Lewis Acidic Salts Research Articles

Conversion of Abundant Aldoses with Acetylacetone: Unveiling the Mechanism and Improving Control in the Formation of Functionalized Furans Using Lewis Acidic Salts in Water

Conversion of Abundant Aldoses with Acetylacetone: Unveiling the Mechanism and Improving Control in the Formation of Functionalized Furans Using Lewis Acidic Salts in Water

Coupling CoIr Nanoalloys with MXene by Lewis Acidic Molten Salt Etching for Wide-pH-Environment Hydrogen Evolution Reaction.

Metal/MXene-based materials show broad prospects in energy conversation through the strong metal-support interaction (SMSI). However, the difficulty and harshness of synthesis heavily limit their further application. Herein, using Lewis acidic molten salt to etch MAX as a precursor of MXene, a more convenient and safer strategy is designed to in situ construct the MXene-supported CoIr nanoalloy (CoIr/MXene) catalyst through Ti─O─M bond. The special layered structure and oxygen-containing functional group of MXene regulate the SMSI upon CoIr nanoalloys. Moreover, the contact angle and in situ Raman test results exhibit good interface hydrophilicity of MXene, enhancing the water adsorption on interfaces, and accelerating the mass transfer process. As a result, CoIr/MXene shows high hydrogen evolution reaction (HER) performance, which only needs overpotentials of 34 and 50mV to drive a current density of 10mA cm-2 in alkaline and acidic media, respectively, with excellent stability. Especially, in alkaline media, CoIr/MXene possesses 6 times higher HER mass activity (4.297 A mgIr -1) than commercial Pt/C catalysts (0.686 A mgPt -1) at the potential of 50mV, indicating larger active site density and intrinsic activity for CoIr/MXene. This work expands the application of the molten salt assist etching strategy and provides new insight for the development of metal/MXene-based catalysts.

Evolution of Surface Chemistry in Two‐Dimensional MXenes: From Mixed to Tunable Uniform Terminations

AbstractSurface chemistry of MXenes is of great interest as the terminations can define the intrinsic properties of this family of materials. The diverse and tunable terminations also distinguish MXenes from many other 2D materials. Conventional fluoride‐containing reagents etching approaches resulted in MXenes with mixed fluoro‐, oxo‐, and hydroxyl surface groups. The relatively strong chemical bonding of MXenes’ surface metal atoms with oxygen and fluorine makes post‐synthetic covalent surface modifications of such MXenes unfavorable. In this minireview, we focus on the recent advances in MXenes with uniform surface terminations. Unconventional methods, including Lewis acidic molten salt etching (LAMS) and bottom‐up direct synthesis, have been proven successful in producing halide‐terminated MXenes. These synthetic strategies have opened new possibilities for MXenes because weaker surface chemical bonds in halide‐terminated MXenes facilitate post‐synthetic covalent surface modifications. Both computational and experimental results on surface termination‐dependent properties are summarized and discussed. Finally, we offer our perspective on the opportunities and challenges in this exciting research field.

Evolution of Surface Chemistry in Two-Dimensional MXenes: From Mixed to Tunable Uniform Terminations.

Surface chemistry of MXenes is of great interest as the terminations can define the intrinsic properties of this family of materials. The diverse and tunable terminations also distinguish MXenes from many other 2D materials. Conventional fluoride-containing reagents etching approaches resulted in MXenes with mixed fluoro-, oxo-, and hydroxyl surface groups. The relatively strong chemical bonding of MXenes' surface metal atoms with oxygen and fluorine makes post-synthetic covalent surface modifications of such MXenes unfavorable. In this minireview, we focus on the recent advances in MXenes with uniform surface terminations. Unconventional methods, including Lewis acidic molten salt etching (LAMS) and bottom-up direct synthesis, have been proven successful in producing halide-terminated MXenes. These synthetic strategies have opened new possibilities for MXenes because weaker surface chemical bonds in halide-terminated MXenes facilitate post-synthetic covalent surface modifications. Both computational and experimental results on surface termination-dependent properties are summarized and discussed. Finally, we offer our perspective on the opportunities and challenges in this exciting research field.

Does Sn2+ play well with Zn2+ as their molten chloride salts react with Cr–Al–B MAB phase?

Novel Zn-MAX phases like Ti2ZnC were synthesized in the literature by the reaction between Lewis acid salts like ZnCl2 and the MAX phase. The interactions between the Cr–Al–B MAB phase contained in the PLS coating formed during HDA of Fe–Cr–B cast steel and five types of molten chloride salts (ZnCl2 and SnCl2) were investigated. The results showed that the Cr-(Al, Zn)–B and Cr-(Al, Sn)–B MAB phase solid solution were formed as the PLS coating reacted with the molten ZnCl2 and SnCl2, respectively, through topochemical reaction. Zn and Sn could not coexist in the A-site Cr–Al–B MAB phase solid solution, and the replacement ability of Sn2+ contained in molten salt was much stronger than that of Zn2+. Zn and Sn whiskers could spontaneously grow on the Cr-(Al, Zn)–B and Cr-(Al, Sn)–B MAB phase solid solution respectively.

Solid-state polymer-particle hybrid electrolytes: Structure and electrochemical properties.

Solid-state electrolytes (SSEs) are challenged by complex interfacial chemistry and poor ion transport through the interfaces they form with battery electrodes. Here, we investigate a class of SSE composed of micrometer-sized lithium oxide (Li2O) particles dispersed in a polymerizable 1,3-dioxolane (DOL) liquid. Ring-opening polymerization (ROP) of the DOL by Lewis acid salts inside a battery cell produces polymer-inorganic hybrid electrolytes with gradient properties on both the particle and battery cell length scales. These electrolytes sustain stable charge-discharge behavior in Li||NCM811 and anode-free Cu||NCM811 electrochemical cells. On the particle length scale, Li2O retards ROP, facilitating efficient ion transport in a fluid-like region near the particle surface. On battery cell length scales, gravity-assisted settling creates physical and electrochemical gradients in the hybrid electrolytes. By means of electrochemical and spectroscopic analyses, we find that Li2O particles participate in a reversible redox reaction that increases the effective CE in anode-free cells to values approaching 100%, enhancing battery cycle life.

In Situ Universal Construction of Thiophosphite/MXene Hybrids via Lewis Acidic Etching for Superior Sodium Storage

AbstractMetal thiophosphite has demonstrated promising application potential as an anode material for sodium‐ion batteries. Nevertheless, the intrinsic low electrical conductivity and drastic volume expansion impede its commercialization. Herein, a series of metal thiophosphite/Ti3C2Tx (metal = Fe, Ni, Co, and Cd) composites are constructed via Lewis acidic molten salt etching followed by synchronous phospho‐sulfurization. The Ti3C2Tx substrate endows the thiophosphite/Ti3C2Tx hybrids with high electrical conductivity. Importantly, thiophosphite grown on the MXene layers exhibits a 3D cross‐linked structure, which not only facilitates electron/ion transport, but also maintains robust structural stability owing to the space confinement effect. As a proof of concept, FePS3/Ti3C2Tx demonstrates remarkable rate performance (827.4 and 598.1 mAh g−1 at 0.1 and 10 A g−1, respectively) along with long‐term cycling stability (capacity retention of 93.7% after 2000 cycles at 5 A g−1). Impressively, the FePS3/Ti3C2Tx//NVPO full cell exhibits a high reversible capacity of 396.8 mAh g−1 over 1350 cycles at 2 A g−1. The sodium storage mechanism of FePS3/Ti3C2Tx anode is further unveiled through in situ XRD/ex situ HRTEM characterizations and theoretical calculations. This work provides a fresh perspective on enhancing the electrochemical performance of thiophosphite through the in situ construction of thiophosphite/Ti3C2Tx hybrids.

Assessing Activation Quality through Evaporative Drying Patterns of Zr-MOF (UiO-66) Colloidal Droplets.

We demonstrate a simple droplet diagnostic approach to monitor the UiO-66 MOF (metal-organic framework) synthesis and its quality using the sessile droplet drying phenomenon. Drying a sessile droplet involves evaporation-driven hydrodynamic flow and particle-nature-dependent self-assembled deposition. In general, the MOF synthesis process involves different sizes and physicochemical nature of particles in every synthesis stage. Equivalent quantities of each of purified pore-activated UiO-66 MOF, yet-to-be-purified pore-inactivated UiO-66 MOF, and reaction precursors of UiO-66 MOF give different deposition patterns when a well-dispersed aqueous droplet of these materials undergoes drying over substrates of varying stiffness and wettability. Yet-to-be-purified, pore-inactivated UiO-66 MOF nanoparticles undergo transport toward the droplet periphery, leading to a thick ring-like deposition at the dried droplet edge. Under appropriate drying conditions, such a deposit leads to desiccation-type mud-like reticular cracking. We study the origin of such ring-like deposits and cracks to understand how the surface charge density of UiO-66 particles controls their stability. We demonstrate that ZrOCl2 salt trapped in a nonpurified pore-inactivated UiO-66 MOF moiety is the principal reason for ring-like deposit formation and subsequent cracking in its dried aqueous droplet edge. Qualitatively, we identified Lewis acid salts that are capable of acting as Bro̷nsted acid upon hydrolysis (like FeCl3, SnCl2, and ZrOCl2), influence surface charge density and colloidal stability of dispersed UiO-66 MOF particles. As a result, immediate particle coagulation is avoided, so those travel to the droplet edge, forming ring-like deposition and subsequent cracking upon drying. Further, we show that crack patterns on such deposits are highly dependent on the stiffness and temperature of depositing substrates via a competition between axial and lateral strains at the deposit-substrate interface.

MXenes with ordered triatomic-layer borate polyanion terminations.

Surface terminations profoundly influence the intrinsic properties of MXenes, but existing terminations are limited to monoatomic layers or simple groups, showing disordered arrangements and inferior stability. Here we present the synthesis of MXenes with triatomic-layer borate polyanion terminations (OBO terminations) through a flux-assisted eutectic molten etching approach. During the synthesis, Lewis acidic salts act as the etching agent to obtain the MXene backbone, while borax generates BO2- species, which cap the MXene surface with an O-B-O configuration. In contrast to conventional chlorine/oxygen-terminated Nb2C with localized charge transport, OBO-terminated Nb2C features band transport described by the Drude model, exhibiting a 15-fold increase in electrical conductivity and a 10-fold improvement in charge mobility at the d.c. limit. This transition is attributed to surface ordering that effectively mitigates charge carrier backscattering and trapping. Additionally, OBO terminations provide Ti3C2 MXene with substantially enriched Li+-hosting sites and thereby a large charge-storage capacity of 420 mAh g-1. Our findings illustrate the potential of intricate termination configurations in MXenes and their applications for (opto)electronics and energy storage.

Enhanced electrochemical performance of molten salt synthesized V2CTx MXene@VOx composite in Zn ion aqueous electrolytes

The development of MXenes using Lewis acidic salts for etching (MS-MXenes) in molten salts presents a novel, fluorine-free method attracting considerable interest. However, the behavior of various MS-MXenes and their derivatives in aqueous environments remains largely unexplored. Particularly, vanadium-based MXenes demonstrate intriguing properties in Zn ion aqueous electrolytes. Herein, a fluorine-free V2CTx MXene@VOx is synthesized by molten salt etching under an argon atmosphere. The electrochemical properties of V2CTx@VOx are tested in various aqueous electrolytes, including Zn(TFSI)2, ZnSO4, ZnBr2, ZnAc2, and ZnCl2 solutions. The V2CTx@VOx shows high redox capacities in all these Zn ion electrolytes, with the peak performance observed in 1 M ZnBr2, achieving a maximum capacity of 172 mAh g−1 at 1 mV s−1. This performance rivals that of V2CTx MXene prepared through wet-chemical methods. These results underscore the potential of MS-MXenes materials in aqueous energy storage applications.

Dual-Phase Structure through Selective Etching of the Double A-Element MAX Phase in Lewis Acidic Molten Salts.

Two-dimensional (2D) MXene materials with innovative properties and versatile applications have gained immense popularity among scientists. The green and environmentally friendly Lewis acid salt etching route has opened up immense possibilities for the advancement of 2D MXene materials. In this study, we precisely etched the Al element from the double A-element MAX phases Ti2(SnyAl1-y)C by employing Lewis molten salt guided by redox potentials. This approach led to the discovery of a novel Ti2SnyCClx dual-phase structure consisting of Ti2SnC and Ti2CClx. We then established that the etching of the MAX phase via Lewis acid salt is facilitated by the oxidation of M-site elements, with the MX sublayer acting as an electron transmission conduit to enable the oxidation of A-site elements. This work is dedicated to unraveling the underlying mechanisms governing the etching processes using Lewis molten salt, thereby contributing to a more profound comprehension of these innovative etching routes.

In situ catalytic polymerization of LiNO3-containing PDOL electrolytes for high-energy quasi-solid-state lithium metal batteries

1,3-Dioxolane (DOL) can be induced to form polymer electrolytes by in situ ring opening polymerization at Lewis acid salts. However, the presence of ion–dipole interactions between NO3− and DOL suppresses this polymerization behavior. Herein, we report that the use of Sc(OTf)3 as an initiator can well disrupt this ion–dipole interaction. The in situ ring-opening polymerization of DOL is achieved to form a LiNO3-modified polyDOL-based electrolyte. New mechanism of Li migration in polymer electrolytes have been revealed by molecular dynamics modeling and constructed molecular interface models. It is found that the electrolyte interior is affected by the ion–dipole interactions of Li+ by NO3−, which made it easier for Li+ to be released from the polar ether-oxygen ligands on the polymer chain for rapid migration, thus exhibiting an ionic conductivity of 1.8 mS cm−1 and tLi+ of 0.78. In addition, NO3− preempts the binding sites around Li+, improves the coordination environment, and prioritizes the formation of a kinetically stable anion-derivative interface, which effectively mitigates the interfacial side reactions between the electrodes and the electrolytes. As a result, the assembled solid-state Li||LiFePO4 cell exhibits an impressive 152.3 mAh/g discharge capacity at 0.5C and maintains 80.3 % of the capacity after 450 cycles at 50 °C. This work not only opens a new avenue for designing high-performance gel polymer electrolytes for more metal-based batteries, but also provides valuable insights into understanding the ion migration mechanism.

N/O co-doped porous carbon synthesized by lewis acid salt activation for high rate performance supercapacitor

Cellulose derivatives are widely used as carbon precursors for porous carbon materials because of their high carbon contents and low cost. Herein, N/O co-doped porous carbon materials were synthesized from sodium carboxymethyl cellulose by one-step carbonization and activation with zinc nitrate hexahydrate, a lewis acid salt, as the chemical foaming agent and activation agent and urea as the nitrogen source. The nitrogen content and oxygen content of the optimized sample reached 7.6 at.% and 12.7 at.%, respectively, and the specific surface area reached 1318 m2 g−1. The doping of nitrogen element increased the specific capacitance by nearly two times. In a three-electrode system, the maximum specific capacitance reached 234 F g−1 at 0.1 A g−1 in 6 M KOH electrolyte. The maximum energy density of symmetric supercapacitor was 15.7 Wh kg−1 at the power density of 497.3 W kg−1. The capacitance retention maintained 90.3 % after 10,000 cycles at 20 A g−1. This work provides a novel method for the preparation of porous carbon materials derived from sodium carboxymethyl cellulose and expands its application in energy storage devices.

Fluorine-Free MXenes: Recent Advances, Synthesis Strategies, and Mechanisms.

MXenes, an exceptional class of 2D materials, possess high conductivity, adaptable surface chemistry, mechanical strength, and tunable bandgaps, making them attractive for diverse applications. Unlocking the potential of MXenes requires precise control over synthesis methods and surface functionality. Conventionally, fluorine-based etchants are used in MXenes synthesis, posing both environmental concerns and alterations to surface properties, along with the introduction of certain defects. This prompts the exploration of innovative fluorine-free strategies for MXenes synthesis. This review focuses on environmentally friendly, fluorine-free techniques for MXene synthesis, emphasizing mechanisms and recent breakthroughs in alternative etching strategies. The comprehensive coverage includes electrochemical etching, Lewis acid-driven molten salt etching, alkaline/hydrothermal techniques, chemical vapor deposition (CVD), and recent innovative methods. Fluorine-free MXenes synthesis yields terminations such as ─O, ─OH, ─Cl, etc., influencing surface chemistry and improving their properties. The presence of ─OH groups in NaOH etched MXenes boosts their energy storage, while ─Cl functionality from Lewis acidic salts optimizes electrochemical performance. Fluorine-free methods mitigate adverse effects of ─F terminations on MXene conductivity, improving electronic properties and broadening their applications. In addition to traditional approaches, this review delves into novel fluorine-free methods for tailoring MXenes properties. It comprehensively addresses challenges, opportunities, and future perspectives in fluorine-free MXenes.

Mechanistic insight into homogeneous catalytic crosslinking behavior between cellulose and epoxide by explicit solvent models

Inspired by recent advances on functional modification of cellulosic materials, the crosslinking behaviors of epoxide with cellulose under the catalysis of different homogeneous catalysts including H2O, Brønsted acid, Brønsted base, Lewis acid and neutral salt were systematically investigated using density functional theory (DFT) methods with hybrid micro-solvation-continuum approach. The results showed that catalytic activity, reaction mechanism and regioselectivity are determined by the combined effect of catalyst type, electronic effect and steric hindrance. All the homogeneous catalysts have catalytic activity for the crosslinking reaction, which decreases in the order of NaOH > HCl > NCl3 > MCl2 > CH3COOH > NaCl (N = Fe3+, Al3+; M = Zn2+, Ca2+). Upon the catalysis of NaOH, hydroxyl group of cellulose is firstly deprotonated to form a carbanion-like intermediate which will further attack the less sterically hindered C atom of epoxide showing excellent regioselectivity. Acidic catalysts readily cause epoxide protonated, which suffers from nucleophilic attack of cellulose and forms the carbocation-like intermediate. Brønsted acid exhibits poor regioselectivity, however, Lewis acid shows an interesting balance between catalytic activity and regioselectivity for the crosslinking reaction, which may be attributed to the unique catalysis and stabilization effects of its coordinated H2O on the transition state structure.

Lewis metal salt synthesis of V2C (MXene) composite nickel diselenide as a high-performance cathode material for secondary aluminum batteries

Rechargeable aluminum batteries are promising energy storage devices that have advantages over lithium-ion batteries in terms of safety, cost, and capacity. Herein, a V2C (MXene) composite nickel diselenide (V2C@NiSe2) cathode is synthesized by etching V2AlC MAX phase via Lewis acidic molten salts and then calcining it with selenium. The V2C@NiSe2 cathode material exhibited reversible redox reactions of Ni2+/Nix+, and Se-/Sex+ in the charge–discharge process. Due to the fact that selenium can also replace functional groups on V2C, it can effectively avoid strong interactions between AlCl4- and V2C layers, thereby increasing their energy density. By using V2C@NiSe2 as the cathode material for aluminum batteries, the first cycle discharge specific capacity reached 486.8 mAh/g and remained at 166 mAh/g after about 3800 cycles at 1 A/g. To compare the performance of the V2C@NiSe2 cathode material, additional cathode materials (V2C@MnSe, V2C@FeSe, V2C@CuSe2, and V2C@CoSe) are created using a similar synthesis process. This work demonstrates a novel synthesis method of intercalation-type composite low-dimensional cathode materials for stable secondary aluminum batteries.

Operando Exploring and Modulating Phase Evolution Chemistry from MAX to MXenes in Molten Salt Synthesis.

Lewis acidic molten salt method is a promising synthesis strategy for achieving MXenes with controllable surface termination from numerous MAX materials. Understanding the phase evolution chemistry during etching and post-processing is highly desirable but remains a key challenge due to the lack of suitable in-situ characterizations and the complexity of the reaction process. Herein, we introduce an operando synchrotron radiation X-ray diffraction (SRXRD) technique to unveil the phase evolution process of Nb2GaC MAX under a molten-salt ambient, proposing a controllable synthesis to achieve optimal etching through precise temperature and time adjustment. Subsequently, the phase structure of Nb2CTx MXenes is successfully tailored from hexagonal to amorphous by time-dependent persulfate oxidation. The resulting amorphous Nb2CTx with a well-patterned morphology and numerous chloride terminations exhibits highly improved specific capacity, rate capability, and long cycling for Li+ storage with a Cl-containing surface protective film. Addressing the time-related phase evolution during the entire molten salt strategy provides new insights into achieving higher efficiency and controllability in preparing MXenes and shows great potential in high-performance energy storage systems based on MXenes.

Vertically Aligned Bismuthene Nanosheets on MXene for High-Performance Capacitive Deionization.

Capacitive deionization has been considered as a promising solution to the challenge of freshwater shortage due to its high efficiency, low environmental footprint, and low energy consumption. However, developing advanced electrode materials to improve capacitive deionization performance remains a challenge. Herein, the hierarchical bismuthene nanosheets (Bi-ene NSs)@MXene heterostructure was successfully prepared by combining the Lewis acidic molten salt etching and the galvanic replacement reaction, which achieves the effective utilization of the molten salt etching byproducts (residual copper). The vertically aligned bismuthene nanosheets array evenly in situ grown on the surface of MXene, which not only facilitate ion and electron transport as well as offer abundant active sites but also provide strong interfacial interaction between bismuthene and MXene. Benefiting from the above advantages, the Bi-ene NSs@MXene heterostructure as a promising capacitive deionization electrode material exhibits high desalination capacity (88.2 mg/g at 1.2 V), fast desalination rate, and good long-term cycling performance. Moreover, the mechanisms involved were elaborated by systematical characterizations and density functional theory calculations. This work provides inspirations for the preparation of MXene-based heterostructures and their application for capacitive deionization.

Multiprincipal Element M2 FeC (M = Ti,V,Nb,Ta,Zr) MAX Phases with Synergistic Effect of Dielectric and Magnetic Loss.

Electromagnetic (EM) wave pollution is harmful to human health and environment, thus it is absolutely important to develop new electromagnetic wave absorbing materials. MAX phases have been attracted more attention as a potential candidate for electromagnetic wave absorbing materials due to their high conductivity and nanolaminated structure. Herein, two new magnetic MAX phases with multiprincipal elements ((Ti1/3 Nb1/3 Ta1/3 )2 FeC and (Ti0.2 V0.2 Nb0.2 Ta0.2 Zr0.2 )2 FeC) in which Fe atoms replace Al atoms in the A sites are successfully synthesized by an isomorphous replacement reaction of multiprincipal (Ti1/3 Nb1/3 Ta1/3 )2 AlC and (Ti0.2 V0.2 Nb0.2 Ta0.2 Zr0.2 )2 AlC MAX phases with Lewis acid salt (FeCl2 ). (Ti1/3 Nb1/3 Ta1/3 )2 FeC and (Ti0.2 V0.2 Nb0.2 Ta0.2 Zr0.2 )2 FeC exhibit ferromagnetic behavior, and the Curie temperature (Tc ) are 302 and 235 K, respectively. The dual electromagnetic absorption mechanisms that include dielectric and magnetic loss, which is realized in these multiprincipal MAX phases. The minimum reflection loss (RL) of (Ti1/3 Nb1/3 Ta1/3 )2 FeC is -44.4dB at 6.56GHz with 3mm thickness, and the effective bandwidth is 2.48GHz. Additionally, the electromagnetic wave absorption properties of the magnetic MAX phases indicate that magnetic loss also plays an important role besides dielectric loss. This work shows a promising composition-design strategy to develop MAX phases with good EM wave absorption performance via simultaneously regulating dielectric and magnetic loss together.

Moiré Superlattice MXene Nanosheets Constructed from Twisted Hexagon-Ti3AlC2 by Microwave-Assisted Lewis Molten Salt Etching: Implications for Structural Stability in Electrochemical Energy Storage

In order to solve the problem of instability of MXene nanosheet structure in electrochemical energy storage processes, inspired by the moiré superlattice effect of magic angle graphene, a twisted hexagon-Ti3AlC2 precursor was synthesized by an in situ reaction method using the molten pool effect of Lewis acid molten salt. In addition, moiré superlattice MXene Ti3C2 nanosheets with rotating moiré pattern lattice structure were innovatively constructed by the substitution reaction etching of the twisted hexagon-Ti3AlC2 precursor with different ionic Lewis acid salts and microwave-assisted etching. The microstructure evolution of moiré superlattice Ti3C2 nanosheets was characterized by a simple and intuitive method. The atomic arrangement and the rotating moiré pattern were modeled. The formation mechanism and relationship between the rotation angle and rotating moiré pattern were also analyzed. This contributes to the enrichment of the moiré superlattice material family. It is expected to further promote the research progress of MXene nanomaterials in the field of electrochemical energy storage.