μA/cmPt2), however, is 8 times lower than for 20 m LiTFSI in 99.95% (4.7 ± 0.2 μA/cmPt2) and 32 times lower than for 1 m LiTFSI in 98% purity (18 ± 1 μA/cmPt2), demonstrating that the WiSE's impurities and concentration are both central in significantly suppressing HER kinetics. The ability and applicability of the reported methods are extended by examining additional WiSEs formulations made of acetates and nitrates.

Atomically dispersed Pd on defective BN nanosheets for nitrite electroreduction to ammonia

Electrocatalytic NO2- reduction to NH3 (NO2RR) offers a prospective strategy to concurrently achieve polluted NO2- removal and effective NH3 electrosynthesis. In this work, we report atomically dispersed Pd on defective BN nanosheets (Pd1/BN) as an efficient catalyst for the NO2RR, achieving the highest NH3-Faradaic efficiency of 91.7% with an NH3 yield rate of 347.1 μmol h-1 cm-2 at -0.6 V vs. RHE, superior to those of most previously reported electrocatalysts. Theoretical computations reveal the isolated Pd sites as catalytic centers to selectively adsorb NO2- and accelerate NO2--to-NH3 hydrogenation process with a minimized reaction barrier, eventually contributing to the considerably enhanced NO2RR selectivity and activity of Pd1/BN.

Selective oxidation of p-phenylenediamine for blood glucose detection enabled by Se-vacancy-rich TiSe2-x@Au nanozyme

Nanozymes with enzyme-like characteristics have drawn wide interest but the catalytic activity and substrate selectivity of nanozymes still need improvement. Herein, Se-vacancy-rich TiSe2-x@Au nanocomposites are designed and demonstrated as nanozymes. The TiSe2-x@Au nanocomposites show excellent peroxidase-like activity and the chromogenic substrate p-phenylenediamine (PPD) can be selectively oxidized to compounds that exhibit an absorption peak at 413 nm that differs from that of self-oxidation or generally oxidized species, suggesting high catalytic activity and strong substrate selectivity. Theoretical calculations reveal that the PPD adsorption geometry at Se vacancies with an adsorption energy of -3.00 eV shows a unique spatial configuration and charge distribution, thereby inhibiting the free reaction and promoting both the activity and selectivity in PPD oxidation. The TiSe2-x@Au colorimetric system exhibits a wide linear range of 0.015 mM-0.6 mM and a low detection limit of 0.0037 mM in the detection of glucose. The blood glucose detection performance for human serum samples is comparable to that of a commercial glucose meter in the hospital (relative standard deviation < 6%). Our findings demonstrate a new strategy for rapid and accurate detection of blood glucose and our results provide insights into the future design of nanozymes.

Vacancy-Mediated Control of Local Electronic Structure for High-Efficiency Electrocatalytic Conversion of N2 to NH3

Ambient electrocatalytic nitrogen (N2 ) reduction has gained significant recognition as a potential substitute for producing ammonia (NH3 ). However, N2 adsorption and *NN protonation for N2 activation reaction with the competing hydrogen evolution reaction remain a daunting challenge. Herein, a defect-rich TiO2 nanosheet electrocatalyst with PdCu alloy nanoparticles (PdCu/TiO2-x ) is designed to elucidate the reactivity and selectivity trends of N2 cleavage path for N2 -to-NH3 catalytic conversion. The introduction of oxygen vacancy (OV) not only acts as active sites but also effectively promotes the electron transfer from Pd-Cu sites to high-concentration Ti3+ sites, and thus lends to the N2 activation via electron donation of PdCu. OVs-mediated control effectively lowers the reaction barrier of *N2 H and *H adsorption and facilitates the first hydrogenation process of N2 activation. Consequently, PdCu/TiO2-x catalyst attains a high rate of NH3 evolution, reaching 5.0 mmol gcat. -1  h-1 . This work paves a pathway of defect-engineering metal-supported electrocatalysts for high-efficient ammonia electrosynthesis.

Nitrogen Reduction Reaction to Ammonia on Transition Metal Carbide Catalysts

The development of a low-cost, energy-efficient, and environmentally friendly alternative to the currently utilized Haber-Bosch process to produce ammonia is of great importance. Ammonia is an essential chemical used in fertilizers and a promising high-density fuel source. The nitrogen reduction reaction (NRR) has been explored intensively as a potential avenue for ammonia production using water as proton source, but to this day a catalyst capable of producing this chemical at high Faradaic efficiency (FE) and commercial yield and rates has not been reported. Here, we investigate the activity of transition metal carbide (TMC) surfaces in the (100) facets of the rocksalt (RS) structure as potential catalysts for the NRR. In this study, we use density functional theory (DFT) to model reaction pathways, estimate stability, assess kinetic barriers, and compare adsorbate energies to determine the overall performance of each TMC surface. For pristine TMC surfaces (with no defects) we find that none of the studied TMCs possess both exergonic adsorption of nitrogen and the capability to selectively protonate nitrogen to form ammonia in the desired aqueous solution. ZrC, however, is shown to be a potential catalyst if used in a non-aqueous electrolyte. To circumvent the endergonic adsorption of nitrogen onto the surface, a carbon vacancy was introduced. This provides a well-defined high coordination active site on the surface. In the presence of a vacancy VC, NbC, and WC showed efficient nitrogen adsorption, selectivity towards ammonia, and a low overpotential (OP). NbC did, however, display an unfeasible kinetic barrier to nitrogen dissociation for ambient-condition purposes, and thus it is suggested for high tempearture/pressure ammonia synthesis. Both WC and VC in their RS (100) structure are promising materials for experimental investigations in aqueous electrolytes, and ZrC could potentially be interesting for non-aqueous electrolytic systems.

A simple hydrothermal synthesis of an oxygen vacancy-rich MnMoO4 rod-like material and its highly efficient electrocatalytic nitrogen reduction

Electrocatalytic nitrogen reduction (NRR) for artificial ammonia synthesis under ambient conditions is considered a promising alternative to the traditional Haber-Bosch process. However, it still faces multiple challenges such as the difficulty of N2 adsorption and activation and limited Faraday efficiency. In this work, a bimetallic oxide MnMoO4 was prepared by a hydrothermal method and low temperature calcination. The influence of the sintering temperature on the microstructure (crystallinity and oxygen vacancies) of the oxide and its NRR properties were systematically explored. The results showed that MnMoO4 sintered at 500 °C had the highest concentration of OVs and showed excellent NRR performance, with the highest NH3 yield (up to 12.28 μg h-1 mgcat-1), high Faraday efficiency (23.04% at -0.30 V vs. RHE), and good stability at -0.40 V vs. RHE, and the catalytic performance was about two times higher than that of Mn2O3 and MoO3. It is also superior to other bimetallic oxide NRR electrocatalysts reported in some cases. In addition, we also explored the ratio between Mn and Mo metals, and the catalytic effect was the best when Mn : Mo = 1 : 1. Due to the synergistic effect between Mn and Mo metals and the large number of OVs present internally, the catalytic activity for the NRR was largely improved. This study suggests that the bimetallic oxide MnMoO4 may be a promising NRR electrocatalyst.

Atomically Mo-Doped SnO2-x for efficient nitrate electroreduction to ammonia

Electrochemical NO3--to-NH3 reduction (NO3RR) emerges as an appealing strategy to alleviate contaminated NO3- and generate valuable NH3 simultaneously. However, substantial research efforts are still needed to advance the development of efficient NO3RR catalysts. Herein, atomically Mo-doped SnO2-x with enriched O-vacancies (Mo-SnO2-x) is reported as a high-efficiency NO3RR catalyst, delivering the highest NH3-Faradaic efficiency of 95.5% with a corresponding NH3 yield rate of 5.3 mg h-1 cm-2 at -0.7 V (RHE). Experimental and theoretical investigations reveal that d-p coupled Mo-Sn pairs constructed on Mo-SnO2-x can synergistically enhance the electron transfer efficiency, activate the NO3- and reduce the protonation barrier of rate-determining step (*NO→*NOH), thereby drastically boosting the NO3RR kinetics and energetics.

Rational design of artificial Lewis pairs coupling with polyethylene glycol for efficient electrochemical ammonia synthesis

Ammonia (NH3) synthesis at mild conditions by electrocatalytic nitrogen reduction (eNRR) has received more attention and has been regarded as a promising alternative to the traditional Haber-Bosch process. Lewis acid-base pairs (LPs) can chemisorb and react with nitrogen by electronic interaction, while the tuning of the microenvironment near electrode can hinder hydrogen evolution reaction (HER) thus improving the selectivity of the eNRR. Herein, the FeOOH nanorod coupled with LPs on the surface (i.e., Fe, Fe-O) was synthesized, which could effectively drive eNRR. Meanwhile, polyethylene glycol (PEG) was introduced to serve as a local non-aqueous electrolyte system to inhibit HER. The prepared FeOOH-150 catalyst achieved outstanding eNRR performance with an NH3 yield rate of 118.07 μg h-1mgcat-1 and a Faradaic efficiency of 51.4 % at -0.6 V vs. RHE in 0.1 M LiClO4 + 20 % PEG. Both the experiment and DFT calculations revealed that the interaction of PEG with Lewis base sites could optimize nitrogen adsorption configuration and activation.

Nitrogen Electrocatalysis: Electrolyte Engineering Strategies to Boost Faradaic Efficiency

The electrochemical activation of dinitrogen at ambient temperature and pressure for the synthesis of ammonia has drawn increasing attention. The faradaic efficiency (FE) as well as ammonia yield in the electrochemical synthesis is far from reaching the requirement of industrial-scale production. In aqueous electrolytes, the competing electron-consuming hydrogen evolution reaction (HER) and poor solubility of nitrogen are the two major bottlenecks. As the electrochemical reduction of nitrogen involves proton-coupled electron transfer reaction, rationally engineered electrolytes are required to boost FE and ammonia yield. In this Review, we comprehensively summarize various electrolyte engineering strategies to boost the FE in aqueous and non-aqueous medium and suggest possible approaches to further improve the performance. In aqueous medium, the performance can be improved by altering the electrolyte pH, transport velocity of protons, and water activity. Other strategies involve the use of hybrid and water-in-salt electrolytes, ionic liquids, and non-aqueous electrolytes. Existing aqueous electrolytes are not ideal for industrial-scale production. Suppression of HER and enhanced nitrogen solubility have been observed with hybrid and non-aqueous electrolytes. The engineered electrolytes are very promising though the electrochemical activation has several challenges. The outcome of lithium-mediated nitrogen reduction reaction with engineered non-aqueous electrolyte is highly encouraging.

Ti-doped iron phosphide nanoarrays grown on carbon cloth as a self-supported electrode for enhanced electrocatalytic nitrogen reduction

The electrocatalytic nitrogen reduction reaction (eNRR) has been widely recognized as a promising method for green ammonia synthesis. However, the inert NN bond, inferior catalytic activity and small electrochemically active area impede its practical application. To circumvent these problems, we proposed self-supported Ti-doped iron phosphide (FeP) nanorod arrays grown on carbon cloth (Ti-FeP/CC) as an electrode for eNRR. The introduction of Ti doping sites regulated the electron structure of FeP, leading to electron migration from Fe to P, which facilitated N2-to-NH3 conversion. The as-prepared Ti-FeP/CC showed an enhancement of electrochemical surface area (ECSA), high electrical conductivity and well-exposed active sites. Ti-FeP/CC was capable of producing a high NH3 yield of 10.93 μg h-1 cm-2 and faradaic efficiency of 10.77% at an optimal voltage of -0.3 V (vs. RHE) in a 0.1 M Na2SO4 solution with excellent stability and durability during the eNRR process. This work not only presents a promising electrode material for eNRR, but also provides a new insight into rational heteroatom doping for electrocatalysis.

Surface-Reconstructed Copper Foil Free-Standing Electrode with Nanoflower Cu/Ce2O3 by In Situ Electrodeposition Reduction for Electrocatalytic Nitrate Reduction to Ammonia

The electrocatalytic reduction of nitrate to ammonia (NRA) is considered to be a promising and environmentally friendly alternative to the Haber-Bosch process. Herein, a simple in situ electrodeposition-reduction method is proposed, which allows for the surface reconstruction and modification of copper foil by introducing cerium. The surface modification results in the formation of Cu/Ce2O3 nanoflowers on the copper electrode surface. Additionally, density functional theory calculations demonstrate that Ce2O3 enhances the adsorption of NO3-. Compared with a pure copper electrode, the NRA performance of surface-reconstructed copper is greatly improved, with the maximum ammonia yield rate and the highest Faradaic efficiency from 24.8% to 73.0% and from 0.081 to 0.386 mmol h-1 cm-2, respectively. The 15N isotope labeling experiment confirms that the nitrogen source in the ammonia originated from nitrate.

Laser Promoting Oxygen Vacancies Generation in Alloy via Mo for HMF Electrochemical Oxidation

It is well known that nickel-based catalysts have high electrocatalytic activity for the 5-hydroxymethylfurfural oxidation reaction (HMFOR), and NiOOH is the main active component. However, the price of nickel and the catalyst's lifetime still need to be solved. In this work, NiOOH containing oxygen vacancies is formed on the surface of Ni alloy by UV laser (1J85-laser). X-ray absorption fine structure (XAFS) analyses indicate an interaction between Mo and Ni, which affects the coordination environment of Ni with oxygen. The chemical valence of Ni is between 0 and 2, indicating the generation of oxygen vacancies. Density functional theory (DFT) suggests that Mo can increase the defect energy and form more oxygen vacancies. In situ Raman electrochemical spectroscopy shows that Mo can promote the formation of NiOOH, thus enhancing the HMFOR activity. The 1J85-laser electrode shows a longer electrocatalytic lifetime than Ni-laser. After 15 cycles, the conversion of HMF is 95.92%.

Heterostructured Co/Co3O4 anchored on N-doped carbon nanotubes as a highly efficient electrocatalyst for nitrate reduction to ammonia

The electrochemical reduction of nitrate (NO₃^-) to ammonia (NH₃) has emerged as an attractive approach for selectively reducing NO₃^- to highly value-added NH₃ and removing NO₃^- pollutants simultaneously. In this work, a heterostructured Co/Co₃O₄ electrocatalyst anchored on N-doped carbon nanotubes was prepared and applied for the NO₃^- reduction towards NH₃ under alkaline conditions. The catalyst achieves outstanding performance with up to 67% NH₃ faradaic efficiency at -1.2 V vs. Hg/HgO and 8.319 mg h^-1 mg_cat^-1 yield at -1.7 V vs. Hg/HgO. In addition, it also exhibits good long-term stability. ¹⁵N isotopic labelling experiments prove that the yielded NH₃ is derived from NO₃^- species. In situ electrochemical Raman spectra revealed that the structure of the as-prepared catalyst showed outstanding stability and identified possible intermediates during the electrocatalytic NO₃^- reduction reaction (NO₃RR).

High-efficiency electrocatalytic nitrite reduction toward ammonia synthesis on CoP@TiO2 nanoribbon array

Electrochemical reduction of nitrite (NO₂^-) can satisfy the necessity for NO₂^- contaminant removal and deliver a sustainable pathway for ammonia (NH₃) generation. Its practical application yet requires highly efficient electrocatalysts to boost NH₃ yield and Faradaic efficiency (FE). In this study, CoP nanoparticle-decorated TiO₂ nanoribbon array on Ti plate (CoP@TiO₂/TP) is verified as a high-efficiency electrocatalyst for the selective reduction of NO₂^- to NH₃. When measured in 0.1 M NaOH with NO₂^-, the freestanding CoP@TiO₂/TP electrode delivers a large NH₃ yield of 849.57 μmol h^-1 cm^-2 and a high FE of 97.01% with good stability. Remarkably, the subsequently fabricated Zn-NO₂^- battery achieves a high power density of 1.24 mW cm^-2 while delivering a NH₃ yield of 714.40 μg h^-1 cm^-2.

Single atomic cerium sites anchored on nitrogen-doped hollow carbon spheres for highly selective electroreduction of nitric oxide to ammonia

Electrocatalytic nitric oxide reduction reaction (NORR) at ambient environments not only offers a promising strategy to yield ammonia (NH₃) but also degrades the NO contaminant; however, its application depends on searching for high-performance catalysts. Herein, we present single atomic Ce sites anchored on nitrogen-doped hollow carbon spheres that are capable of electro-catalyzing NO reduction to NH₃ in an acidic solution, achieving a maximal Faradaic efficiency of 91 % and a yield rate of 1023 μg h^-1 mg_cat.^-1 at -0.7 V vs RHE for NH₃ formation, both of which outperform these on Ce nanoclusters and approach the best-reported results. Meanwhile, the single atomic Ce catalyst shows good structural and electrochemical stability during the 30-h NO electrolysis. Furthermore, when the single atomic Ce catalyst was used as cathodic material in a proof-of-concept of Zn-NO battery, it delivers a maximal power density of 3.4 mW cm^-2 and a high NH₃ yield rate of 309 μg h^-1 mg_cat.^-1. Theoretical simulations suggest that the Ce-N4 active moiety can not only activate NO molecules via a strong electronic interaction but also reduce the free energy barrier of *NO transition to *NOH intermediate as the limiting step, and therefore boosting the NORR kinetics and suppressing the competitive hydrogen evolution.

A metal-free catalyst for electrocatalytic NO reduction to NH3

Metal-free boron phosphide (BP) is explored for the first time as an effective catalyst for electrocatalytic NO reduction to NH₃, showing a high NH₃-faradaic efficiency of 83.3% with an NH₃ yield rate of 96.6 μmol h^-1 cm^-2, surpassing most metal-based catalysts. Theoretical results reveal that the B and P atoms of BP can serve as dual-active centers to synergistically activate NO, promote the NORR hydrogenation process and inhibit the competing hydrogen evolution reaction.

High-Performance Artificial Synapse Based on CVD-Grown WSe2 Flakes with Intrinsic Defects

High-performance artificial synaptic devices with rich functions are highly desired for the development of an advanced brain-like neuromorphic system. Here, we prepare synaptic devices based on a CVD-grown WSe₂ flake, which has an unusual morphology of nested triangles. The WSe₂ transistor exhibits robust synaptic behaviors such as excitatory postsynaptic current, paired-pulse facilitation, short-time plasticity, and long-time plasticity. Furthermore, due to its high sensitivity to light illumination, the WSe₂ transistor exhibits excellent light-dosage-dependent and light wavelength-dependent plasticity, which endow the synaptic device with more intelligent learning and memory functions. In addition, WSe₂ optoelectronic synapses can mimic "learning experience" behavior and associative learning behavior like the brain. An artificial neural network is simulated for pattern recognition of hand-written digital images in the MNIST data set and the best recognition accuracy could reach 92.9% based on weight updating training of our WSe₂ device. Detailed surface potential analysis and PL characterization reveal that the intrinsic defects generated in growth are dominantly responsible for the controllable synaptic plasticity. Our work suggests that the CVD-grown WSe₂ flake with intrinsic defects capable of robust trapping/de-trapping charges holds great application prospects in future high-performance neuromorphic computation.

Tandem Electrocatalytic Nitrate Reduction to Ammonia on MBenes

We demonstrate the great feasibility of MBenes as a new class of tandem catalysts for electrocatalytic nitrate reduction to ammonia (NO₃ RR). As a proof of concept, FeB₂ is first employed as a model MBene catalyst for the NO₃ RR, showing a maximum NH₃ -Faradaic efficiency of 96.8 % with a corresponding NH₃ yield of 25.5 mg h^-1  cm^-2 at -0.6 V vs. RHE. Mechanistic studies reveal that the exceptional NO₃ RR activity of FeB₂ arises from the tandem catalysis mechanism, that is, B sites activate NO₃ ^- to form intermediates, while Fe sites dissociate H₂ O and increase *H supply on B sites to promote the intermediate hydrogenation and enhance the NO₃ ^- -to-NH₃ conversion.

Ambient ammonia synthesis via nitrite electroreduction over NiS2 nanoparticles-decorated TiO2 nanoribbon array

Nitrite (NO₂^-), as a N-containing pollutant, widely exists in aqueous solution, causing a series of environmental and health problems. Electrocatalytic NO₂^- reduction is a promising and sustainable strategy to remove NO₂^-, meanwhile, producing high value-added ammonia (NH₃). But the NO₂^- reduction reaction (NO₂^-RR) involves complex 6-electron transfer process that requires high-efficiency electrocatalysts to accomplish NO₂^--to-NH₃ conversion. Herein, we report NiS₂ nanoparticles decorated TiO₂ nanoribbon array on titanium mesh (NiS₂@TiO₂/TM) as a fantastic NO₂^-RR electrocatalyst for ambient NH₃ synthesis. When tested in NO₂^--containing solution, NiS₂@TiO₂/TM achieves a satisfactory NH₃ yield of 591.9 µmol h^-1 cm^-2 and a high Faradaic efficiency of 92.1 %. Besides, it shows remarkable stability during 12-h electrolysis test.

Self-Tandem Electrocatalytic NO Reduction to NH3 on a W Single-Atom Catalyst

We design single-atom W confined in MoO_3-x amorphous nanosheets (W₁/MoO_3-x) comprising W₁-O₅ motifs as a highly active and durable NORR catalyst. Theoretical and operando spectroscopic investigations reveal the dual functions of W₁-O₅ motifs to (1) facilitate the activation and protonation of NO molecules and (2) promote H₂O dissociation while suppressing *H dimerization to increase the proton supply, eventually resulting in a self-tandem NORR mechanism of W₁/MoO_3-x to greatly accelerate the protonation energetics of the NO-to-NH₃ pathway. As a result, W₁/MoO_3-x exhibits the highest NH₃-Faradaic efficiency of 91.2% and NH₃ yield rate of 308.6 μmol h^-1 cm^-2, surpassing that of most previously reported NORR catalysts.

High-efficiency electroreduction of nitrite to ammonia on a Cu@TiO2 nanobelt array

Electrochemical nitrite (NO₂^-) reduction is a potential and sustainable route to produce high-value ammonia (NH₃), but it requires highly active electrocatalysts. Herein, Cu nanoparticles anchored on a TiO₂ nanobelt array on a titanium plate (Cu@TiO₂/TP) are reported as a high-efficiency electrocatalyst for NO₂^--to-NH₃ conversion. The designed Cu@TiO₂/TP catalyst exhibits outstanding catalytic performance toward the NO₂^-RR, with a high NH₃ yield of 760.5 μmol h^-1 cm^-2 (237.7 μmol h^-1 mg_cat.^-1) and an excellent faradaic efficiency of 95.3% in neutral solution. Meanwhile, it also presents strong electrochemical stability during cyclic tests and long-term electrolysis.

Electrocatalytic NO Reduction to NH3 on Mo2C Nanosheets

Electrocatalytic reduction of NO to NH₃ (NORR) emerges as a promising route for achieving harmful NO treatment and sustainable NH₃ generation. In this work, we first report that Mo₂C is an active and selective NORR catalyst. The developed Mo₂C nanosheets deliver a high NH₃ yield rate of 122.7 μmol h^-1 cm^-2 with an NH₃ Faradaic efficiency of 86.3% at -0.4 V. Theoretical computations unveil that the surface-terminated Mo atoms on Mo₂C can effectively activate NO, promote protonation energetics, and suppress proton adsorption, resulting in high NORR activity and selectivity of Mo₂C.

Sub-nm RuOx Clusters on Pd Metallene for Synergistically Enhanced Nitrate Electroreduction to Ammonia

The electrochemical nitrate reduction to ammonia reaction (NO3RR) has emerged as an appealing route for achieving both wastewater treatment and ammonia production. Herein, sub-nm RuOx clusters anchored on a Pd metallene (RuOx/Pd) are reported as a highly effective NO3RR catalyst, delivering a maximum NH3-Faradaic efficiency of 98.6% with a corresponding NH3 yield rate of 23.5 mg h-1 cm-2 and partial a current density of 296.3 mA cm-2 at -0.5 V vs RHE. Operando spectroscopic characterizations combined with theoretical computations unveil the synergy of RuOx and Pd to enhance the NO3RR energetics through a mechanism of hydrogen spillover and hydrogen-bond interactions. In detail, RuOx activates NO3- to form intermediates, while Pd dissociates H2O to generate *H, which spontaneously migrates to the RuOx/Pd interface via a hydrogen spillover process. Further hydrogen-bond interactions between spillovered *H and intermediates makes spillovered *H desorb from the RuOx/Pd interface and participate in the intermediate hydrogenation, contributing to the enhanced activity of RuOx/Pd for NO3--to-NH3 conversion.

Microenvironment Regulation of the Ti3C2Tx MXene Surface for Enhanced Electrochemical Nitrogen Reduction

The overwhelmingly competitive hydrogen evolution reaction (HER) is a bottleneck challenge in the electrocatalytic nitrogen reduction reaction (eNRR) process. Herein, we develop a general and effective strategy to suppress the HER via covalent surface functionalization to modulate the local microenvironment of the electrocatalyst. A hydrophobic molecular layer with tunable coverage density was coated on the surface of Ti₃C₂T_x MXene, and the one with appropriate coverage density significantly improved the eNRR efficiency with an excellent faradaic efficiency (FE) of 38.01% at -0.35 V and a high NH₃ yield rate of 17.81 μg h^-1mg_cat^-1 at -0.55 V (vs RHE) in a Na₂SO₄ solution, which were 3.5-fold in FE and 6.5-fold in NH₃ yield rate higher than those of the pristine Ti₃C₂T_x. Experimental results combined with molecular dynamics (MD) simulations reveal that the hydrophobic molecular layer on the surface greatly limits the proton transfer and benefits higher exposure of active sites with enhanced N₂ chemisorption ability, which cumulatively contribute to the boosted eNRR efficiency.

Single-Step Synthesis of Fe-Fe3 O4 Catalyst for Highly Efficient and Selective Electrochemical Nitrogen Reduction

Nitrogen reduction electrocatalysts are highly attractive for catalytic science. However, most electrocatalysts are limited by their low faradaic efficiency, poor ammonia yield, and tedious and costly catalyst synthesis process. In this work, Fe-based oxide composite nanoparticles with steady chemical states are prepared by a single-step green procedure under ambient conditions. The resulting Fe-Fe₃ O₄ demonstrates remarkable activity and selectivity for nitrogen reduction reaction (NRR) with the highest faradaic efficiency of 53.2±1.8 % and NH₃ yield rate of 24.6±0.8 μg h^-1  mg_cat. ^-1 at -0.4 V (vs. RHE) in 0.1 m Na₂ SO₄ electrolyte. Characterization experiments and theoretical calculation reveal that Fe-Fe₃ O₄ exhibits significantly enhanced charge transfer capability and suppresses the competitive HER process.

Oxygen Functionalization-Induced Charging Effect on Boron Active Sites for High-Yield Electrocatalytic NH3 Production

Ammonia has been recognized as the future renewable energy fuel because of its wide-ranging applications in H₂ storage and transportation sector. In order to avoid the environmentally hazardous Haber-Bosch process, recently, the third-generation ambient ammonia synthesis has drawn phenomenal attention and thus tremendous efforts are devoted to developing efficient electrocatalysts that would circumvent the bottlenecks of the electrochemical nitrogen reduction reaction (NRR) like competitive hydrogen evolution reaction, poor selectivity of N₂ on catalyst surface. Herein, we report the synthesis of an oxygen-functionalized boron carbonitride matrix via a two-step pyrolysis technique. The conductive BNCO₍₁₀₀₀₎ architecture, the compatibility of B-2p_z orbital with the N-2p_z orbital and the charging effect over B due to the C and O edge-atoms in a pentagon altogether facilitate N₂ adsorption on the B edge-active sites. The optimum electrolyte acidity with 0.1 M HCl and the lowered anion crowding effect aid the protonation steps of NRR via an associative alternating pathway, which gives a sufficiently high yield of ammonia (211.5 μg h^-1 mg_cat^-1) on the optimized BNCO₍₁₀₀₀₎ catalyst with a Faradaic efficiency of 34.7% at - 0.1 V vs RHE. This work thus offers a cost-effective electrode material and provides a contemporary idea about reinforcing the charging effect over the secured active sites for NRR by selectively choosing the electrolyte anions and functionalizing the active edges of the BNCO₍₁₀₀₀₎ catalyst.

Vacancy Defects in 2D Transition Metal Dichalcogenide Electrocatalysts: From Aggregated to Atomic Configuration

Vacancy defect engineering has been well leveraged to flexibly shape comprehensive physicochemical properties of diverse catalysts. In particular, growing research effort has been devoted to engineering chalcogen anionic vacancies (S/Se/Te) of 2D transition metal dichalcogenides (2D TMDs) toward the ultimate performance limit of electrocatalytic hydrogen evolution reaction (HER). In spite of remarkable progress achieved in the past decade, systematic and in-depth insights into the state-of-the-art vacancy engineering for 2D-TMDs-based electrocatalysis are still lacking. Herein, this review delivers a full picture of vacancy engineering evolving from aggregated to atomic configurations covering their development background, controllable manufacturing, thorough characterization, and representative HER application. Of particular interest, the deep-seated correlations between specific vacancy regulation routes and resulting catalytic performance improvement are logically clarified in terms of atomic rearrangement, charge redistribution, energy band variation, intermediate adsorption-desorption optimization, and charge/mass transfer facilitation. Beyond that, a broader vision is cast into the cutting-edge research fields of vacancy-engineering-based single-atom catalysis and dynamic structure-performance correlations across catalyst service lifetime. Together with critical discussion on residual challenges and future prospects, this review sheds new light on the rational design of advanced defect catalysts and navigates their broader application in high-efficiency energy conversion and storage fields.

Ni nanoparticle-decorated biomass carbon for efficient electrocatalytic nitrite reduction to ammonia

Electrocatalytic nitrite (NO₂^-) reduction to ammonia (NH₃) can not only synthesize value-added NH₃, but also remove NO₂^- pollutants from the environment. However, the low efficiency of NO₂^--to-NH₃ conversion hinders its applications. Here, Ni nanoparticle-decorated juncus-derived biomass carbon prepared at 800 °C (Ni@JBC-800) serves as an efficient catalyst for NH₃ synthesis by selective electroreduction of NO₂^-. This catalyst shows a remarkable NH₃ yield of 4117.3 μg h^-1 mg_cat.^-1 and a large faradaic efficiency of 83.4% in an alkaline electrolyte. The catalytic mechanism is further investigated by theoretical calculations.

Heterostructured Bi2S3/MoS2 Nanoarrays for Efficient Electrocatalytic Nitrate Reduction to Ammonia Under Ambient Conditions

Developing efficient electrocatalysts to realize the nitrate reduction reaction (eNO₃^-RR) for ammonia synthesis as an alternative to the traditional Haber-Bosch production process is of great significance. Herein, the heterostructured Bi₂S₃/MoS₂ nanoarrays were successfully synthesized by Bi₂S₃ nanowires anchored on MoS₂ nanosheets. Owing to the interfacial coupling effect, both particular surface area and exposure active sites increase. Density functional theory further uncovered that the excellent activity originates from charge transfer of the interface and a low potential barrier of 0.58 eV for hydrogenation of *NO to *NOH on Bi₂S₃/MoS₂. Compared with pure Bi₂S₃ and MoS₂ catalysts, the heterostructured Bi₂S₃/MoS₂ nanoarrays exhibit a superior NH₃ yield of 15.04 × 10^-2 mmol·h^-1·cm^-2 and a Faraday efficiency of 88.4% at -0.8 V versus the reversible hydrogen electrode. This work provides a new avenue to explore advanced electrocatalysts, which is expected to shorten the distance from the practical application of the eNO₃^-RR technology.

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.

High-Efficiency Electrosynthesis of Ammonia with Selective Reduction of Nitrate in Neutral Media Enabled by Self-Supported Mn2CoO4 Nanoarray

Ambient ammonia synthesis by electroreduction of nitrate (NO3-) provides us a sustainable and environmentally friendly alternative to the traditional Haber-Bosch process. In this work, Mn2CoO4 nanoarray grown on carbon cloth (Mn2CoO4/CC) serves as a superior electrocatalyst for efficient NH3 synthesis by selective reduction of NO3-. When operated in 0.1 M PBS with 0.1 M NaNO3, Mn2CoO4/CC reaches a high Faraday efficiency of 98.6% and a large NH3 yield up to 11.19 mg/h/cm2. Moreover, it exhibits excellent electrocatalytic stability. Theory calculations show that the Mn2CoO4 surface has strong interaction with NO3-, which can effectively inhibit the occurrence of hydrogen evolution, beneficial for NO3--to-NH3 conversion.