MXene

MXenes are the emerging class of 2D materials, widely explored as supercapacitor electrodes with their explicit properties. Using the pseudocapacitive nature of the widely reported MXene-Ti 3 C 2 T x, an asymmetric cell is designed with Ti 3 C 2 T x as the negative electrode and Vanadium nitride/Porous carbon as the positive electrode. The asymmetric cell provides a high cell voltage of 1.8 V with a specific capacitance of 105 F/g at 1 A/g in 6 M KOH. It also presents a capacitance retention of 73% even after 10,000 charge-discharge cycles. The asymmetric cell exhibits a high potential window, which is about 3 times higher when compared to symmetric supercapacitors. The asymmetric electrode system exhibits an energy density of 12.81 Wh/kg and a corresponding power density of 985.8 W/kg at 1 A/g. This value of energy density is greater when compared to various symmetric supercapacitors. Thus Ti 3 C 2 T x MXene electrodes can effectively replace the conventional carbon electrodes used...

Hydrogen evolution reaction (HER) via electrocatalysis is one method of enabling sustainable production of molecular hydrogen as a clean and promising energy carrier. Previous theoretical and experimental results have shown that some two-dimensional (2D) transition metal carbides (MXenes) can be effective electrocatalysts for the HER, based on the assumption that they are functionalized entirely with oxygen or hydroxyl groups on the basal plane. However, it is known that MXenes can contain other basal plane functionalities, e.g., fluorine, due to the synthesis process, yet the influence of fluorine termination on their HER activity remains unexplored. In this paper, we investigate the role and effect of basal plane functionalization (T x) on the HER activity of 5 different MXenes using a combination of experimental and theoretical approaches. We first studied Ti 3 C 2 T x produced by different fluorine-containing etchants and found that those with higher fluorine coverage on the bas...

The recent development of nanoscale fillers, such as carbon nanotube, graphene, and nanocellulose, allows the functionality of polymer nanocomposites to be controlled and enhanced. However, conventional synthesis methods of polymer nanocomposites cannot maximise the reinforcement of these nanofillers at high filler content. Approaches to synthesis of high content filler polymer nanocomposites are suggested to facilitate future applications. The fabrication methods address design of the polymer nanocomposite architecture, which encompass one, two, and three dimensional morphology. Factors that hamper the reinforcement of nanostructures, such as alignment, dispersion of filler as well as interfacial bonding between filler and polymer are outlined. Using suitable approaches, maximum potential reinforcement of nanoscale filler can be anticipated without limitations in orientation, dispersion, and the integrity of the filler particle-matrix interface. The high filler content polymer composite containing emerging materials such as 2D transition metal carbides, nitrides, and carbonitrides (MXenes) are expected in the future.

Transition metal carbides and nitrides (MXenes), a family of two-dimensional (2D) inorganic compounds, are materials composed of a few atomic layers of transition metal carbides, nitrides, or carbonitrides. Ti3C2, the first 2D layered MXene, was isolated in 2011. This material, which is a layered bulk material analogous to graphite, was derived from its 3D phase, Ti3AlC2 MAX. Since then, material scientists have either determined or predicted the stable phases of >200 different MXenes based on combinations of various transition metals such as Ti, Mo, V, Cr, and their alloys with C and N. Extensive experimental and theoretical studies have shown their exciting potential for energy conversion and electrochemical storage. To this end, we comprehensively summarize the current advances in MXene research. We begin by reviewing the structure types and morphologies and their fabrication routes. The review then discusses the mechanical, electrical, optical, and electrochemical properties of MXenes. The focus then turns to their exciting potential in energy storage and conversion. Energy storage applications include electrodes in rechargeable lithium- and sodium-ion batteries, lithium-sulfur batteries, and supercapacitors. In terms of energy conversion, photocatalytic fuel production, such as hydrogen evolution from water splitting, and carbon dioxide reduction are presented. The potential of MXenes for the photocatalytic degradation of organic pollutants in water, such as dye waste, is also addressed, along with their promise as catalysts for ammonium synthesis from nitrogen. Finally, their application potential is summarized.

Growing interest in the potential applications of two-dimensional (2D) materials has fueled advancement in the identification of 2D systems with exotic properties. Increasingly, the bottleneck in this field is the synthesis of these materials. Although theoretical calculations have predicted a myriad of promising 2D materials, only a few dozen have been experimentally realized since the initial discovery of graphene. Here, we adapt the state-of-the-art positive and unlabeled (PU) machine learning framework to predict which theoretically proposed 2D materials have the highest likelihood of being successfully synthesized. Using elemental information and data from high-throughput density functional theory calculations, we apply the PU learning method to the MXene family of 2D transition metal carbides, carbonitrides, and nitrides, and their layered precursor MAX phases, and identify 18 MXene compounds that are highly promising candidates for synthesis. By considering both the MXenes and their precursors, we further propose 20 synthesizable MAX phases that can be chemically exfoliated to produce MXenes. T he explosion of progress in high-throughput computational screening of materials has led to accelerated identification of many promising candidates for energy storage, 1 electrocatalysis, 2 photovoltaic absorbers, 3 and a staggering variety of other applications. 4 There is a growing availability of experimental and theoretical data on 3D crystals in popular databases, 5,6 and recent efforts have dramatically increased the collection of proposed 2D materials and their predicted properties. 7−10 While high-throughput density functional theory (DFT) studies are a powerful method for targeted materials design and characterization without requiring costly and time-consuming synthesis, 4,11−13 an imposing challenge remains: using computation to accelerate the synthesis of materials. Machine learning (ML) has emerged as a promising way forward in this respect, as shown in studies on predicting thermodynamic stability of arbitrary compositions 14 and reaction successes in inorganic−organic hybrid material synthesis, 15 identifying trends in synthesis conditions for metal oxides, 16 and searching for high-temperature ferroelectric perovskites. 17 However, ML has not yet been exploited to provide insights into the synthesis of 2D materials. To make actionable predictions about the synthesis of possible 2D materials, we first identify a family of 2D materials that is well-suited for analysis via ML, namely, a family comprising a large chemical search space, with examples of successful synthesis. The 2D transition metal carbides, carbonitrides, and nitrides (MXenes) 18,19 with the general formula M n+1 X n T x (n = 1−3) and their parent MAX phases (layers of MXenes interleaved with A-element atoms) 20,21 are an ideal choice to satisfy these constraints. The large variety of chemical compositions, number of layers, pure (single

Hydrogen evolution reaction (HER) via electrocatalysis is one method of enabling sustainable production of molecular hydrogen as a clean and promising energy carrier. Previous theoretical and experimental results have shown that some two-dimensional (2D) transition metal carbides (MXenes) can be effective electrocatalysts for the HER, based on the assumption that they are functionalized entirely with oxygen or hydroxyl groups on the basal plane. However, it is known that MXenes can contain other basal plane functionalities, e.g., fluorine, due to the synthesis process, yet the influence of fluorine termination on their HER activity remains unexplored. In this paper, we investigate the role and effect of basal plane functionalization (T x) on the HER activity of 5 different MXenes using a combination of experimental and theoretical approaches. We first studied Ti 3 C 2 T x produced by different fluorine-containing etchants and found that those with higher fluorine coverage on the basal plane exhibited lower HER activity. We then controllably prepared Mo 2 CT x with very low basal plane fluorine coverage, achieving a geometric current density of −10 mA cm −2 at 189 mV overpotential in acid. More importantly, our results indicate that the oxygen groups on the basal planes of Mo 2 CT x are catalytically active toward the HER, unlike in the case of widely studied 2H-phase transition metal dichalcogenides such as MoS 2 , in which only the edge sites are active. These results pave the way for the rational design of 2D materials for either the HER, when minimal overpotential is desired, or for energy storage, when maximum voltage window is needed.

The chemical bonding in the carbide core and the surface chemistry in a new group of transition-metal carbides Ti n+1 C n-T x (n = 1,2) called MXenes have been investigated by surface-sensitive valence band X-ray photoelectron spectroscopy. Changes in band structures of stacked nano sheets of different thicknesses are analyzed in connection to known hybridization regions of TiC and TiO 2 that affect elastic and transport properties. By employing high excitation energy, the photoelectron cross-section for the C 2s – Ti 3d hybridization region at the bottom of the valence band is enhanced. As shown in this work, the O 2p and F 2p bands strongly depend both on the bond lengths to the surface groups and the adsorption sites. The effect of surface oxidation and Ar + sputtering on the electronic structure is also discussed.

Transition-metal carbides and nitrides (MXenes) have attracted significant interest owing to their desirable properties, abundance, and high electrocatalytic activity. Tremendous studies have demonstrated the potential of MXenes for energy conversion and storage. However, further development of this potential must address various aspects of MXenes, including finite modification strategies, undefined reaction mechanisms, working life cycle, and additional untapped applications. In this Focus Review, we summarize the most recent progress in applications of MXenes and general guidelines for the rational design of MXene structures. The coupling of mixed-dimensional nanomaterials and the introduction of single atoms are discussed in the production of hydrogen, ammonia, and multi-carbon compounds, as well as energy storage applications. In addition, in situ characterization tools are briefly introduced as a potential means of shedding light on MXenes’ chemical activity. Finally, an outlook on future opportunities is provided to stimulate discussion and further research into MXene material synthesis and mechanisms.

MXenes are the emerging class of 2D materials, widely explored as supercapacitor electrodes with their explicit properties. Using the pseudocapacitive nature of the widely reported MXene-Ti 3 C 2 T x, an asymmetric cell is designed with Ti 3 C 2 T x as the negative electrode and Vanadium nitride/Porous carbon as the positive electrode. The asymmetric cell provides a high cell voltage of 1.8 V with a specific capacitance of 105 F/g at 1 A/g in 6 M KOH. It also presents a capacitance retention of 73% even after 10,000 charge-discharge cycles. The asymmetric cell exhibits a high potential window, which is about 3 times higher when compared to symmetric supercapacitors. The asymmetric electrode system exhibits an energy density of 12.81 Wh/kg and a corresponding power density of 985.8 W/kg at 1 A/g. This value of energy density is greater when compared to various symmetric supercapacitors. Thus Ti 3 C 2 T x MXene electrodes can effectively replace the conventional carbon electrodes used in asymmetric supercapacitors.

Two-dimensional (2D) nanomaterials are an emerging platform with unique structural and physiochemical properties attracted intense interest in developing high-performance gas and liquid separation membranes. This review provides an overview on the latest breakthrough studies in the fabrication of 2D nanomaterials membranes including atomically thin membranes, laminar and mixed matrix membranes. Especially, we focus on structural features, permeation mechanism as well as gas and liquid separation performance of 2D nanomaterials membranes. Additionally, we highlighted the unique role of 2D nanomaterials in mitigating plasticization and physical aging phenomena in polymer-based membranes. Finally, we underline current challenges and future prospects of this intriguing materials platform for developing next generation of membranes.

Structural design on the atomic level can provide novel chemistries of hybrid MAX phases and their MXenes. Herein, density functional theory is used to predict phase stability of quaternary i-MAX phases with in-plane chemical order and a general chemistry (W M ) AC, where M = Sc, Y (W), and A = Al, Si, Ga, Ge, In, and Sn. Of over 18 compositions probed, only two-with a monoclinic C2/c structure-are predicted to be stable: (W Sc ) AlC and (W Y ) AlC and indeed found to exist. Selectively etching the Al and Sc/Y atoms from these 3D laminates results in W C-based MXene sheets with ordered metal divacancies. Using electrochemical experiments, this MXene is shown to be a new, promising catalyst for the hydrogen evolution reaction. The addition of yet one more element, W, to the stable of M elements known to form MAX phases, and the synthesis of a pure W-based MXene establishes that the etching of i-MAX phases is a fruitful path for creating new MXene chemistries that has hitherto been no...

MXenes (Titanium Carbide, Ti 3 C 2-MXene) are two-dimensional nanomaterials that are known for their conductivity, film-forming ability, and elasticity. Though literature reports the possibility of usage of Ti 3 C 2-MXenes for sensor development, the material properties and response need be studied in detail for designing sensors to measure dynamic variables like force, displacement, etc., in a dynamic environment. Ti 3 C 2-MXenes due to their good electro-mechanical properties can be used for manufacturing sensing elements for engineering and biomedical applications. This paper focuses on an investigation of the dynamic response properties of Ti 3 C 2-MXenes subjected to shockwave and impact forces. A supersonic shockwave (Mach number: 1.68, peak overpressure: 234.3 kPa) produced in a shock tube acts as an external force on the Ti 3 C 2-MXene film placed inside the shock tube. In the experiment performed, the response time of the Ti 3 C 2-MXene film sample has been observed to be i...

Chemical bonding of termination species in 2D carbides investigated through valence band UPS/XPS of Ti 3 C 2 T x MXene To cite this article: Lars-Åke Näslund et al 2021 2D Mater. 8 045026 View the article online for updates and enhancements.

In the present study, an adsorbent material for removal of organic contaminants in wastewater is synthetized by a green and facile mechanochemical method. It is composed of Ti 3 C 2 T x MXene layers (obtained by mechanochemical etching of MAX phase with concentrated HF) pillared with terephthalate by rapid direct reaction. Such material shows high specific surface area (135.7 m 2 g −1) and excellent adsorption capability of methylene blue (209 mg g −1) because of the larger interlayer space among MXene sheets and free carboxylate groups of terephthalate. The spent adsorbent is reutilized (with addition of sole aluminum) to synthetize the MAX phase by mechanochemical procedure, where the terephthalate and the pollutant are carbonized into the carbide. In this way, new MXene-based adsorbent can be re-synthetized for further use.

Functional three-dimensional (3D) structures based on 2D Ti3C2Tx
MXene nanosheets are promising candidates for high-performance applications.
However, the repulsive electrical double layer forces among MXene nanosheets
make their 3D assembly challenging. Herein, we demonstrate that the diffusion of
positively charged poly(allylamine hydrochloride) (PAH) into the single-layer
MXene suspension allows for a range of free-standing 3D porous structures. The
assemblies are developed by the adsorption of PAH chains from an aqueous
solution on the MXene surfaces at the interface and their subsequent diffusion into
the bulk of MXene suspension. Our morphological analysis shows that an
optimum PAH concentration results in smaller and more circular pores. Moreover,
120−200 kDa PAH is found to be more effective than 17.5 kDa in developing
MXene assemblies with well-defined pores. A significant change in the porous morphology is observed by manipulating the ionization degrees of the components, illustrating the importance of electrostatic interactions in morphology development. The MXene/PAH hybrids are electroconductive and can be processed into different shapes, including porous fibers that exhibit shape memory properties. Altogether, our results provide a basis for the rational design of hybrid MXene-based macro assemblies with potential applications in electromagnetic shielding, environmental remediation, and tissue engineering.