Electroreduction of nitrogen with almost 100% current-to-ammonia efficiency.

In addition to its use in the fertilizer and chemical industries1, ammonia is currently seen as a potential replacement for carbon-based fuels and as a carrier for worldwide transportation of renewable energy2. Implementation of this vision requires transformation of the existing fossil-fuel-based technology for NH3 production3 to a simpler, scale-flexible technology, such as the electrochemical lithium-mediated nitrogen-reduction reaction3,4. This provides a genuine pathway from N2 to ammonia, but it is currently hampered by limited yield rates and low efficiencies4-12. Here we investigate the role of the electrolyte in this reaction and present a high-efficiency, robust process that is enabled by compact ionic layering in the electrode-electrolyte interface region. The interface is generated by a high-concentration imide-based lithium-salt electrolyte, providing stabilized ammonia yield rates of 150 ± 20 nmol s-1 cm-2 and a current-to-ammonia efficiency that is close to 100%. The ionic assembly formed at the electrode surface suppresses the electrolyte decomposition and supports stable N2 reduction. Our study highlights the interrelation between the performance of the lithium-mediated nitrogen-reduction reaction and the physicochemical properties of the electrode-electrolyte interface. We anticipate that these findings will guide the development of a robust, high-performance process for sustainable ammonia production.

Phase Change Materials for Renewable Energy Storage at Intermediate Temperatures.

Thermal energy storage technologies utilizing phase change materials (PCMs) that melt in the intermediate temperature range, between 100 and 220 °C, have the potential to mitigate the intermittency issues of wind and solar energy. This technology can take thermal or electrical energy from renewable sources and store it in the form of heat. This is of particular utility when the end use of the energy is also as heat. For this purpose, the material should have a phase change between 100 and 220 °C with a high latent heat of fusion. Although a range of PCMs are known for this temperature range, many of these materials are not practically viable for stability and safety reasons, a perspective not often clear in the primary literature. This review examines the recent development of thermal energy storage materials for application with renewables, the different material classes, their physicochemical properties, and the chemical structural origins of their advantageous thermal properties. Perspectives on further research directions needed to reach the goal of large scale, highly efficient, inexpensive, and reliable intermediate temperature thermal energy storage technologies are also presented.

Advanced Electrolyte Formula for Robust Operation of Vanadium Redox Flow Batteries at Elevated Temperatures.

Insufficient thermal stability of vanadium redox flow battery (VRFB) electrolytes at elevated temperatures (>40 °C) remains a challenge in the development and commercialization of this technology, which otherwise presents a broad range of technological advantages for the long-term storage of intermittent renewable energy. Herein, a new concept of combined additives is presented, which significantly increases thermal stability of the battery, enabling safe operation to the highest temperature (50 °C) tested to date. This is achieved by combining two chemically distinct additives-inorganic ammonium phosphate and polyvinylpyrrolidone (PVP) surfactant, which collectively decelerate both protonation and agglomeration of the oxo-vanadium species in solution and thereby significantly suppress detrimental formation of precipitates. Specifically, the precipitation rate is reduced by nearly 75% under static conditions at 50° C. This improvement is reflected in the robust operation of a complete VRFB device for over 300 h of continuous operation at 50 °C, achieving an impressive 83% voltage efficiency at 100 mA cm‒2 current density, with no precipitation detected in either the electrode/flow-frame or electrolyte tank.

Concluding remarks: Sustainable nitrogen activation - are we there yet?

The activation of dinitrogen as a fundamental step in reactions to produce nitrogen compounds, including ammonia and nitrates, has a cornerstone role in chemistry. Bringing together research from disparate fields where this can be achieved sustainably, this Faraday Discussion seeks to build connections between approaches that can stimulate further advances. In this paper we set out to provide an overview of these different approaches and their commonalities. We explore experimental aspects including the positive role of increasing nitrogen pressure in some fields, as well as offering perspectives on when 15N2 experiments might, and might not, be necessary. Deconstructing the nitrogen reduction reaction, we attempt to provide a common framework of energetic scales within which all of the different approaches and their components can be understood. On sustainability, we argue that although green ammonia produced from a green-H2-fed Haber-Bosch process seems to fit the bill, there remain many real-world contexts in which other, sustainable, approaches to this vital reaction are urgently needed.

Fluoroborate ionic liquids as sodium battery electrolytes.

High-voltage sodium batteries are an appealing solution for economical energy storage applications. Currently available electrolyte materials have seen limited success in such applications therefore the identification of high-performing and safer alternatives is urgently required. Herein we synthesise six novel ionic liquids derived from two fluoroborate anions which have shown great promise in recent battery literature. This study reports for the first time the electrochemically applicable room-temperature ionic liquid (RTIL) N-ethyl-N,N,N-tris(2-(2-methoxyethoxy)ethyl)ammonium (tetrakis)hexafluoroisopropoxy borate ([N2(2O2O1)3][B(hfip)4]). The RTIL shows promising physical properties with a very low glass-transition at -73 °C and low viscosity. The RTIL exhibits an electrochemical window of 5.3 V on a glassy carbon substrate which enables high stability electrochemical cycling of sodium in a 3-electrode system. Of particular note is the strong passivation behaviour of [N2(2O2O1)3][B(hfip)4] on aluminium current-collector foil at potentials as high as 7 V (vs. Na+/Na) which is further improved with the addition of 50 mol% Na[FSI]. This study shows [B(hfip)4]- ionic liquids have the desired physical and electrochemical properties for high-voltage sodium electrolytes.

Stable Acidic Water Oxidation with a Cobalt-Iron-Lead Oxide Catalyst Operating via a Cobalt-Selective Self-Healing Mechanism.

The instability and expense of anodes for water electrolyzers with acidic electrolytes can be overcome through the implementation of a cobalt-iron-lead oxide electrocatalyst, [Co-Fe-Pb]Ox , that is self-healing in the presence of dissolved metal precursors. However, the latter requirement is pernicious for the membrane and especially the cathode half-reaction since Pb2+ and Fe3+ precursors poison the state-of-the-art platinum H2 evolving catalyst. To address this, we demonstrate the invariably stable operation of [Co-Fe-Pb]Ox in acidic solutions through a cobalt-selective self-healing mechanism without the addition of Pb2+ and Fe3+ and investigate the kinetics of the process. Soft X-ray absorption spectroscopy reveals that low concentrations of Co2+ in the solution stabilize the catalytically active Co(Fe) sites. The highly promising performance of this system is showcased by steady water electrooxidation at 80±1 °C and 10 mA cm-2 , using a flat electrode, at an overpotential of 0.56±0.01 V on a one-week timescale.

Structure Effects on the Ionicity of Protic Ionic Liquids.

We report on the characterisation of 16 protic ionic liquids (PILs) prepared by neutralisation of primary or tertiary amines with a range of simple carboxylic acids, or salicylic acid. The extent of proton transfer was greater for simple primary amine ILs compared to tertiary amines. For the latter case, proton transfer was increased by providing a better solvation environment for the ions through the addition of a hydroxyl group, either on the tertiary amine, or by formation of PIL/molecular solvent mixtures. The library of PILs was characterised by differential scanning calorimetry and a range of transport properties (i. e. viscosity, conductivity and diffusivity) were measured. Using the (fractional) Walden rule, the conductivity and viscosity results were analysed with respect to their deviation from ideal behaviour. The validity of the Walden plot for PILs containing ions of varying sizes was also verified for a number of samples by directly measuring self-diffusion coefficients using pulsed-field gradient spin-echo (PGSE) NMR. Ionicity was found to decrease as the alkyl chain length and degree of branching of both the cations and anions was increased. These results aim to develop a better understanding of the relationship between PIL properties and structure, to help design ILs with optimal properties for applications.

Thin films of poly(vinylidene fluoride-co-hexafluoropropylene)-ionic liquid mixtures as amperometric gas sensing materials for oxygen and ammonia.

Gas sensors are important devices used to monitor the type and amount of gas present. Amperometric gas sensors - based on measuring the current upon an applied potential - have been progressing towards miniaturised designs that are smaller, lower cost, faster responding and more robust compared to commercially available sensors. In this work, a planar thin-film electrode device is employed for gas sensing with a thin layer of gel polymer electrolyte (GPE). The GPE consists of a room temperature ionic liquid (RTIL, with two different imidazolium cations and the tetrafluoroborate [BF4]- anion) mixed with poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP). The polymer acts as a scaffold, with the RTIL ions able to flow within the porous percolated channels, resulting in a highly robust gel with high conductivity. The chemical nature of the polymer allows thin-films (ca. 6 µm) to be evenly dropcast onto planar electrode devices, using minimal amounts of material. Remarkably, no significant effect of resistance was observed in the voltammetric response with such thin films. Oxygen (O2) and ammonia (NH3) gases were detected in the concentration ranges 1-20% O2 and 1-10 ppm NH3 in the two GPEs using both linear sweep voltammetry (LSV) and long-term chronoamperometry (LTCA). LTCA was the preferred detection method for both gases due to the steady-state current response compared to the sloping current response from LSV. The thin nature of the film gave fast response times for both gases - less than 10 seconds for O2 and ca. 40 seconds for NH3 - easily rivaling the commercially available porous electrode designs and allowing for continuous monitoring of gas concentrations. These materials appear to be highly promising candidates as gas detection electrolytes in miniaturised devices, with accurate and fast responses in both the cathodic and anodic potential regions.

Single-Boron Catalysts for Nitrogen Reduction Reaction.

Boron has been explored as p-block catalysts for nitrogen reduction reaction (NRR) by density functional theory. Unlike transition metals, on which the active centers need empty d orbitals to accept the lone-pair electrons of the nitrogen molecule, the sp3 hybrid orbital of the boron atom can form B-to-N π-back bonding. This results in the population of the N-N π* orbital and the concomitant decrease of the N-N bond order. We demonstrate that the catalytic activity of boron is highly correlated with the degree of charge transfer between the boron atom and the substrate. Among the 21 concept-catalysts, single boron atoms supported on graphene and substituted into h-MoS2 are identified as the most promising NRR catalysts, offering excellent energy efficiency and selectivity against hydrogen evolution reaction.

Hydrogen Evolution in [NiFe] Hydrogenases: A Case of Heterolytic Approach between Proton and Hydride.

The mechanism for Hydrogen Evolution Reaction (HER) in [NiFe] hydrogenase enzymes distinguishes them from inorganic catalysts. The first H+/e- pair injected to the active site of the hydrogenases transforms into hydride, while the second H+/e- pair injection leads to the formation of the H-/H+ pair both binding to the active site. The two opposite charged hydrogens heterolytically approach each other in order to form dihydrogen (H2), which is enhanced by the Coulomb force. Two previously proposed reaction routes for this process have been examined by Conceptual Density Functional Theory (DFT) in this work. One presents better agreement with experimental spectra, while the other is thermodynamically more favorable. Both paths suggest that the approach and the charge transfer between the proton and hydride are motivated by the stabilization of the electronic activity and the electrophilicity of Ni. After the heterolytic approach of the proton and hydride moieties, the two hydrogen atoms attach to the Ni ion and combine homolytically.

Self-assembled structure and dynamics of imidazolium-based protic salts in water solution.

Protic ionic liquids containing cations with long alkyl chains can form self-assembled micelles, vesicles, microemulsions, and lyotropic liquid crystal structures in water, acid water or tetrahydrofuran, etc. As a result of this unique property, they are regarded as a novel category of amphiphiles, and are gaining importance in the field of colloid and interface chemistry. The critical micelle concentration (CMC) of protic salts, e.g., alkyl-ammonium nitrates in water, was found to increase with decreasing chain length. It is generally believed that a long alkyl chain length is essential for the formation of self-assembled structures. So far, no self-assembled structure has been reported for protic ionic liquids with an alkyl chain length of n < 4. This paper reports on the structure and dynamics of two imidazolium based protic organic salts with no alkyl chain or a methyl group (n = 1) attached to the cation in water solution, determined through a detailed analysis of NMR spectra and pulsed-field gradient NMR data. We demonstrate that these imidazolium cations with no or a short alkyl chain (n = 1) can form a self-assembled clustering structure in water solution, which has a strong influence on the diffusion behavior of imidazolium molecular ions. It is speculated that this self-assembled structure is likely to be present in other similar solutions of ionic liquids with short alkyl chains.

Preparation of chiral graphene oxides by covalent attachment of chiral cysteines for voltammetric recognition of tartrates.

The authors describe the preparation of a chiral graphene oxides (GOs) by covalent attachment of D- or L-cysteine using a one-step hydrothermal method. The resulting chiral functionalized GOs shows circular dichroism with intensities similar to those produced by the cysteines. This indicates that the chirality of cysteines is well preserved in the chiral GOs. The material is reasonably stable at temperatures from 20 to 200 °C and at pH values from 0 to 14. A glassy carbon electrode (GCE) was modified with the chiral GOs and then tested for recognition capability for L- and D-tartrate (0.5 mM). The enantioselectivity of the chiral GOs appears to be the result of a synergistic effect where GO increases the conductivity and cysteine provides the chiral environment. The method is assumed to provide a useful general scheme for development of advanced carbonaceous materials with chiral recognition capabilities. Graphical abstract Chiral graphene oxides produced by covalently attaching chiral amino acids displays effective enantioselective recognition. Graphical abstract contains poor quality of text inside the artwork. Please do not re-use the file that we have rejected or attempt to increase its resolution and re-save. It is originally poor, therefore, increasing the resolution will not solve the quality problem. We suggest that you provide us the original format. We prefer replacement figures containing vector/editable objects rather than embedded images. Preferred file formats are eps, ai, tiff and pdf.We have uploaded the modified version as Graphical abstract.

A Hybrid Anion for Ionic Liquid and Battery Electrolyte Applications: Half Triflamide, Half Carbonate.

The synthesis of a new imide type anion, methylcarbonate(trifluoromethylsulfonyl)imide (MCTFSI) is described and the physicochemical properties of its sodium and N-butyl-N-methyl pyrrolidinium salts as well as structural information obtained by X-ray diffraction studies of the sodium salt are discussed in terms of charge delocalisation, coordination chemistry and electrochemical behaviour with respect to the analogous imdides bis(trifluoromethanesulfonyl)imide (TFSI) and bis(fluorosulfonyl)imide (FSI). The insight obtained from studying the new anion informs and reemphasizes the concept of weakly coordinating anions and coordination chemistry in designing electrolyte salts.

The oxygen reduction reaction on [NiFe] hydrogenases.

Oxygen tolerance capacity is critical for hydrogen oxidation/evolution catalysts. In nature, [NiFe] hydrogenases show excellent O2-tolerance and can rapidly reactivate the active site. This work aims to understand the reduction of O2 on the active site of [NiFe] hydrogenases. From the density functional theory (DFT) calculations, the free energy diagram for the oxygen reduction reaction (ORR) has been derived and the rate-determining step is found to be the Ni-B to Ni-SIb' step. Our calculation explains the slow reactivation for the Ni-A state compared to the Ni-B state, which is due to the particularly stable structure of the Ni-A state.

Hydrogen bonding effect between active site and protein environment on catalysis performance in H2-producing [NiFe] hydrogenases.

The interaction between the active site and the surrounding protein environment plays a fundamental role in the hydrogen evolution reaction (HER) in [NiFe] hydrogenases. Our density functional theory (DFT) findings demonstrate that the reaction Gibbs free energy required for the rate determining step decreases by 7.1 kcal mol-1 when the surrounding protein environment is taken into account, which is chiefly due to free energy decreases for the two H+/e- addition steps (the so-called Ni-SIa to I1, and Ni-C to Ni-R), being the largest thermodynamic impediments of the whole reaction. The variety of hydrogen bonds (H-bonds) between the amino acids and the active site is hypothesised to be the main reason for such stability: H-bonds not only work as electrostatic attractive forces that influence the charge redistribution, but more importantly, they act as an electron 'pull' taking electrons from the active site towards the amino acids. Moreover, the electron 'pull' effect through H-bonds via the S- in cysteine residues shows a larger influence on the energy profile than that via the CN- ligands on Fe.

The influence of anion chemistry on the ionic conductivity and molecular dynamics in protic organic ionic plastic crystals.

Proton conductors are widely used in different electrochemical devices including fuel cells and redox flow batteries. Compared to conventional proton conducting polymer membranes, protic organic ionic plastic crystal (POIPC) is a novel solid-state proton conductor with high proton conductivity even under anhydrous conditions. In this work, different organic protic salts based on the same parent di-functional cation with different anions were synthesized and characterized. It is found that the di-protonated cation plays an important role in defining the thermal properties, leading to stronger plastic crystal behavior and a higher melting point. Static solid-state NMR and the synchrotron XRD results show that the di-protonated cation allows greater dynamics in the crystal in contrast to the mono-protonated counterparts. The 1-(N,N-dimethylammonium)-2-(ammonium)ethane triflate ([DMEDAH2][Tf]2) has the highest ionic conductivity of 1.1 × 10-4 S cm-1 at 50 °C, whereas the bis(trifluoromethanesulfonyl)amide counterpart [DMEDAH2][TFSA]2 has the lowest ionic conductivity (2.8 × 10-7 S cm-1 at 50 °C) with no measureable mobile ion component at this temperature. The fraction of mobile species is significantly suppressed in the TFSA containing salts as against the Tf systems.

Feasibility of N2 Binding and Reduction to Ammonia on Fe-Deposited MoS2 2D Sheets: A DFT Study.

Based on the structure of the nitrogenase FeMo cofactor (FeMoco), it is reported that Fe deposited on MoS2 2D sheets exhibits high selectivity towards the spontaneous fixation of N2 against chemisorption of CO2 and H2 O. DFT predictions also indicate the ability of this material to convert N2 into NH3 with a maximum energy input of 1.02 eV as an activation barrier for the first proton-electron pair transfer.

New dimensions in salt-solvent mixtures: a 4th evolution of ionic liquids.

In the field of ionic liquids (ILs) it has long been of fundamental interest to examine the transition from salt-in-solvent behaviour to pure liquid-salt behaviour, in terms of structures and properties. At the same time, a variety of applications have beneficially employed IL-solvent mixtures as media that offer an optimal set of properties. Their properties in many cases can be other than as expected on the basis of simple mixing concepts. Instead, they can reflect the distinct structural and interaction changes that occur as the mixture passes through the various stages from pure coulombic medium, to "plasticised" coulombic medium, into a meso-region where distinct molecular and ionic domains can co-exist. Such domains can persist to quite a high dilution into the salt-in-solvent regime and their presence manifests itself in a number of important synergistic interaction effects in diverse areas such as membrane transport and corrosion protection. Similarly, the use of ionic liquids in synthetic processes where there is a significant volume fraction of molecular species present can produce a variety of distinct and unexpected effects. The range of these salt-solvent mixtures is considerably broader than just those based on ionic liquids, since there is only minor value in the pure salt being a liquid at the outset. In other words, the extensive families of organic and metal salts become candidates for study and use. Our perspective then is of an evolution of ionic liquids into a broader field of fundamental phenomena and applications. This can draw on an even larger family of tuneable salts that exhibit an exciting combination of properties when mixed with molecular liquids.