Ionic liquids intercalation in titanium carbide MXenes: A first-principles investigation.

Herein, we present a density functional theory with dispersion correction (DFT-D) calculations that focus on the intercalation of ionic liquids (ILs) electrolytes into the two-dimensional (2D) Ti3C2Tx MXenes. These ILs include the cation 1-ethyl-3-methylimidazolium (Emim+), accompanied by three distinct anions: bis(trifluoromethylsulfonyl)imide (TFSA-), (fluorosulfonyl)imide (FSA-) and fluorosulfonyl(trifluoromethanesulfonyl)imide (FTFSA-). By altering the surface termination elements, we explore the intricate geometries of IL intercalation in neutral, negative, and positive pore systems. Accurate estimation of charge transfer is achieved through five population analysis models, such as Hirshfeld, Hirshfeld-I, DDEC6 (density derived electrostatic and chemical), Bader, and VDD (voronoi deformation density) charges. In this work, we recommend the DDEC6 and Hirshfeld-I charge models, as they offer moderate values and exhibit reasonable trends. The investigation, aimed at visualizing non-covalent interactions, elucidates the role of cation-MXene and anion-MXene interactions in governing the intercalation phenomenon of ionic liquids within MXenes. The magnitude of this role depends on two factors: the specific arrangement of the cation, and the nature of the anionic species involved in the process.

Nitrogen-Doped Graphene-Like Carbon Intercalated MXene Heterostructure Electrodes for Enhanced Sodium- and Lithium-Ion Storage.

MXene is investigated as an electrode material for different energy storage systems due to layered structures and metal-like electrical conductivity. Experimental results show MXenes possess excellent cycling performance as anode materials, especially at large current densities. However, the reversible capacity is relatively low, which is a significant barrier to meeting the demands of industrial applications. This work synthesizes N-doped graphene-like carbon (NGC) intercalated Ti3C2Tx (NGC-Ti3C2Tx) van der Waals heterostructure by an in situ method. The as-prepared NGC-Ti3C2Tx van der Waals heterostructure is employed as sodium-ion and lithium-ion battery electrodes. For sodium-ion batteries, a reversible specific capacity of 305 mAh g-1 is achieved at a specific current of 20 mA g-1, 2.3 times higher than that of Ti3C2Tx. For lithium-ion batteries, a reversible capacity of 400 mAh g-1 at a specific current of 20 mA g-1 is 1.5 times higher than that of Ti3C2Tx. Both sodium-ion and lithium-ion batteries made from NGC-Ti3C2Tx shows high cycling stability. The theoretical calculations also verify the remarkable improvement in battery capacity within the NGC-Ti3C2O2 system, attributed to the additional adsorption of working ions at the edge states of NGC. This work offers an innovative way to synthesize a new van der Waals heterostructure and provides a new route to improve the electrochemical performance significantly.

Supramolecular Complexation-enhanced CO2 Chemisorption in Amine-derived Sorbents.

A supramolecular complexation approach is developed to improve the CO2 chemisorption performance of solvent-lean amine sorbents. Operando spectroscopy techniques reveal the formation of carbamic acid in the presence of a crown ether. The reaction pathway is confirmed by theoretical simulation, in which the crown ether acts as proton acceptor and shuttle to drive the formation and stabilization of carbamic acid. Improved CO2 capacity and diminished energy consumption in sorbent regeneration was achieved.

Ionic Pairs-Engineered Fluorinated Covalent Organic Frameworks Toward Direct Air Capture of CO2.

The covalent organic frameworks (COFs) possessing high crystallinity and capability to capture low-concentration CO2 (400 ppm) from air are still underdeveloped. The challenge lies in simultaneously incorporating high-density active sites for CO2 insertion and maintaining the ordered structure. Herein, a structure engineering approach is developed to afford an ionic pair-functionalized crystalline and stable fluorinated COF (F-COF) skeleton. The ordered structure of the F-COF is well maintained after the integration of abundant basic fluorinated alcoholate anions, as revealed by synchrotron X-ray scattering experiments. The breakthrough test demonstrates its attractive performance in capturing (400 ppm) CO2 from gas mixtures via O─C bond formation, as indicated by the in situ spectroscopy and operando nuclear magnetic resonance spectroscopy using 13C-labeled CO2 sources. Both theoretical and experimental thermodynamic studies reveal the reaction enthalpy of ≈-40 kJ mol-1 between CO2 and the COF scaffolds. This implies weaker interaction strength compared with state-of-the-art amine-derived sorbents, thus allowing complete CO2 release with less energy input. The structure evolution study from synchrotron X-ray scattering and small-angle neutron scattering confirms the well-maintained crystalline patterns after CO2 insertion. The as-developed proof-of-concept approach provides guidance on anchoring binding sites for direct air capture (DAC) of CO2 in crystalline scaffolds.

Insights into coordination and ligand trends of lanthanide complexes from the Cambridge Structural Database.

Understanding lanthanide coordination chemistry can help develop new ligands for more efficient separation of lanthanides for critical materials needs. The Cambridge Structural Database (CSD) contains tens of thousands of single crystal structures of lanthanide complexes that can serve as a training ground for both fundamental chemical insights and future machine learning and generative artificial intelligence models. This work aims to understand the currently available structures of lanthanide complexes in CSD by analyzing the coordination shell, donor types, and ligand types, from the perspective of rare-earth element (REE) separations. We obtain four sets of lanthanide complexes from CSD: Subset 1, all Ln-containing complexes (49472 structures); Subset 2, mononuclear Ln complexes (27858 structures); Subset 3, mononuclear Ln complexes without cyclopentadienyl ligands (Cp) (26156 structures); Subset 4, Ln complexes with at least one 1,10-phenanthroline (phen) or its derivative as a coordinating ligand (2226 structures). The subsequent analysis of lanthanide complexes in these subsets examines the trends in coordination numbers and first shell distances as well as identifies and characterizes the ligands and donor groups. In addition, examples of Ln-complexes with commercially available complexants and phen-based ligands are interrogated in detail. This systematic investigation lays the groundwork for future data-driven ligand designs for REE separations based on the structural insights into the lanthanide coordination chemistry.

Unlocking iron metal as a cathode for sustainable Li-ion batteries by an anion solid solution.

Traditional cathode chemistry of Li-ion batteries relies on the transport of Li-ions within the solid structures, with the transition metal ions and anions acting as the static components. Here, we demonstrate that a solid solution of F- and PO43- facilitates the reversible conversion of a fine mixture of iron powder, LiF, and Li3PO4 into iron salts. Notably, in its fully lithiated state, we use commercial iron metal powder in this cathode, departing from electrodes that begin with iron salts, such as FeF3. Our results show that Fe-cations and anions of F- and PO43- act as charge carriers in addition to Li-ions during the conversion from iron metal to a solid solution of iron salts. This composite electrode delivers a reversible capacity of up to 368 mAh/g and a specific energy of 940 Wh/kg. Our study underscores the potential of amorphous composites comprising lithium salts as high-energy battery electrodes.

Remote Activation of H-H Bonds by Platinum in Dilute Alloy Catalysts.

With heterogeneous catalysts, chemical promotion takes place at their surfaces. Even in the case of single-atom alloys, where small quantities of a reactive metal are dispersed within the main host, it is assumed that both elements are exposed and available to bond with the reactants. Here, we show, on the basis of in situ X-ray absorption spectroscopy data, that in alloy catalysts made from Pt highly diluted in Cu the Pt atoms are located at the inner interface between the metal nanoparticles and the silica support instead. Kinetic experiments indicated that these catalysts still display better selectivity for the hydrogenation of unsaturated aldehydes to unsaturated alcohols than the pure metals. Density functional theory calculations corroborated the stability of Pt at the metal-support interface and explained the catalytic performance as being due to a remote lowering of the activation barrier for the dissociation of H2 at Cu sites by the internal Pt atoms.

Enzyme-Inspired Ligand Engineering of Gold Nanoclusters for Electrocatalytic Microenvironment Manipulation.

Natural enzymes intricately regulate substrate accessibility through specific amino acid sequences and folded structures at their active sites. Achieving such precise control over the microenvironment has proven to be challenging in nanocatalysis, especially in the realm of ligand-stabilized metal nanoparticles. Here, we use atomically precise metal nanoclusters (NCs) as model catalysts to demonstrate an effective ligand engineering strategy to control the local concentration of CO2 on the surface of gold (Au) NCs during electrocatalytic CO2 reduction reactions (CO2RR). The precise incorporation of two 2-thiouracil-5-carboxylic acid (TCA) ligands within the pocket-like cavity of [Au25(pMBA)18]- NCs (pMBA = para-mercaptobenzoic acid) leads to a substantial acceleration in the reaction kinetics of CO2RR. This enhancement is attributed to a more favorable microenvironment in proximity to the active site for CO2, facilitated by supramolecular interactions between the nucleophilic Nδ- of the pyrimidine ring of the TCA ligand and the electrophilic Cδ+ of CO2. A comprehensive investigation employing absorption spectroscopy, mass spectrometry, isotopic labeling measurements, electrochemical analyses, and quantum chemical computation highlights the pivotal role of local CO2 enrichment in enhancing the activity and selectivity of TCA-modified Au25 NCs for CO2RR. Notably, a high Faradaic efficiency of 98.6% toward CO has been achieved. The surface engineering approach and catalytic fundamentals elucidated in this study provide a systematic foundation for the molecular-level design of metal-based electrocatalysts.

Minimal Kinetic Model of Direct Air Capture of CO2 by Supported Amine Sorbents in Dry and Humid Conditions.

Dilute concentration (∼400 ppm) and humidity are two important factors in the direct air capture (DAC) of CO2 by supported sorbents. In this work, a minimal DAC CO2 adsorption-kinetics model was formulated for supported amine sorbents under dry and humid conditions. Our model fits well with a recent DAC experiment with supported amine sorbent in both dry and humid conditions. Temperature and flow rate effects on breakthrough curves were quantitatively captured, and increasing temperature led to faster CO2 adsorption kinetics. Moisture was shown to broaden the breakthrough curve with slower CO2 adsorption kinetics but significantly improve the uptake capacity. The present minimal model provides a versatile platform for kinetic modeling of the DAC of CO2 on supported amine and other chemisorption systems.

The Influence of Ions on the Electrochemical Stability of Aqueous Electrolytes.

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

Celebrating the 150th anniversary of Vanderbilt University.

An introduction to the Nanoscale Horizons, Nanoscale and Nanoscale Advances themed collection celebrating the 150th anniversary of Vanderbilt University, featuring work from researchers currently affiliated with Vanderbilt University, esteemed alumni, and researchers with strong connections and extensive collaborations with the university.

Atomically synergistic Zn-Cr catalyst for iso-stoichiometric co-conversion of ethane and CO2 to ethylene and CO.

Developing atomically synergistic bifunctional catalysts relies on the creation of colocalized active atoms to facilitate distinct elementary steps in catalytic cycles. Herein, we show that the atomically-synergistic binuclear-site catalyst (ABC) consisting of [Formula: see text]-O-Cr6+ on zeolite SSZ-13 displays unique catalytic properties for iso-stoichiometric co-conversion of ethane and CO2. Ethylene selectivity and utilization of converted CO2 can reach 100 % and 99.0% under 500 °C at ethane conversion of 9.6%, respectively. In-situ/ex-situ spectroscopic studies and DFT calculations reveal atomic synergies between acidic Zn and redox Cr sites. [Formula: see text] ([Formula: see text]) sites facilitate ß-C-H bond cleavage in ethane and the formation of Zn-Hδ- hydride, thereby the enhanced basicity promotes CO2 adsorption/activation and prevents ethane C-C bond scission. The redox Cr site accelerates CO2 dissociation by replenishing lattice oxygen and facilitates H2O formation/desorption. This study presents the advantages of the ABC concept, paving the way for the rational design of novel advanced catalysts.

Dynamic Monkey Bar Mechanism of Superionic Li-ion Transport in LiTaCl6.

The LiTaCl6 solid electrolyte has the lowest activation energy of ionic conduction at ambient conditions (0.165 eV), with a record high ionic conductivity for a ternary compound (11 mS cm-1 ). However, the mechanism has been unclear. We train machine-learning force fields (MLFF) on ab initio molecular dynamics (AIMD) data on-the-fly and perform MLFF MD simulations of AIMD quality up to the nanosecond scale at the experimental temperatures, which allows us to predict accurate activation energy for Li-ion diffusion (at 0.164 eV). Detailed analyses of trajectories and vibrational density of states show that the large-amplitude vibrations of Cl- ions in TaCl6 - enable the fast Li-ion transport by allowing dynamic breaking and reforming of Li-Cl bonds across the space in between the TaCl6 - octahedra. We term this process the dynamic-monkey-bar mechanism of superionic Li+ transport which could aid the development of new solid electrolytes for all-solid-state lithium batteries.

Surpassing the Performance of Phenolate-derived Ionic Liquids in CO2 Chemisorption by Harnessing the Robust Nature of Pyrazolonates.

Superbase-derived ionic liquids (SILs) are promising sorbents to tackle the carbon challenge featured by tunable interaction strength with CO2 via structural engineering, particularly the oxygenate-derived counterparts (e. g., phenolate). However, for the widely deployed phenolate-derived SILs, unsolved stability issues severely limited their applications leading to unfavorable and diminished CO2 chemisorption performance caused by ylide formation-involved side reactions and the phenolate-quinone transformation via auto-oxidation. In this work, robust pyrazolonate-derived SILs possessing anti-oxidation nature were developed by introducing aza-fused rings in the oxygenate-derived anions, which delivered promising and tunable CO2 uptake capacity surpassing the phenolate-based SIL via a carbonate formation pathway (O-C bond formation), as illustrated by detailed spectroscopy studies. Further theoretical calculations and experimental comparisons demonstrated the more favorable reaction enthalpy and improved anti-oxidation properties of the pyrazolonate-derived SILs compared with phenolate anions. The achievements being made in this work provides a promising approach to achieve efficient carbon capture by combining the benefits of strong interaction strength of oxygenate species with CO2 and the stability improvement enabled by aza-fused rings introduction.

Significant Roles of Surface Hydrides in Enhancing the Performance of Cu/BaTiO2.8 H0.2 Catalyst for CO2 Hydrogenation to Methanol.

Tuning the anionic site of catalyst supports can impact reaction pathways by creating active sites on the support or influencing metal-support interactions when using supported metal nanoparticles. This study focuses on CO2 hydrogenation over supported Cu nanoparticles, revealing a 3-fold increase in methanol yield when replacing oxygen anions with hydrides in the perovskite support (Cu/BaTiO2.8 H0.2 yields ~146 mg/h/gCu vs. Cu/BaTiO3 yields ~50 mg/h/gCu). The contrast suggests that significant roles are played by the support hydrides in the reaction. Temperature programmed reaction and isotopic labelling studies indicate that BaTiO2.8 H0.2 surface hydride species follow a Mars van Krevelen mechanism in CO2 hydrogenation, promoting methanol production. High-pressure steady-state isotopic transient kinetic analysis (SSITKA) studies suggest that Cu/BaTiO2.8 H0.2 possesses both a higher density and more active and selective sites for methanol production compared to Cu/BaTiO3 . An operando high-pressure diffuse reflectance infrared spectroscopy (DRIFTS)-SSITKA study shows that formate species are the major surface intermediates over both catalysts, and the subsequent hydrogenation steps of formate are likely rate-limiting. However, the catalytic reactivity of Cu/BaTiO2.8 H0.2 towards the formate species is much higher than Cu/BaTiO3 , likely due to the altered electronic structure of interface Cu sites by the hydrides in the support as validated by density functional theory (DFT) calculations.

Hydrogen-mediated polarity compensation on the (110) surface terminations of ABO3 perovskites.

Polar surfaces undergo polarity compensation, which can lead to significantly different surface chemistry from their nonpolar counterparts. This process in turn can substantially alter the binding of adsorbates on the surface. Here, we find that hydrogen binds much more strongly to the polar (110) surface than the nonpolar (100) surface for a wide range of ABO3 perovskites, forming a hydroxyl layer on the O24- termination and a hydride layer on the ABO4+ termination of the (110) surface. The stronger adsorption on the polar surfaces can be explained by polarity compensation: hydrogen atoms can act as electron donors or acceptors to compensate for the polarity of perovskite surfaces. The relative stability of the surface terminations is further compared under different gas environments and several perovskites have been found to form stable surface hydride layers under oxygen-poor conditions. These results demonstrate the feasibility of creating stable surface hydrides on perovskites by polarity compensation which might lead to new hydrogenation catalysts based on ABO3 perovskites.

Reversible Cl/Cl- redox in a spinel Mn3O4 electrode.

A unique prospect of using halides as charge carriers is the possibility of the halides undergoing anodic redox behaviors when serving as charge carriers for the charge-neutrality compensation of electrodes. However, the anodic conversion of halides to neutral halogen species has often been irreversible at room temperature due to the emergence of diatomic halogen gaseous products. Here, we report that chloride ions can be reversibly converted to near-neutral atomic chlorine species in the Mn3O4 electrode at room temperature in a highly concentrated chloride-based aqueous electrolyte. Notably, the Zn2+ cations inserted in the first discharge and trapped in the Mn3O4 structure create an environment to stabilize the converted chlorine atoms within the structure. Characterization results suggest that the Cl/Cl- redox is responsible for the observed large capacity, as the oxidation state of Mn barely changes upon charging. Computation results corroborate that the converted chlorine species exist as polychloride monoanions, e.g., [Cl3]- and [Cl5]-, inside the Zn2+-trapped Mn3O4, and the presence of polychloride species is confirmed experimentally. Our results point to the halogen plating inside electrode lattices as a new charge-storage mechanism.

Active sites of atomically dispersed Pt supported on Gd-doped ceria with improved low temperature performance for CO oxidation.

"Single - atom" catalysts (SACs) have been the focus of intense research, due to debates about their reactivity and challenges toward determining and designing "single - atom" (SA) sites. To address the challenge, in this work, we designed Pt SACs supported on Gd-doped ceria (Pt/CGO), which showed improved activity for CO oxidation compared to its counterpart, Pt/ceria. The enhanced activity of Pt/CGO was associated with a new Pt SA site which appeared only in the Pt/CGO catalyst under CO pretreatment at elevated temperatures. Combined X-ray and optical spectroscopies revealed that, at this site, Pt was found to be d-electron rich and bridged with Gd-induced defects via an oxygen vacancy. As explained by density functional theory calculations, this site opened a new path via a dicarbonyl intermediate for CO oxidation with a greatly reduced energy barrier. These results provide guidance for rationally improving the catalytic properties of SA sites for oxidation reactions.

First principles insights into stability of defected MXenes in water.

Two-dimensional transition metal carbides and nitrides, known as MXenes, have demonstrated remarkable performance in electrochemical energy storage and various other applications. Despite their potential, MXenes exhibit instability in aqueous solutions, and the role of defects in their aqueous stability is unclear. In this study, we report on the interfacial chemistry between water and defected Ti3C2O2 MXene at room temperature using first principles molecular dynamics simulations. We investigate how defects such as O vacancy, Ti vacancy, F terminal groups, and Ti-O vacancy pair influence the chemical interaction of water molecules with the basal plane of Ti3C2O2. Our results show that water molecules can repair the surface O vacancies, by dissociating to hydroxide and hydronium. On the other hand, F terminal groups cannot effectively block water chemisorption on the surface Ti, while Ti vacancies behave as a spectator species on the surface with respect to interaction with water. Ti3C2O2 with a Ti-O vacancy pair is found to behave like Ti3C2O2 with an O vacancy where a water molecule dissociates and refills the vacancy. These findings enrich our understanding of water interaction with defects on the MXene surfaces.

Hybrid organic-inorganic two-dimensional metal carbide MXenes with amido- and imido-terminated surfaces.

Two-dimensional (2D) transition-metal carbides and nitrides (MXenes) combine the electronic and mechanical properties of 2D inorganic crystals with chemically modifiable surfaces, which provides an ideal platform for both fundamental and applied studies of interfaces. Good progress has been achieved in the functionalization of MXenes with small inorganic ligands, but relatively little work has been reported on the covalent bonding of various organic groups to MXene surfaces. Here we synthesize a family of hybrid MXenes (h-MXenes) that incorporate amido- and imido-bonding between organic and inorganic parts by reacting halogen-terminated MXenes with deprotonated organic amines. The resulting hybrid structures unite tailorability of organic molecules with electronic connectivity and other properties of inorganic 2D materials. Describing the structure of h-MXene necessitates the integration of concepts from coordination chemistry, self-assembled monolayers and surface science. The optical properties of h-MXenes reveal coherent coupling between the organic and inorganic constituents. h-MXenes also exhibit superior stability against hydrolysis.