Identification and elimination of false positives in electrochemical nitrogen reduction studies

Chemistry of Materials Underpinning Photoelectrochemical Solar Fuel Production

Since its inception, photoelectrochemistry has sought to power the generation of fuels, particularly hydrogen, using energy from sunlight. Efficient and durable photoelectrodes, however, remain elusive. Here we review the current state of the art, focusing our discussion on advances in photoelectrodes made in the past decade. We open by briefly discussing fundamental photoelectrochemical concepts and implications for photoelectrode function. We next review a broad range of semiconductor photoelectrodes broken down by material class (oxides, nitrides, chalcogenides, and mature photovoltaic semiconductors), identifying intrinsic properties and discussing their influence on performance. We then identify innovative in situ and operando techniques to directly probe the photoelectrode|electrolyte interface, enabling direct assessment of structure-property relationships for catalytic surfaces in active reaction environments. We close by considering more complex photoelectrochemical fuel-forming reactions (carbon dioxide and nitrogen reduction, as well as alternative oxidation reactions), where product selectivity imposes additional criteria on electrochemical driving force and photoelectrode architecture. By contextualizing recent literature within a fundamental framework, we seek to provide direction for continued progress toward achieving efficient and stable fuel-forming photoelectrodes.

Recent advances in metal single-atom catalysts for ammonia electrosynthesis

Electrochemical ammonia synthesis is a promising alternative to the Haber-Bosch process, offering significant potential for sustainable agricultural production and the development of portable, carbon-free energy carriers. The development of electrocatalytic systems is currently dependent on the exploration of electrocatalysts with high activity, selectivity, and stability. Metal single-atom catalysts (SACs) have become a new attractive frontier for ammonia electrosynthesis, owing to their maximized atom utilization, unsaturated atom coordination, and tunable electronic structure. In this review, we focused on different metal sites inside the single-atom catalysts and summarized recent advances in SACs for ammonia electrosynthesis. The properties of small nitrogenous substances (including N2, NO, NO2-, and NO3-) are summarized. In addition, the SACs for different catalytic systems are reviewed, with a particular focus on the special and common grounds of metal atom sites. Finally, the perspectives and challenges of SACs for ammonia electrosynthesis are comprehensively discussed, aspiring to provide insights into the development of electrochemical ammonia synthesis.

Laser-Regulated CoFeRu-LDH Nanostructures: Nitrite-to-Ammonia Production in Zn-Nitrite Battery and Oxygen Evolution in Water Electrolysis

Herein, the design and synthesis of Ru-doped CoFe-layered double hydroxide (CoFeRu─LDH) nanostructures is presented via an innovative yet straightforward pulsed laser method. The CoFeRu─LDH catalyst demonstrates outstanding electrocatalytic performance, achieving a high NH4 + Faradaic efficiency (FE) of 89.65% at -0.7 V versus reversible hydrogen electrode for nitrite reduction reaction (NO2 -RR) and a low overpotential of 297 mV at 10 mA cm-2 for oxygen evolution reaction (OER). Comprehensive in situ and ex situ analyses reveal the electrochemically energetic species formed on the CoFeRu─LDH surface during the NO2 -RR and OER. Theoretical studies confirm that Ru doping plays an imperative role in tuning the electronic structure of CoFeRu─LDH, lowering its reaction barriers, and thereby remarkably enhancing its NO2 -RR and OER performance. Specifically, a galvanic Zn-nitrite battery using CoFeRu─LDH as the cathode efficiently converts NO2 - to NH4 + with an FE of 96.8% while concurrently generating electricity with a power density of 4.14 mV cm-2. Furthermore, pairing CoFeRu─LDH as the anode with Pt/C as the cathode in water electrolysis enables H2 production at a low cell voltage of 1.57 V at 10 mA cm-2. This study presents a new pathway to designing versatile, high-performance electrocatalysts for sustainable energy conversion and the production of carbon-free NH3 and H2 fuels.

Size Effect of Surface Defects Dictates Reactivity for Nitrogen Electrofixation

Electrocatalytic nitrogen reduction reaction (eNRR) offers a sustainable pathway for ammonia (NH3) production. Defect engineering enhances eNRR activity but can concurrently amplify the competing hydrogen evolution reaction (HER), posing challenges for achieving high selectivity. Herein, VOx with systematically tuned defect sizes is engineered to establish a structure-activity relationship between defect size and eNRR performance. In situ spectroscopy and theoretical calculations reveal that medium-sized defects (VOx-MD, 1-2 nm) provide an optimal electronic environment for enhanced N2 adsorption and activation while maintaining spatial flexibility to facilitate efficient hydrogenation. Consequently, VOx-MD exhibits outstanding eNRR performance, achieving an NH3 yield rate of 81.94 ± 1.45 µg h-1 mg-1 and a Faradaic efficiency of 31.97 ± 0.75 % at -0.5 V (vs RHE). These findings highlight the critical role of defect size in governing eNRR activity, offering a scalable strategy for designing advanced catalysts for competitve electrocatalytic reactions.

Harnessing Lithium-Mediated Green Ammonia Synthesis with Water Electrolysis Boosted by Membrane Electrolyzer with Polyoxometalate Proton Shuttles

Integrating water electrolysis (WE) with lithium-mediated nitrogen reduction (Li-NRR) offers a sustainable route for green ammonia production by directly utilizing protons from water oxidation, eliminating reliance on grey or blue hydrogen. Here, polyoxometalates (POMs) function as electron-coupled proton buffers (ECPBs) to seamlessly link WE with Li-NRR in a three-compartment flow reactor comprising an aqueous anode, an organic cathode, and a gas feed chamber. POMs serve as proton shuttles while suppressing the competing hydrogen evolution reaction (HER), facilitating efficient ammonia synthesis. The addition of polymethyl methacrylate (PMMA) enhances catholyte hydrophobicity, mitigating water contamination. By optimizing ECPB concentration, a dynamic balance is achieved between lithium nitride intermediates (LiNxHy) formation and consumption, yielding ammonia at 573.7 ± 5.2 µg h⁻¹ cm⁻2 with a Faradaic efficiency of 54.2%. This design advances flow reactor technology by uniquely utilizing water oxidation as a direct proton source, bypassing conventional hydrogen oxidation methods. The use of POMs as proton shuttles establishes a new benchmark for green ammonia production, reinforcing its potential in sustainable chemistry.

Fact or fiction: What is in your 15N2 and 15NH3 cylinders for sustainable ammonia and urea research?

Attaining reliable performance indicators via 15N-labeling is a daunting research endeavor for sustainable ammonia and urea synthesis processes, as currently obtained reaction rates and yields are often low. However, artifact and misinterpretation can be induced by ppm-level impurities in commercial 15N2 and 15NH3 cylinders, which are not well understood. Here, we report quantitative in-line gas chromatography-mass spectrometry (GC-MS) analysis of impurities in commercial 15N2 and 15NH3 cylinders at ppm level, in conjunction with 1H-NMR methods, exemplified by 808-16,252 ppm of 14N15N, 0-4,891 ppm 15NH3, 3-319 ppm 14NH3, 0-231 ppm of 14NO2, 0-176 ppm 15NO2, and 0-566 ppm 15N16O impurities, some of which were not reported in cylinder certification nor previous studies. The collected results have formed recommended protocols applicable to all future 15N labeling experiments intended to eliminate false positives.

Revealing Mechanisms of Lithium-Mediated Nitrogen Reduction Reaction from First-Principles Simulations

Recently, lithium-mediated nitrogen reduction reaction (Li-NRR) in nonaqueous electrolytes has proven to be an environmentally friendly and feasible route for ammonia electrosynthesis, revealing tremendous economic and social advantages over the industrial Haber-Bosch process which consumes enormous fossil fuels and generates massive carbon dioxide emissions, and direct electrocatalytic nitrogen reduction reaction (NRR) which suffers from sluggish kinetics and poor faradaic efficiencies. However, reaction mechanisms of Li-NRR and the role of solid electrolyte interface (SEI) layer in activating N2 remain unclear, impeding its further development. Here, using electronic structure theory, we discover a nitridation-coupled reduction mechanism and a nitrogen cycling reduction mechanism on lithium and lithium nitride surfaces, respectively, which are major components of SEI in experimental characterization. Our work reveals divergent pathways in Li-NRR from conventional direct electrocatalytic NRR, highlights the role of surface reconstruction in improving reactivity, and sheds light on further enhancing efficiency of ammonia electrosynthesis.

Microfluidics for Electrochemical Energy Conversion and Storage: Prospects Toward Sustainable Ammonia Production

Ammonia is a key chemical in the production of fertilizers, refrigeration and an emerging hydrogen-carrying fuel. However, the Haber-Bosch process, the industrial standard for centralized ammonia production, is energy-intensive and indirectly generates significant carbon dioxide emissions. Electrochemical nitrogen reduction offers a promising alternative for green ammonia production. Yet, current reaction rates remain well below economically feasible targets. This work examines the application of electrochemical microfluidics for the enhancement of the rates of electrochemical ammonia synthesis. The review is built on the introduction to electrochemical microfluidics, corresponding cell designs, and the main applications of microfluidics in electrochemical energy conversion/storage. Based on recent advances in electrochemical ammonia synthesis, with an emphasis on the critical role of robust experimental controls, electrochemical microfluidics represents a promising route to environmentally friendly, on-site and on-demand ammonia production. This review aims to bridge the knowledge gap between the disciplines of electrochemistry and microfluidics and promote interdisciplinary understanding and innovation in this transformative field.

Electrocatalytic CN Coupling: Advances in Urea Synthesis and Opportunities for Alternative Products

Urea is an essential fertilizer produced through the industrial synthesis of ammonia (NH3) via the Haber-Bosch process, which contributes ≈1.2% of global annual CO2 emissions. Electrocatalytic urea synthesis under ambient conditions via CN coupling from CO2 and nitrogen species such as nitrate (NO3 -), nitrite (NO2 -), nitric oxide (NO), and nitrogen gas (N2) has gained interest as a more sustainable route. However, challenges remain due to the unclear reaction pathways for urea formation, competing reactions, and the complexity of the resulting product matrix. This review highlights recent advances in catalyst design, urea quantification, and intermediate identification in the CN coupling reaction for electrocatalytic urea synthesis. Furthermore, this review explores future prospects for industrial CN coupling, considering potential nitrogen and carbon sources and examining alternative CN coupling products, such as amides and amines.

Promoting *NO2 Hydrogeneration on Cobalt-Doped Copper Oxides for Selective Ammonia Electrosynthesis from Nitrate

Electrochemical nitrate reduction reaction (NO3RR) driven by renewable energy has attracted extensive attention, which can realize nitrate degradation and ammonia production simultaneously under ambient conditions. Copper-based catalysts are the most widely used due to their high activity but are subject to low selectivity owing to intermediate nitrite accumulation. This work has reported cobalt-doped copper oxide (Co/CuO) composites, while introduced Co sites accelerate the hydrogenation of nitrogen species. The prepared Co/CuO composites realize a 93.9 % Faradaic efficiency at the applied potential of -0.4 V (vs. RHE), yielding ammonia at 0.5441 mmol⋅h-1 cm-2. The exceptional stability of Co/CuO composites has also been affirmed by following continuous recycling tests. In situ characterization confirms that the existence of Co sites is prone to conversion of *NO2 to *NH2, thus exhibiting high performance for NO3RR.

Tuning Metal-Phthalocyanine 2D Covalent Organic Frameworks for the Nitrogen Reduction Reaction: Cooperativity of Transition Metals and Organic Linkers

Two-dimensional covalent organic frameworks (2D-COFs) exhibit high activity in the electrochemical nitrogen reduction reaction (NRR), while the structure-performance relationship toward which remains an unsolved problem, therefore hindering the development of efficient catalysts. In this work, a total of 145 candidate transition metal phthalocyanine (TMPc-DX) materials comprised of 29 transition metals and X-phthalocyanine (X = N, O, F, S, and Cl) are evaluated for their capabilities in the NRR by density functional theory calculations. Our computations showed that metal atoms in TMPc-DN/DF/DCl materials could efficiently activate N2 molecules by doping with electron-withdrawing groups. The catalysts MoPc-DN and NbPc-DN are recognized to exhibit the highest electrocatalytic performance for the NRR with the lowest limit potentials of 0.31 and 0.32 V, respectively. Moreover, NbPc-DN showed a significant advantage by suppressing the hydrogen evolution reaction, making it a top contender among TMPc-DX materials for the electrocatalytic NRR. Through detailed orbital interaction analysis, our research has demonstrated that embedding transition metals into the substrate (Pc-DX) results in a direct correlation between the number of electrons transferred to the substrate and the reduction of energy splitting of the metal's d orbital, which directly causes a decrease in the metal's magnetic moment. Consequently, the affinity of the metal for N2 adsorption is directly proportional to the extent of the electron transfer. Our findings underscore the significance of the material's magnetic moment as a preliminary indicator for gauging the effectiveness of N2 adsorption.

Samarium as a Catalytic Electron-Transfer Mediator in Electrocatalytic Nitrogen Reduction to Ammonia

Samarium diiodide (SmI2) exhibits high selectivity for N2R catalyzed by molybdenum complexes; however, it has so far been employed only as a stoichiometric reagent (0.3 equiv of NH3 per Sm) combined with coordinating proton sources (e.g., H2O, ROH). The latter inhibit catalytic turnover of SmIII owing to buildup of stable hydroxide/alkoxide sinks. Here, we report a tandem Sm/Mo-catalyzed eN2R system that achieves the lowest overpotential and highest Faradaic efficiency (82%) reported to date for nonaqueous eN2R at ambient pressure. Up to 8.4 equiv of NH3 is produced per Sm, representing a 25-fold increase over N2R with stoichiometric SmI2. A noncoordinating proton source enables electrochemical SmI3/SmI2 cycling at the applied potential of -1.45 V vs Fc+/0.

Highly Accessible Electrocatalyst with In Situ Formed Copper-Cluster Active Sites for Enhanced Nitrate-to-Ammonia Conversion

Ammonia synthesis via nitrate electroreduction is more attractive and sustainable than the energy-extensive Haber-Bosch process and intrinsically sluggish nitrogen electroreduction. Herein, we have designed a single-site Cu catalyst on hierarchical nitrogen-doped carbon nanocage support (Cu1/hNCNC) for nitrate electroreduction, which achieves an ultrahigh ammonia yield rate (YRNH3) of 99.4 mol h-1 gCu-1 (2.30 mol h-1 gcat.-1) with ammonia Faradaic efficiency (FENH3) of 99.3%, far beyond the most reported single-site catalysts on carbon-based supports. The combined operando characterization and theoretical studies indicate that the in situ formed Cu-cluster active sites are responsible for the high YRNH3 and FENH3 due to the enhanced NO3- adsorption and subsequent protonation on the unique Cu3-N4 moieties, and meanwhile, the hierarchical hNCNC support facilitates the mass/charge transfer kinetics, thus promoting the high expression of intrinsic activity. The demonstration of plasma N2 oxidization and nitrate electroreduction cascade reaction manifests the great potential of the Cu1/hNCNC electrocatalyst in sustainable NH3 synthesis. These findings offer valuable insights into the design of effective catalysts for electrosynthetic reactions.

Electrochemical Ammonia Synthesis: The Energy Efficiency Challenge

We discuss the challenges associated with achieving high energy efficiency in electrochemical ammonia synthesis at near-ambient conditions. The current Li-mediated process has a theoretical maximum energy efficiency of ∼28%, since Li deposition gives rise to a very large effective overpotential. As a starting point toward finding electrocatalysts with lower effective overpotentials, we show that one reason why Li and alkaline earth metals work as N2 reduction electrocatalysts at ambient conditions is that the thermal elemental processes, N2 dissociation and NH3 desorption, are both facile at room temperature for these metals. Many transition metals, which have less negative reduction potentials and thus lower effective overpotentials, can dissociate N2 at these conditions but they all bind NH3 too strongly. Strategies to circumvent this problem are discussed, as are the other requirements for a good N2 reduction electrocatalyst.

La doped-Fe2(MoO4)3 with the synergistic effect between Fe2+/Fe3+ cycling and oxygen vacancies enhances the electrocatalytic synthesizing NH3

The electrocatalytic nitrogen reduction reaction (NRR) is a crucial process in addressing energy shortages and environmental concerns by synthesizing the NH3. However, the difficulty of N2 activation and fewer NRR active sites limit the application of NRR. Therefore, the NRR performance can be improved by rapid electron transport paths to participate in multi-electron reactions and N2 activation. Doping with transition metal element is a viable strategy to provide electrons and electronic channels in the NRR. This study focuses on the synthesis of Fe2(MoO4)3 (FeMo) and x%La-doped FeMo (x = 3, 5, 7, and 10) using the hydrothermal method. La-doping creates electron transport channels Fe2+-O2--Fe3+ and oxygen vacancies, achieving an equal molar ratio of Fe2+/Fe3+. This strategy enables the super-exchange in Fe2+-O2--Fe3+, and then enhances electron transport speed for a rapid hydrogenation reaction. Therefore, the synergistic effect of Fe2+/Fe3+ cycling and oxygen vacancies improves the NRR performance. Notably, 5%La-FeMo demonstrates the superior NRR performance (NH3 yield rate: 29.6 μg h-1 mgcat-1, Faradaic efficiency: 5.8%) at -0.8 V (vs. RHE). This work analyzes the influence of the catalyst electronic environment on the NRR performance based on the effect on different valence states of ions on electron transport.

Bi2Ti2O7 Quantum Dots for Efficient Photocatalytic Fixation of Nitrogen to Ammonia: Impacts of Shallow Energy Levels

Photocatalytic fixation of nitrogen to ammonia represents an attractive alternative to the Haber-Bosch process under ambient conditions, and the performance can be enhanced by defect engineering of the photocatalysts, in particular, formation of shallow energy levels due to oxygen vacancies that can significantly facilitate the adsorption and activation of nitrogen. This calls for deliberate size engineering of the photocatalysts. In the present study, pyrochlore Bi2Ti2O7 quantum dots and (bulk-like) nanosheets are prepared hydrothermally by using bismuth nitrate and titanium sulfate as the precursors. Despite a similar oxygen vacancy concentration, the quantum dots exhibit a drastically enhanced photocatalytic performance toward nitrogen fixation, at a rate of 332.03 µmol g-1 h-1, which is 77 times higher than that of the nanosheet counterpart. Spectroscopic and computational studies based on density functional theory calculations show that the shallow levels arising from oxygen vacancies in the Bi2Ti2O7 quantum dots, in conjunction with the moderately constrained quantum confinement effect, facilitate the chemical adsorption and activation of nitrogen.

Unraveling the Active Sites on Mesoporous CuFe2O4@N-Carbon Catalysts with Abundant Oxygen Vacancies and M-N-C Content for Boosted Nitrogen Reduction Toward Electrosynthesis of Ammonia

Transition metal centers dispersed over nitrogen-doped carbon (M-NC) supports have been widely explored for electrocatalytic reactions; however, sparsely reported for electrochemical nitrogen reduction reaction (ENRR). Particularly, the single-atom catalysts (SACs) have shown reasonable ammonia yield rate and faradaic efficiency (FE), but their complex synthesis and low durability for long-term electrocatalysis runs restrict their use on a larger scale. Importantly, the catalytic active sites in metal nanostructured-based M-NC catalysts toward enhanced N2 adsorption and activation are still not clear as they are highly challenging to reveal. A few studies have predicted that the surface oxygen vacancies (Ovac) favor an enhanced ENRR performance. Herein, a strategy using tailored M-NC content and Ovac in a single catalyst for enhanced ammonia electrosynthesis is devised. A mesoporous bimetallic spinel oxide (CuFe2O4) supported over N-doped carbon (CuFe2O4@NC) derived from Prussian blue analog (PBA) via controlled pyrolysis possess is found to show boosted ENRR activity. Moreover, operando NH3 formation over the catalyst is observed using four electrode set up. This approach enables rapid evaluation ofelectrocatalytic efficacy and avoids false positive results. The rotating disc electrode results reveal that mass transport in acidic media and surface absorption in alkline media primarily regulate ENRR over CuFe2O4@NC electrocatalyst.

Electrochemical Nitrogen Reduction: The Energetic Distance to Lithium

Energy-efficient electrochemical reduction of nitrogen to ammonia could help in mitigating climate change. Today, only Li- and recently Ca-mediated systems can perform the reaction. These materials have a large intrinsic energy loss due to the need to electroplate the metal. In this work, we present a series of calculated energetics, formation energies, and binding energies as fundamental features to calculate the energetic distance between Li and Ca and potential new electrochemical nitrogen reduction systems. The featured energetic distance increases with the standard potential. However, dimensionality reduction using principal component analysis provides an encouraging picture; Li and Ca are not exceptional in this feature space, and other materials should be able to carry out the reaction. However, it becomes more challenging the more positive the plating potential is.

Insight into Fluoride Additives to Enhance Ammonia Production from Lithium-Mediated Electrochemical Nitrogen Reduction Reaction

Demands for green ammonia production increase due to its application as a proton carrier, and recent achievements in electrochemical Li-mediated nitrogen reduction reactions (Li-NRRs) show promising reliability. Here, it is demonstrated that F-containing additives in the electrolyte improve ammonia production by modulating the solid electrolyte interphase (SEI). It is suggested that the anionic additives with low lowest unoccupied molecular orbital levels enhance efficiency by contributing to the formation of a conductive SEI incorporated with LiF. Specifically, as little as 0.3 wt.% of BF4 - additive to the electrolyte, the Faradaic efficiency (FE) for ammonia production is enhanced by over 15% compared to an additive-free electrolyte, achieving a high yield of 161 ± 3 nmol s-1 cm-2. The BF4 - additive exhibits advantages, with decreased overpotential and improved FE, compared to its use as the bulk electrolyte. The observation of the Li3N upper layer implies that active Li-NRR catalytic cycles are occurring on the outermost SEI, and density functional theory simulations propose that an SEI incorporated with LiF facilitates energy profiles for the protonation by adjusting the binding energies of the intermediates compared to bare copper. This study unlocks the potential of additives and offers insights into the SEIs for efficient Li-NRRs.

Challenges and Opportunities for Single-Atom Electrocatalysts: From Lab-Scale Research to Potential Industry-Level Applications

Single-atom electrocatalysts (SACs) are a class of promising materials for driving electrochemical energy conversion reactions due to their intrinsic advantages, including maximum metal utilization, well-defined active structures, and strong interface effects. However, SACs have not reached full commercialization for broad industrial applications. This review summarizes recent research achievements in the design of SACs for crucial electrocatalytic reactions on their active sites, coordination, and substrates, as well as the synthesis methods. The key challenges facing SACs in activity, selectivity, stability, and scalability, are highlighted. Furthermore, it is pointed out the new strategies to address these challenges including increasing intrinsic activity of metal sites, enhancing the utilization of metal sites, improving the stability, optimizing the local environment, developing new fabrication techniques, leveraging insights from theoretical studies, and expanding potential applications. Finally, the views are offered on the future direction of single-atom electrocatalysis toward commercialization.

A sustainable CVD approach for ZrN as a potential catalyst for nitrogen reduction reaction

In pursuit of developing alternatives for the highly polluting Haber-Bosch process for ammonia synthesis, the electrocatalytic nitrogen reduction reaction (NRR) using transition metal nitrides such as zirconium mononitride (ZrN) has been identified as a potential pathway for ammonia synthesis. In particular, specific facets of ZrN have been theoretically described as potentially active and selective for NRR. Major obstacles that need to be addressed include the synthesis of tailored catalyst materials that can activate the inert dinitrogen bond while suppressing hydrogen evolution reaction (HER) and not degrading during electrocatalysis. To tackle these challenges, a comprehensive understanding of the influence of the catalyst's structure, composition, and morphology on the NRR activity is required. This motivates the use of metal-organic chemical vapor deposition (MOCVD) as the material synthesis route as it enables catalyst nanoengineering by tailoring the process parameters. Herein, we report the fabrication of oriented and facetted crystalline ZrN thin films employing a single source precursor (SSP) MOCVD approach on silicon and glassy carbon (GC) substrates. First principles density functional theory (DFT) simulations elucidated the preferred decomposition pathway of SSP, whereas ab initio molecular dynamics simulations show that ZrN at room temperature undergoes surface oxidation with ambient O2, yielding a Zr-O-N film, which is consistent with compositional analysis using Rutherford backscattering spectrometry (RBS) in combination with nuclear reaction analysis (NRA) and X-ray photoelectron spectroscopy (XPS) depth profiling. Proof-of-principle electrochemical experiments demonstrated the applicability of the developed ZrN films on GC for NRR and qualitatively hint towards a possible activity for the electrochemical NRR in the sulfuric acid electrolyte.

Strategic Design of Hemi-Isoindigo Polymer for a Highly Sensitive and Selective All-Printed Flexible Nitrogen Dioxide Chemiresistive Sensor

The study has developed two hemi-isoindigo (HID)-based polymers for printed flexible resistor-type nitrogen oxide (NO2) sensors: poly[2-ethylhexyl 3-((3'",4'-bis(dodecyloxy)-3,4-dimethoxy-[2,2':5',2'"-terthiophen]-5-yl)methylene)-2-oxoindoline-1-carboxylate] (P1) and poly[2-ethylhexyl 2-oxo-3-((3,3'",4,4'-tetrakis(dodecyloxy)-[2,2':5',2'"-terthiophen]-5-yl)methylene)indoline-1-carboxylate] (P2). These polymers feature thermally removable carbamate side chains on the HID units, providing solubility and creating molecular cavities after thermal annealing. These cavities enhance NO2 diffusion, and the liberated unsubstituted amide ─C(═O)NH─ groups readily form robust double hydrogen bonds (DHB), as demonstrated by computer simulations. Furthermore, both polymers possess elevated highest occupied molecular orbital (HOMO) energy levels of -4.74 and -4.77 eV, making them highly susceptible to p-doping by NO2. Gas sensors fabricated from P1 and P2 films, anneal under optimized conditions to partially remove carbamate side chains, exhibit remarkable sensitivities of +1400% ppm-1 and +3844% ppm-1, and low detection limit (LOD) values of 514 ppb and 38.9 ppb toward NO2, respectively. These sensors also demonstrate excellent selectivity for NO2 over other gases.

Optimizing the Lattice Nitrogen Coordination to Break the Performance Limitation of Metal Nitrides for Electrocatalytic Nitrogen Reduction

Metal nitrides (MNs) are attracting enormous attention in the electrocatalytic nitrogen reduction reaction (NRR) because of their rich lattice nitrogen (Nlat) and the unique ability of Nlat vacancies to activate N2. However, continuing controversy exists on whether MNs are catalytically active for NRR or produce NH3 via the reductive decomposition of Nlat without N2 activation in the in situ electrochemical conditions, let alone the rational design of high-performance MN catalysts. Herein, we focus on the common rocksalt-type MN(100) catalysts and establish a quantitative theoretical framework based on the first-principles microkinetic simulations to resolve these puzzles. The results show that the Mars-van Krevelen mechanism is kinetically more favorable to drive the NRR on a majority of MNs, in which Nlat plays a pivotal role in achieving the Volmer process and N2 activation. In terms of stability, activity, and selectivity, we find that MN(100) with moderate formation energy of Nlat vacancy (E vac) can achieve maximum activity and maintain electrochemical stability, while low- or high-E vac ones are either unstable or catalytically less active. Unfortunately, owing to the five-coordinate structural feature of Nlat on rocksalt-type MN(100), this maximum activity is limited to a yield of NH3 of only ∼10-15 mol s-1 cm-2. Intriguingly, we identify a volcano-type activity-regulating role of the local structural features of Nlat and show that the four-coordinate Nlat can exhibit optimal activity and overcome the performance limitation, while less coordinated Nlat fails. This work provides, arguably for the first time, an in-depth theoretical insight into the activity and stability paradox of MNs for NRR and underlines the importance of reaction kinetic assessment in comparison with the prevailing simple thermodynamic analysis.

Fast and Sensitive Detection of Ammonia from Electrochemical Nitrogen Reduction Reactions by 1H NMR with Radiation Damping

A facile NMR method is reported for analysis of ammonia from the electrochemical reduction of nitrogen, which compares a calibrated colorimetric method, a calibrated 1H NMR method and two 1H NMR direct measurements using external reference materials. Unlike spectrophotometric methods, 1H NMR requires less bench time and does not require separation of ammonia from the electrolyte. A novel approach to the problem of radiation damping in NMR measurements considered the specific role of hardware tuning. Radiation damping is suppressed improving signal-to-noise ratio and detection limit (1.5 µg L-1). The method is demonstrated to be effective for the analysis of ammonia from direct electrochemical nitrogen reduction in KOH, and from lithium-mediated nitrogen reduction in a non-aqueous solution. An uncertainty budget is prepared for the measurement of ammonia.

Electrochemical Nitrate-to-Ammonia Conversion Enabled by Carbon-Decoration of Ni─GaOOH Synthesized via Plasma-Assisted CO2 Reduction

The release of nitrates into the environment leads to contaminated soil and water that poses a health risk to humans and animals. Due to the transition to renewable energy-based technologies, an electrochemical approach is an emerging option that can selectively produce valuable ammonia from nitrate sources. However, traditional metal-based electrocatalysts often suffer from low nitrate adsorption that reduces NH3 production rates. Here, a Ni-GaOOH-C/Ga electrocatalyst for electrochemical nitrate conversion into NH3 is synthesized via a low energy atmospheric-pressure plasma process that reduces CO2 into highly dispersed activated carbon on dispersed Ni─GaOOH particles produced from a liquid metal Ga─Ni alloy precursor. Nitrate conversion rates of up to 26.3 µg h-1 mg-1 cat are achieved with good stability of up to 20 h. Critically, the presence of carbon centers is central to improved performance where both Ni─C and NiO─C interfaces act as NO3- adsorption and reduction centers during the reaction. Density functional theory (DFT) calculations indicate that the NiO─C and Ni─C reaction sites reduce the Gibbs free energy required for NO3- reduction to NH3 compared to NiO and Ni. Importantly, catalysts without carbon centers do not produce NH3, emphasizing the unique effects of incorporating carbon nanoparticles into the electrocatalyst.

Engineered Electron-Deficient Sites at Boron-Doped Strontium Titanate/Electrolyte Interfaces Accelerate the Electrocatalytic Reduction of N2 to NH3: A Combined DFT and Experimental Electrocatalysis Study

The development of an efficient, selective, and durable catalysis system for the electrocatalytic N2 reduction reaction (ENRR) is a promising strategy for the sustainable production of ammonia. The high-performance ENRR is limited by two major challenges: poor adsorption of N2 over the catalyst surface and abysmal N2 solubility in aqueous electrolytes. Herein, with the help of our combined density functional theory (DFT) calculations and experimental electrocatalysis study, we demonstrate that concurrently induced electron-deficient Lewis acid sites in an electrocatalyst and in an electrolyte medium can significantly boost the ENRR performance. The DFT calculations, ex situ X-ray photoelectron and FTIR spectroscopy, electrochemical measurements, and N2-TPD (temperature-programmed desorption) over boron-doped strontium titanate (BSTO) samples reveal that the Lewis acid-base interactions of N2 synergistically enhance the adsorption and activation of N2. Besides, the B-dopant induces the defect sites (oxygen vacancies and Ti3+) that assist in enhanced N2 adsorption and results in suppressed hydrogen evolution due to B-induced electron-deficient sites for H+ adsorption. The insights from the DFT study evince that B prefers the Srtop position (on top of Sr) where N2 adsorbs in an end-on configuration, which favors the associative alternating pathway and suppresses the competitive hydrogen evolution. Thus, our combined experimental and DFT study demonstrates an insight toward enhancing the ENRR performance along with the suppressed hydrogen evolution via concurrently engineered electron-deficient sites at electrode and electrolyte interfaces.

Self-enhanced localized alkalinity at the encapsulated Cu catalyst for superb electrocatalytic nitrate/nitrite reduction to NH3 in neutral electrolyte

The electrocatalytic nitrate/nitrite reduction reaction (eNOx-RR) to ammonia (NH3) is thermodynamically more favorable than the eye-catching nitrogen (N2) electroreduction. To date, the high eNOx-RR-to-NH3 activity is limited to strong alkaline electrolytes but cannot be achieved in economic and sustainable neutral/near-neutral electrolytes. Here, we construct a copper (Cu) catalyst encapsulated inside the hydrophilic hierarchical nitrogen-doped carbon nanocages (Cu@hNCNC). During eNOx-RR, the hNCNC shell hinders the diffusion of generated OH- ions outward, thus creating a self-enhanced local high pH environment around the inside Cu nanoparticles. Consequently, the Cu@hNCNC catalyst exhibits an excellent eNOx-RR-to-NH3 activity in the neutral electrolyte, equivalent to the Cu catalyst immobilized on the outer surface of hNCNC (Cu/hNCNC) in strong alkaline electrolyte, with much better stability for the former. The optimal NH3 yield rate reaches 4.0 moles per hour per gram with a high Faradaic efficiency of 99.7%. The strong-alkalinity-free advantage facilitates the practicability of Cu@hNCNC catalyst as demonstrated in a coupled plasma-driven N2 oxidization with eNOx-RR-to-NH3.

Synergizing Fe2O3 Nanoparticles on Single Atom Fe-N-C for Nitrate Reduction to Ammonia at Industrial Current Densities

The electrochemical reduction of nitrates (NO3 -) enables a pathway for the carbon neutral synthesis of ammonia (NH3), via the nitrate reduction reaction (NO3RR), which has been demonstrated at high selectivity. However, to make NH3 synthesis cost-competitive with current technologies, high NH3 partial current densities (jNH3) must be achieved to reduce the levelized cost of NH3. Here, the high NO3RR activity of Fe-based materials is leveraged to synthesize a novel active particle-active support system with Fe2O3 nanoparticles supported on atomically dispersed Fe-N-C. The optimized 3×Fe2O3/Fe-N-C catalyst demonstrates an ultrahigh NO3RR activity, reaching a maximum jNH3 of 1.95 A cm-2 at a Faradaic efficiency (FE) for NH3 of 100% and an NH3 yield rate over 9 mmol hr-1 cm-2. Operando XANES and post-mortem XPS reveal the importance of a pre-reduction activation step, reducing the surface Fe2O3 (Fe3+) to highly active Fe0 sites, which are maintained during electrolysis. Durability studies demonstrate the robustness of both the Fe2O3 particles and Fe-Nx sites at highly cathodic potentials, maintaining a current of -1.3 A cm-2 over 24 hours. This work exhibits an effective and durable active particle-active support system enhancing the performance of the NO3RR, enabling industrially relevant current densities and near 100% selectivity.

Electrochemical Nitrate Reduction: Ammonia Synthesis and the Beyond

Natural nitrogen cycle has been severely disrupted by anthropogenic activities. The overuse of N-containing fertilizers induces the increase of nitrate level in surface and ground waters, and substantial emission of nitrogen oxides causes heavy air pollution. Nitrogen gas, as the main component of air, has been used for mass ammonia production for over a century, providing enough nutrition for agriculture to support world population increase. In the last decade, researchers have made great efforts to develop ammonia processes under ambient conditions to combat the intensive energy consumption and high carbon emission associated with the Haber-Bosch process. Among different techniques, electrochemical nitrate reduction reaction (NO3RR) can achieve nitrate removal and ammonia generation simultaneously using renewable electricity as the power, and there is an exponential growth of studies in this research direction. Here, a timely and comprehensive review on the important progresses of electrochemical NO3RR, covering the rational design of electrocatalysts, emerging CN coupling reactions, and advanced energy conversion and storage systems is provided. Moreover, future perspectives are proposed to accelerate the industrialized NH3 production and green synthesis of chemicals, leading to a sustainable nitrogen cycle via prosperous N-based electrochemistry.

Phenol as proton shuttle and buffer for lithium-mediated ammonia electrosynthesis

Ammonia is a crucial component in the production of fertilizers and various nitrogen-based compounds. Now, the lithium-mediated nitrogen reduction reaction (Li-NRR) has emerged as a promising approach for ammonia synthesis at ambient conditions. The proton shuttle plays a critical role in the proton transfer process during Li-NRR. However, the structure-activity relationship and design principles for effective proton shuttles have not yet been established in practical Li-NRR systems. Here, we propose a general procedure for verifying a true proton shuttle and established design principles for effective proton shuttles. We systematically evaluate several classes of proton shuttles in a continuous-flow reactor with hydrogen oxidation at the anode. Among the tested proton shuttles, phenol exhibits the highest Faradaic efficiency of 72 ± 3% towards ammonia, surpassing that of ethanol, which has been commonly used so far. Experimental investigations including operando isotope-labelled mass spectrometry proved the proton-shuttling capability of phenol. Further mass transport modeling sheds light on the mechanism.

Molybdenum/selenium based heterostructure catalyst for efficient hydrogen evolution: Effects of ionic dissolution and repolymerization on catalytic performance

Transition metal chalcogenides (TMCs) are recognized as highly efficient electrocatalysts and have wide applications in the field of hydrogen production by electrolysis of water, but the real catalytic substances and catalytic processes of these catalysts are not clear. The species evolution of Mo and Se during alkaline hydrogen evolution was investigated by constructing MoSe2@CoSe2 heterostructure. The real-time evolution of Mo and Se in MoSe2@CoSe2 was monitored using in situ Raman spectroscopy to determine the origin of the activity. Mo and Se in MoSe2@CoSe2 were dissolved in the form of MoO42- and SeO32-, respectively, and subsequently re-adsorbed and polymerized on the electrode surface to form new species Mo2O72- and SeO42-. Theoretical calculations show that adsorption of Mo2O72- and SeO42- can significantly enhance the HER catalytic activity of Co(OH)2. The addition of additional MoO42- and SeO32- to the electrolyte with Co(OH)2 electrodes both enhances its HER activity and promotes its durability. This study helps to deepen our insight into mechanisms involved in the structural changes of catalyst surfaces and offers a logical basis for revealing the mechanism of the influence of species evolution on catalytic performance.

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.

Ammonia Synthesis via Electrocatalytic Nitrate Reduction Using NiCoO2 Nanoarrays on a Copper Foam

Ammonia (NH3) plays a vital role in industrial and agricultural development. The electrocatalytic nitrate reduction reaction (eNO3RR) is an effective method to produce NH3 under environmental conditions but also requires considerably active and selective electrocatalysts. Herein, a copper foam was used as a conductive substrate for the electrode materials. Specifically, a Co metal-organic framework (Co-MOF) was in situ grown on the copper foam, etched, and calcined to form NiCoO2@Cu nanosheets, which were used as cathode electrodes for the eNO3RR. In 0.1 M Na2SO4 with 0.1 M NaNO3 electrolyte, NiCoO2@Cu nanosheets realized an NH3 yield of 5940.73 μg h-1 cm-2 at -0.9 V vs reversible hydrogen electrode (RHE), with a Faradaic efficiency of 94.2% at -0.7 V vs RHE. After 33 h of the catalytic reaction, the selectivity of NH3-N increased to 99.7%. The excellent electrocatalytic performance of NiCoO2@Cu nanosheets was attributed to the apparent synergistic effect between the Ni atoms and the Co atoms of bimetallic materials. This study shows that the Ni doping of NiCoO2@Cu nanosheets effectively facilitated the adsorption of NO3- on NiCoO2@Cu, and it promoted the eNO3RR.

Nonmetal Atom Doping Induced Orbital Shifts and Charge Modulation at the Edge of Two-Dimensional Boron Carbonitride Leading to Enhanced Photocatalytic Nitrogen Reduction

Electronic structure, particularly charge state analysis, plays a crucial role in comprehending catalytic mechanisms. This study focuses on metal-free boron carbonitride (BCN) nanosheets as a case study to investigate the impact of heteroatom doping on the charge state of active sites at the edge of two-dimensional (2D) metal-free nanomaterials. Our observations revealed that the doping induces a shift in the frontier py orbital near the Fermi level, accompanied by alterations in its charge state. These changes provide insights into the nitrogen adsorption descriptors and the critical hydrogenation step, ultimately leading to the proposal of a competitive charge transfer mechanism. Additionally, this exploration has led to the screening of five BCN-type structures (P@T1-C1, S@T1-B1, O@T1-B1, P@T1-B1C2, and P@T1-B1C3) with promising nitrogen reduction reaction (NRR) performances. The BCN structure (S@T1-B1) exhibited the lowest NRR overpotential reaching -0.2 V, which is associated with the proposed charge competition mechanism. Furthermore, the investigation delves into the key step hydrogenation mechanism, descriptors, and volcano diagrams of the conformational relationships. In addition, the proposed doping strategy endows the 2D-BCN with more sensitivity toward the solar spectrum, suggesting its application as a potential photocatalyst. Overall, this study establishes a strong foundation for the advancement of nonmetal-atom-doped BCN nanosheets in nitrogen reduction applications, while also providing a versatile framework for fine-tuning edge-site activity within the broader context of two-dimensional photo/electrocatalytic materials.

Lewis Acid Mediated Alteration of Electron Density on a Copper Site via pπ-dπ Mixing Enhances the Electrochemical Nitrogen Reduction Reaction

Strategic modulation of the electronic structure of the catalyst to foster the electrochemical nitrogen reduction reaction (eNRR) to the ammonia process significantly is still an area that needs to be explored. Herein, we report the incorporation of the Lewis acid into an electron-rich copper site regulating the electron density of the metal, which has been experimentally proved from the d-band center position to have a direct influence on the adsorption of N2 compared to the protons. The catalyst boron doped copper-cuprous oxide hybrid system (B-Cu/Cu2O) has shown promising Faradaic efficiency of 32% at -0.2 V vs reversible hydrogen electrode (RHE) compared to the pristine cuprous oxide (Cu2O(N)) system. The in situ Fourier transform infrared study confirms the presence of intermediates evolved during the electroreduction process. This study demonstrates the design of the active center with a specific push-pull interaction via the pπ-dπ bonding-antibonding approach and can shed light on the electrochemical activation and reduction of dinitrogen to produce ammonia.

Interstitial Boron-Modulated Porous Pd Nanotubes for Ammonia Electrosynthesis

Electrochemical conversion of nitrogen into ammonia at ambient conditions as a sustainable approach has gained significant attention, but it is still extremely challenging to simultaneously obtain a high faradaic efficiency (FE) and NH3 yield. In this work, the interstitial boron-doped porous Pd nanotubes (B-Pd PNTs) are constructed by combining the self-template reduction method with boron doping. Benefiting from distinctive one-dimensional porous nanotube architectonics and the incorporation of the interstitial B atoms, the resulting B-Pd PNTs exhibit high NH3 yield (18.36 μg h-1 mgcat.-1) and FE (21.95%) in neutral conditions, outperforming the Pd/PdO PNTs (10.4 μg h-1 mgcat.-1 and 8.47%). The present study provides an attractive method to enhance the efficiency of the electroreduction of nitrogen into ammonia by incorporating interstitial boron into porous Pd-based catalysts.

Calcium-mediated nitrogen reduction for electrochemical ammonia synthesis

Ammonia (NH3) is a key commodity chemical for the agricultural, textile and pharmaceutical industries, but its production via the Haber-Bosch process is carbon-intensive and centralized. Alternatively, an electrochemical method could enable decentralized, ambient NH3 production that can be paired with renewable energy. The first verified electrochemical method for NH3 synthesis was a process mediated by lithium (Li) in organic electrolytes. So far, however, elements other than Li remain unexplored in this process for potential benefits in efficiency, reaction rates, device design, abundance and stability. In our demonstration of a Li-free system, we found that calcium can mediate the reduction of nitrogen for NH3 synthesis. We verified the calcium-mediated process using a rigorous protocol and achieved an NH3 Faradaic efficiency of 40 ± 2% using calcium tetrakis(hexafluoroisopropyloxy)borate (Ca[B(hfip)4]2) as the electrolyte. Our results offer the possibility of using abundant materials for the electrochemical production of NH3, a critical chemical precursor and promising energy vector.

Ultra-Low Loading of Gold on Nickel Foam for Nitrogen Electrochemistry

Ammonia (NH3) is widely used in various fields, and it is also considered a promising carbon free energy carrier, due to its high hydrogen content. The nitrogen reduction reaction (NRR), which converts nitrogen into ammonia by using protons from water as the hydrogen source, is receiving a lot of attention, since effective process optimization would make it possible to overcome the Haber-Bosch method. In this study, we used a solution-based approach to obtain functionalized porous Ni foam substrates with a small amount of gold (<0.1 mg cm-1). We investigated several deposition conditions and obtained different morphologies. The electrochemical performance of various catalysts on the hydrogen evolution reaction (HER) and NRR has been characterized. The ammonia production yield was determined by chronoamperometry experiments at several potentials, and the results showed a maximum ammonia yield rate of 20 µg h-1 mgcat-1 and a Faradaic efficiency of 5.22%. This study demonstrates the potential of gold-based catalysts for sustainable ammonia production and highlights the importance of optimizing deposition conditions to improve the selectivity toward HER.

Electrocatalytic Reduction of Dinitrogen to Ammonia with Water as Proton and Electron Donor Catalyzed by a Combination of a Tri-ironoxotungstate and an Alkali Metal Cation

The electrification of ammonia synthesis is a key target for its decentralization and lowering impact on atmospheric CO2 concentrations. The lithium metal electrochemical reduction of nitrogen to ammonia using alcohols as proton/electron donors is an important advance, but requires rather negative potentials, and anhydrous conditions. Organometallic electrocatalysts using redox mediators have also been reported. Water as a proton and electron donor has not been demonstrated in these reactions. Here a N2 to NH3 electrocatalytic reduction using an inorganic molecular catalyst, a tri-iron substituted polyoxotungstate, {SiFe3W9}, is presented. The catalyst requires the presence of Li+ or Na+ cations as promoters through their binding to {SiFe3W9}. Experimental NMR, CV and UV-vis measurements, and MD simulations and DFT calculations show that the alkali metal cation enables the decrease of the redox potential of {SiFe3W9} allowing the activation of N2. Controlled potential electrolysis with highly purified 14N2 and 15N2 ruled out formation of NH3 from contaminants. Importantly, using Na+ cations and polyethylene glycol as solvent, the anodic oxidation of water can be used as a proton and electron donor for the formation of NH3. In an undivided cell electrolyzer under 1 bar N2, rates of NH3 formation of 1.15 nmol sec-1 cm-2, faradaic efficiencies of ∼25%, 5.1 equiv of NH3 per equivalent of {SiFe3W9} in 10 h, and a TOF of 64 s-1 were obtained. The future development of suitable high surface area cathodes and well solubilized N2 and the use of H2O as the reducing agent are important keys to the future deployment of an electrocatalytic ammonia synthesis.

Eliminating Concentration Polarization with Cationic Covalent Organic Polymer to Promote Effective Overpotential of Nitrogen Fixation

Electrocatalytic nitrogen reduction reaction offers a sustainable alternative to the conventional Haber-Bosch process. However, it is currently restricted by low effective overpotential due to the concentration polarization, which arises from accumulated products, ammonium, at the reaction interface. Here, a novel covalent organic polymer with ordered periodic cationic sites is proposed to tackle this challenge. The whole network exhibits strong positive charge and effectively repels the positively charged ammonium, enabling an ultra-low interfacial product concentration, and successfully driving the reaction equilibrium to the forward direction. With the given potential unchanged, the suppressed overpotential can be much liberated, ultimately leading to a continuous high-level reaction rate. As expected, when this tailored microenvironment is coupled with a transition metal-based catalyst, a 24-fold improvement is generated in the Faradaic efficiency (73.74 %) as compared with the bare one. The proposed strategy underscores the importance of optimizing dynamic processes as a means of improving overall performance in electrochemical syntheses.

Adsorbate-Induced Adatom Formation on Lithium, Iron, Cobalt, Ruthenium, and Rhenium Surfaces

Recent experimental and theoretical studies have demonstrated the reaction-driven metal-metal bond breaking in metal catalytic surfaces even under relatively mild conditions. Here, we construct a density functional theory (DFT) database for the adsorbate-induced adatom formation energy on the close-packed facets of three hexagonal close-packed metals (Co, Ru, and Re) and two body-centered cubic metals (Li and Fe), where the source of the ejected metal atom is either a step edge or a close-packed surface. For Co and Ru, we also considered their metastable face-centered cubic structures. We studied 18 different adsorbates relevant to catalytic processes and predicted noticeably easier adatom formation on Li and Fe compared to the other three metals. The NH3- and CO-induced adatom formation on Fe(110) is possible at room temperature, a result relevant to NH3 synthesis and Fischer-Tropsch synthesis, respectively. There also exist other systems with favorable adsorbate effects for adatom formation relevant to catalytic processes at elevated temperatures (500-700 K). Our results offer insight into the reaction-driven formation of metal clusters, which could play the role of active sites in reactions catalyzed by Li, Fe, Co, Ru, and Re catalysts.

Electrochemical Nitrogen Fixation for Green Ammonia: Recent Progress and Challenges

Ammonia, a key feedstock used in various industries, has been considered a sustainable fuel and energy storage option. However, NH3 production via the conventional Haber-Bosch process is costly, energy-intensive, and significantly contributing to a massive carbon footprint. An electrochemical synthetic pathway for nitrogen fixation has recently gained considerable attention as NH3 can be produced through a green process without generating harmful pollutants. This review discusses the recent progress and challenges associated with the two relevant electrochemical pathways: direct and indirect nitrogen reduction reactions. The detailed mechanisms of these reactions and highlight the recent efforts to improve the catalytic performances are discussed. Finally, various promising research strategies and remaining tasks are presented to highlight future opportunities in the electrochemical nitrogen reduction reaction.

Elucidating electrochemical nitrate and nitrite reduction over atomically-dispersed transition metal sites

Electrocatalytic reduction of waste nitrates (NO₃^-) enables the synthesis of ammonia (NH₃) in a carbon neutral and decentralized manner. Atomically dispersed metal-nitrogen-carbon (M-N-C) catalysts demonstrate a high catalytic activity and uniquely favor mono-nitrogen products. However, the reaction fundamentals remain largely underexplored. Herein, we report a set of 14; 3d-, 4d-, 5d- and f-block M-N-C catalysts. The selectivity and activity of NO₃^- reduction to NH₃ in neutral media, with a specific focus on deciphering the role of the NO₂^- intermediate in the reaction cascade, reveals strong correlations (R=0.9) between the NO₂^- reduction activity and NO₃^- reduction selectivity for NH₃. Moreover, theoretical computations reveal the associative/dissociative adsorption pathways for NO₂^- evolution, over the normal M-N₄ sites and their oxo-form (O-M-N₄) for oxyphilic metals. This work provides a platform for designing multi-element NO₃RR cascades with single-atom sites or their hybridization with extended catalytic surfaces.

Selective Electrocatalytic Oxidation of Nitrogen to Nitric Acid Using Manganese Phthalocyanine

Ammonia is produced through the energy-intensive Haber-Bosch process, which undergoes catalytic oxidation for the production of commercial nitric acid by the senescent Ostwald process. The two energy-intensive industrial processes demand for process sustainability. Hence, single-step electrocatalysis offers a promising approach toward a more environmentally friendly solution. Herein, we report a 10-electron pathway associated one-step electrochemical dinitrogen oxidation reaction (N₂OR) to nitric acid by manganese phthalocyanine (MnPc) hollow nano-structures under ambient conditions. The catalyst delivers a nitric acid yield of 513.2 μmol h^-1 g_cat^-1 with 33.9% Faradaic efficiency @ 2.1 V versus reversible hydrogen electrode. The excellent N₂OR performances are achieved due to the specific-selectivity, presence of greater number of exposed active sites, recyclability, and long period stability. The extended X-ray absorption fine structure confirms that Mn atoms are coordinated to the pyrrolic and pyridinic nitrogen via Mn-N₄ coordination. Density functional theory-based theoretical calculations confirm that the Mn-N₄ site of MnPc is the main active center for N₂OR, which suppresses the oxygen evolution reaction. This work provides a new arena about the successful example of one step nitric acid production utilizing a Mn-N₄ active site-based metal phthalocyanine electrocatalyst by dinitrogen oxidation for the development of a carbon-neutral sustainable society.

The role of ion solvation in lithium mediated nitrogen reduction

Since its verification in 2019, there have been numerous high-profile papers reporting improved efficiency of lithium-mediated electrochemical nitrogen reduction to make ammonia. However, the literature lacks any coherent investigation systematically linking bulk electrolyte properties to electrochemical performance and Solid Electrolyte Interphase (SEI) properties. In this study, we discover that the salt concentration has a remarkable effect on electrolyte stability: at concentrations of 0.6 M LiClO4 and above the electrode potential is stable for at least 12 hours at an applied current density of -2 mA cm-2 at ambient temperature and pressure. Conversely, at the lower concentrations explored in prior studies, the potential required to maintain a given N2 reduction current increased by 8 V within a period of 1 hour under the same conditions. The behaviour is linked more coordination of the salt anion and cation with increasing salt concentration in the electrolyte observed via Raman spectroscopy. Time of flight secondary ion mass spectrometry and X-ray photoelectron spectroscopy reveal a more inorganic, and therefore more stable, SEI layer is formed with increasing salt concentration. A drop in faradaic efficiency for nitrogen reduction is seen at concentrations higher than 0.6 M LiClO4, which is attributed to a combination of a decrease in nitrogen solubility and diffusivity as well as increased SEI conductivity as measured by electrochemical impedance spectroscopy.

Hydrophobic Nanoporous Silver with ZIF Encapsulation for Nitrogen Reduction Electrocatalysis

Electrochemical nitrogen reduction reaction (ENRR) offers a sustainable alternative to the environmentally hazardous Haber-Bosch process for producing ammonia. However, it suffers from an unsatisfactory performance due to its limited active sites and competitive hydrogen evolution reaction. Herein, we design a hydrophobic oleylamine-modified zeolitic imidazolate framework-coated nanoporous silver composite structure (NPS@O-ZIF). The composite achieves a high ammonia yield of (41.3 ± 0.9) μg·h^-1·cm^-2 and great Faradaic efficiency of (31.7 ± 1.2)%, overcoming the performances of NPS@ZIF and traditional silver nanoparticles@O-ZIF. Our strategy affords more active sites and accessible channels for reactant species due to the porous structure of NPS cores and restrains the evolution of hydrogen by introducing the hydrophobic molecule coated on the ZIF surfaces. Hence, the design of the hydrophobic core-shell composite catalyst provides a valuably practical strategy for ENRR as well as other water-sensitive reactions.

Robust Copper-Based Nanosponge Architecture Decorated by Ruthenium with Enhanced Electrocatalytic Performance for Ambient Nitrogen Reduction to Ammonia

Electrochemical conversion of nitrogen to green ammonia is an attractive alternative to the Haber-Bosch process. However, it is currently bottlenecked by the lack of highly efficient electrocatalysts to drive the sluggish nitrogen reduction reaction (N₂RR). Herein, we strategically design a cost-effective bimetallic Ru-Cu mixture catalyst in a nanosponge (NS) architecture via a rapid and facile method. The porous NS mixture catalysts exhibit a large electrochemical active surface area and enhanced specific activity arising from the charge redistribution for improved activation and adsorption of the activated nitrogen species. Benefiting from the synergistic effect of the Cu constituent on morphology decoration and thermodynamic suppression of the competing hydrogen evolution reaction, the optimized Ru_0.15Cu_0.85 NS catalyst presents an impressive N₂RR performance with an ammonia yield rate of 26.25 μg h^-1 mg_cat.^-1 (corresponding to 10.5 μg h^-1 cm^-2) and Faradic efficiency of 4.39% as well as superior stability in alkaline medium, which was superior to that of monometallic Ru and Cu nanostructures. Additionally, this work develops a new bimetallic combination of Ru and Cu, which promotes the strategy to design efficient electrocatalysts for electrochemical ammonia production under ambient conditions.

Defect-induced charge redistribution of MoO3-x nanometric wires for photocatalytic ammonia synthesis

Photocatalytic ammonia synthesis technology has become one of the effective methods to replace the Haber method for nitrogen fixation in the future for its low energy consumption and green environment. However, limited by the weak adsorption/activation ability of N₂ molecules at the photocatalyst interface, the efficient nitrogen fixation still remains a daunting job. Defect-induced charge redistribution as a catalytic site for N₂ molecules is the most prominent strategy to enhance the adsorption/activation of N₂ molecules at the interface of catalysts. In this study, MoO_3-x nanowires containing asymmetric defects were prepared by a one-step hydrothermal method via using glycine as a defect inducer. It is shown that at the atomic scale, the defect-induced charge reconfiguration can significantly improve the nitrogen adsorption and activation capacity and enhance the nitrogen fixation capacity; at the nanoscale, the charge redistribution induced by asymmetric defects effectively improved the photogenerated charge separation. Given the charge redistribution on the atomic and nanoscale of MoO_3-x nanowires, the optimal nitrogen fixation rate of MoO_3-x reached 200.35 µmol g^-1h^-1.