Is lithium the key for nitrogen electroreduction?

Reshape Iron Nanoparticles Using a Zinc Oxide Nanowire Array for High Efficiency and Stable Electrocatalytic Nitrogen Fixation

As a type of century-old catalyst, the use of iron-based materials runs through the Haber-Bosch process and electrochemical synthesis of ammonia because of its excellent capability, low cost, and abundant reserves. How to continuously improve its catalytic activity and stability for electrochemical nitrogen fixation has always been a goal pursued by scientific researchers. Herein, we develop a free-standing iron-based catalyst, i.e., the iron nanoparticles with zinc oxide nanowire array support (Fe/ZnO NA), which exhibits a high ammonia yield of ∼54.81 μg h-1 mgcat.-1 and a Faradaic efficiency (FE) of ∼9.56% in a 0.5 M potassium hydroxide solution, along with good reusability and durability. Its electrocatalytic ability is superior to that of commercial Fe materials and most reported Fe-based catalysts, thus showing great competitiveness. This is because the ZnO NA not only supplies stable support for the homogeneous dispersion of Fe nanoparticles but also provides a very beneficial synergy to their catalytic activity. The work renews traditional iron-based catalysts and is thus of great significance for promoting the industrialization of electrochemical ammonia synthesis.

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.

Cu(N2)-Li Battery for Ammonia Synthesis

The cathodic mechanism of Li-N2 batteries is similar to Li-mediated N2 reduction (LiNR). Herein, the Li-N2, LiNR, and Cu-Li battery were amalgamated to a milliliter-scale Cu(N2)-Li system. The utilization of a lithium anode with lithium oxidation reaction (LiOR), ensures an uninterrupted supply of lithium ions to active N2. LiOR not only enhances electrolyte stability but also reduces voltage by stripping Li ions, in contrast to the inert platinum anode, commonly employed in LiNR. Notably, an unusual observation of ammonia accumulation within the anode chamber elucidates the presence and role of reaction intermediates. The charging process aimed at lithium regeneration faces high polarization, and a cycling procedure involving low-current charging was proposed to improve cycling. This study integrates insights from three distinct research directions to leverage their respective advantages and scientific insights. The Li-N2 battery emerges as a highly advantageous strategy for ammonia synthesis due to the progressiveness of lithium anode.

Ambient Electrochemical Ammonia Synthesis: From Theoretical Guidance to Catalyst Design

Ammonia, a vital component in the synthesis of fertilizers, plastics, and explosives, is traditionally produced via the energy-intensive and environmentally detrimental Haber-Bosch process. Given its considerable energy consumption and significant greenhouse gas emissions, there is a growing shift toward electrocatalytic ammonia synthesis as an eco-friendly alternative. However, developing efficient electrocatalysts capable of achieving high selectivity, Faraday efficiency, and yield under ambient conditions remains a significant challenge. This review delves into the decades-long research into electrocatalytic ammonia synthesis, highlighting the evolution of fundamental principles, theoretical descriptors, and reaction mechanisms. An in-depth analysis of the nitrogen reduction reaction (NRR) and nitrate reduction reaction (NitRR) is provided, with a focus on their electrocatalysts. Additionally, the theories behind electrocatalyst design for ammonia synthesis are examined, including the Gibbs free energy approach, Sabatier principle, d-band center theory, and orbital spin states. The review culminates in a comprehensive overview of the current challenges and prospective future directions in electrocatalyst development for NRR and NitRR, paving the way for more sustainable methods of ammonia production.

Probing Degradation in Lithium Ion Batteries with On-Chip Electrochemistry Mass Spectrometry

The rapid uptake of lithium ion batteries (LIBs) for large scale electric vehicle and energy storage applications requires a deeper understanding of the degradation mechanisms. Capacity fade is due to the complex interplay between phase transitions, electrolyte decomposition and transition metal dissolution; many of these poorly understood parasitic reactions evolve gases as a side product. Here we present an on-chip electrochemistry mass spectrometry method that enables ultra-sensitive, fully quantified and time resolved detection of volatile species evolving from an operating LIB. The technique's electrochemical performance and mass transport is described by a finite element model and then experimentally used to demonstrate the variety of new insights into LIB performance. We show the versatility of the technique, including (a) observation of oxygen evolving from a LiNiMnCoO2 cathode and (b) the solid electrolyte interphase formation reaction on graphite in a variety of electrolytes, enabling the deconvolution of lithium inventory loss (c) the first direct evidence, by virtue of the improved time resolution of our technique, that carbon dioxide reduction to ethylene takes place in a lithium ion battery. The emerging insight will guide and validate battery lifetime models, as well as inform the design of longer lasting batteries.

Near-Atomic-Scale Perspective on the Oxidation of Ti3 C2 Tx MXenes: Insights from Atom Probe Tomography

MXenes are a family of 2D transition metal carbides and nitrides with remarkable properties, bearing great potential for energy storage and catalysis applications. However, their oxidation behavior is not yet fully understood, and there are still open questions regarding the spatial distribution and precise quantification of surface terminations, intercalated ions, and possible uncontrolled impurities incorporated during synthesis and processing. Here, atom probe tomography (APT) analysis of as-synthesized Ti3 C2 Tx MXenes reveals the presence of alkali (Li, Na) and halogen (Cl, F) elements as well as unetched Al. Following oxidation of the colloidal solution of MXenes, it is observed that the alkalis are enriched in TiO2 nanowires. Although these elements are tolerated through the incorporation by wet chemical synthesis, they are often overlooked when the activity of these materials is considered, particularly during catalytic testing. This work demonstrates how the capability of APT to image these elements in 3D at the near-atomic scale can help to better understand the activity and degradation of MXenes, in order to guide their synthesis for superior functional properties.

Fundamental Drivers of Electrochemical Barriers

We find that ion creation and destruction dominate the behavior of electrochemical reaction barriers, through grand-canonical electronic structure calculations of proton deposition on transition metal surfaces. We show that barriers respond to potential in a nonlinear manner and trace this to the continuous degree of electron transfer as an ion is created or destroyed. This explains both Marcus-like curvature and Hammond-like shifts. Across materials, we find the barrier energy to be driven primarily by the charge presented on the surface, which, in turn, is dictated by the native work function, a fundamentally different driving force than in nonelectrochemical systems.

Economics of Emerging Ammonia Fertilizer Production Methods - a Role for On-Farm Synthesis?

Prospects of recent promising methods of producing ammonia without fossil fuels are discussed. Despite demonstrating efficiency gains over previous similar approaches, the novel biological and electrochemical pathways require further large improvements to compete with electricity-powered Haber-Bosch. As some literature asserts that future production will shift to smaller scales, such as on-farm, we qualitatively discuss the economics of scale of future green ammonia production.

Searching for the Rules of Electrochemical Nitrogen Fixation

Li-mediated ammonia synthesis is, thus far, the only electrochemical method for heterogeneous decentralized ammonia production. The unique selectivity of the solid electrode provides an alternative to one of the largest heterogeneous thermal catalytic processes. However, it is burdened with intrinsic energy losses, operating at a Li plating potential. In this work, we survey the periodic table to understand the fundamental features that make Li stand out. Through density functional theory calculations and experimentation on chemistries analogous to lithium (e.g., Na, Mg, Ca), we find that lithium is unique in several ways. It combines a stable nitride that readily decomposes to ammonia with an ideal solid electrolyte interphase, balancing reagents at the reactive interface. We propose descriptors based on simulated formation and binding energies of key intermediates and further on hard and soft acids and bases (HSAB principle) to generalize such features. The survey will help the community toward electrochemical systems beyond Li for nitrogen fixation.

Development of a versatile electrochemical cell for in situ grazing-incidence X-ray diffraction during non-aqueous electrochemical nitrogen reduction

In situ techniques are essential to understanding the behavior of electrocatalysts under operating conditions. When employed, in situ synchrotron grazing-incidence X-ray diffraction (GI-XRD) can provide time-resolved structural information of materials formed at the electrode surface. In situ cells, however, often require epoxy resins to secure electrodes, do not enable electrolyte flow, or exhibit limited chemical compatibility, hindering the study of non-aqueous electrochemical systems. Here, a versatile electrochemical cell for air-free in situ synchrotron GI-XRD during non-aqueous Li-mediated electrochemical N2 reduction (Li-N2R) has been designed. This cell not only fulfills the stringent material requirements necessary to study this system but is also readily extendable to other electrochemical systems. Under conditions relevant to non-aqueous Li-N2R, the formation of Li metal, LiOH and Li2O as well as a peak consistent with the α-phase of Li3N was observed, thus demonstrating the functionality of this cell toward developing a mechanistic understanding of complicated electrochemical systems.

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.

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.

Li-N2 Battery for Ammonia Synthesis and Computational Insight

Electrochemical synthesis of ammonia is deemed as an alternative to the fossil-fuel-driven Haber-Bosch (HB) process, in which Li-mediated nitrogen reduction (LiNR) is the most promising scheme. Continuous lithium-mediated nitrogen reduction for ammonia synthesis (C-LiNR) has recently been reported in high-level journals with many foggy internal reactions. Synthesizing ammonia in a separate way may be profitable for understanding the mechanism of LiNR. Herein, an intermittent lithium-mediated nitrogen reduction for ammonia synthesis (I-LiNR) was proposed, three steps required for I-LiNR were achieved in the cathode chamber of a Li-N₂ battery. Discharge, stand, and charge in the Li-N₂ battery correspond to N₂ lithification, protonation, and lithium regeneration, respectively. It can also realize the quasi-continuous process with practical significance because it could be carried out through identical batteries. Products such as Li₃N, LiOH, and NH₃ are detected experimentally, which demonstrate a definite reaction pathway. The mechanism of the Li-N₂ battery, the Li-mediated synthesis of ammonia, and LiOH decomposition are explored through density functional theory calculations. The role of Li in dinitrogen activation is highlighted. It expands the range of LiOH-based Li-air batteries and may guide the study from Li-air to Li-N₂; attention has been given to the reaction mechanism of Li-mediated nitrogen reduction. The challenges and opportunities of the procedure are discussed in the end.

Water Increases the Faradaic Selectivity of Li-Mediated Nitrogen Reduction

The lithium-mediated system catalyzes nitrogen to ammonia under ambient conditions. Herein we discover that trace amount of water as an electrolyte additive-in contrast to prior reports from the literature-can effect a dramatic improvement in the Faradaic selectivity of N2 reduction to NH3. We report that an optimal water concentration of 35.9 mM and LiClO4 salt concentration of 0.8 M allows a Faradaic efficiency up to 27.9 ± 2.5% at ambient pressure. We attribute the increase in Faradaic efficiency to the incorporation of Li2O in the solid electrolyte interphase, as suggested by our X-ray photoelectron spectroscopy measurements. Our results highlight the extreme sensitivity of lithium-mediated N2 reduction to small changes in the experimental conditions.

Near ambient N2 fixation on solid electrodes versus enzymes and homogeneous catalysts

The Mo/Fe nitrogenase enzyme is unique in its ability to efficiently reduce dinitrogen to ammonia at atmospheric pressures and room temperature. Should an artificial electrolytic device achieve the same feat, it would revolutionize fertilizer production and even provide an energy-dense, truly carbon-free fuel. This Review provides a coherent comparison of recent progress made in dinitrogen fixation on solid electrodes, homogeneous catalysts and nitrogenases. Specific emphasis is placed on systems for which there is unequivocal evidence that dinitrogen reduction has taken place. By establishing the cross-cutting themes and synergies between these systems, we identify viable avenues for future research.

Emerging p-Block-Element-Based Electrocatalysts for Sustainable Nitrogen Conversion

Artificial nitrogen conversion reactions, such as the production of ammonia via dinitrogen or nitrate reduction and the synthesis of organonitrogen compounds via C-N coupling, play a pivotal role in the modern life. As alternatives to the traditional industrial processes that are energy- and carbon-emission-intensive, electrocatalytic nitrogen conversion reactions under mild conditions have attracted significant research interests. However, the electrosynthesis process still suffers from low product yield and Faradaic efficiency, which highlight the importance of developing efficient catalysts. In contrast to the transition-metal-based catalysts that have been widely studied, the p-block-element-based catalysts have recently shown promising performance because of their intriguing physiochemical properties and intrinsically poor hydrogen adsorption ability. In this Perspective, we summarize the latest breakthroughs in the development of p-block-element-based electrocatalysts toward nitrogen conversion applications, including ammonia electrosynthesis from N₂ reduction and nitrate reduction and urea electrosynthesis using nitrogen-containing feedstocks and carbon dioxide. The catalyst design strategies and the underlying reaction mechanisms are discussed. Finally, major challenges and opportunities in future research directions are also proposed.

Copper Particle-Enhanced Lithium-Mediated Synthesis of Green Ammonia from Water and Nitrogen

Ammonia (NH₃) is one of the most frequently produced chemical products in the world, and it plays an indispensable role in life on Earth. However, its synthesis by the Haber-Bosch (H-B) process is highly energy intensive, resulting in extensive carbon emissions that are unsustainable due to their ability to harm the environment. Herein, we propose a facile and mass-producible strategy for increasing the rate and efficiency of nitrogen fixation through the use of copper particle-catalyzed Li nitridation and a solid electrolyte as a medium to reduce Li salt; the above strategy results in the conversion of water and nitrogen into NH₃ through the use of renewable electrical energy at room temperature and atmospheric pressure. Copper particles are uniformly pressed into Li metal by a simple rolling method, and their critical role in accelerating the nitrogen fixation process is revealed by both electrochemical tests and simulations. The nitridation of the Li in the composite is reduced to a few minutes instead of the more than 40 h that are needed for bare Li and N₂ at room temperature and atmospheric pressure. Our new method provides three important advantages over the H-B method: (1) the new method can be operated at atmospheric pressure, thereby lowering equipment requirements and increasing security; (2) the use of water instead of fossil fuels as a hydrogen source decreases the consumption of these fuels and the emission of CO₂; and (3) the low equipment requirements lead to the ready miniaturization and decentralization of the NH₃ synthesizing process, thus promoting the possible use of renewable sources of electricity (e.g., wind or solar energy).

Solvation of Isoelectronic Halide and Alkali Metal Ions by Argon Atoms

This work systematically examines the interactions of alkali metal cations and their isoelectronic halide counterparts with up to six solvating Ar atoms (M⁺Ar_n and X^-Ar_n, where M = Li, Na, K, and Rb; X = H, F, Cl, and Br; and n = 1-6) via full geometry optimizations with the MP2 method and robust, correlation-consistent quadruple-ζ (QZ) basis sets. 116 unique M⁺Ar_n and X^-Ar_n stationary points have been characterized on the MP2/QZ potential energy surface. To the best of our knowledge, approximately two dozen of these stationary points have been reported here for the first time. Some of these new structures are either the lowest-energy stationary point for a particular cluster or energetically competitive with it. The CCSD(T) method was employed to perform additional single-point energy computations upon all MP2/QZ-optimized structures using the same basis set. CCSD(T)/QZ results indicate that internally solvated structures with the ion at/near the geometric center of the cluster have appreciably higher energies than those placing the ion on the periphery. While this study extends the prior investigations of M⁺Ar_n clusters found within the literature, it notably provides one of the first thorough characterizations of and comparisons to the corresponding negatively charged X^-Ar_n clusters.