The family of 2D transition metal carbides, carbonitrides and nitrides (collectively referred to as MXenes) has expanded rapidly since the discovery of Ti3C2 in 2011. The materials reported so far always have surface terminations, such as hydroxyl, oxygen or fluorine, which impart hydrophilicity to their surfaces. About 20 different MXenes have been synthesized, and the structures and properties of dozens more have been theoretically predicted. The availability of solid solutions, the control of surface terminations and a recent discovery of multi-transition-metal layered MXenes offer the potential for synthesis of many new structures. The versatile chemistry of MXenes allows the tuning of properties for applications including energy storage, electromagnetic interference shielding, reinforcement for composites, water purification, gas- and biosensors, lubrication, and photo-, electro- and chemical catalysis. Attractive electronic, optical, plasmonic and thermoelectric properties have also been shown. In this Review, we present the synthesis, structure and properties of MXenes, as well as their energy storage and related applications, and an outlook for future research. More than twenty 2D carbides, nitrides and carbonitrides of transition metals (MXenes) have been synthesized and studied, and dozens more predicted to exist. Highly electrically conductive MXenes show promise in electrical energy storage, electromagnetic interference shielding, electrocatalysis, plasmonics and other applications.

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2D metal carbides and nitrides (MXenes) for energy storage

Recently a new, large family of two-dimensional (2D) early transition metal carbides and carbonitrides, called MXenes, was discovered. MXenes are produced by selective etching of the A element from the MAX phases, which are metallically conductive, layered solids connected by strong metallic, ionic, and covalent bonds, such as Ti2AlC, Ti3AlC2, and Ta4AlC3. MXenes ­combine the metallic conductivity of transition metal carbides with the hydrophilic nature of their hydroxyl or oxygen terminated surfaces. In essence, they behave as “conductive clays”. This article reviews progress—both ­experimental and theoretical—on their synthesis, structure, properties, intercalation, delamination, and potential applications. MXenes are expected to be good candidates for a host of applications. They have already shown promising performance in electrochemical energy storage systems. A detailed outlook for future research on MXenes is also presented.

25th Anniversary Article: MXenes: A New Family of Two‐Dimensional Materials

Safe and powerful energy storage devices are becoming increasingly important. Charging times of seconds to minutes, with power densities exceeding those of batteries, can in principle be provided by electrochemical capacitors--in particular, pseudocapacitors. Recent research has focused mainly on improving the gravimetric performance of the electrodes of such systems, but for portable electronics and vehicles volume is at a premium. The best volumetric capacitances of carbon-based electrodes are around 300 farads per cubic centimetre; hydrated ruthenium oxide can reach capacitances of 1,000 to 1,500 farads per cubic centimetre with great cyclability, but only in thin films. Recently, electrodes made of two-dimensional titanium carbide (Ti3C2, a member of the 'MXene' family), produced by etching aluminium from titanium aluminium carbide (Ti3AlC2, a 'MAX' phase) in concentrated hydrofluoric acid, have been shown to have volumetric capacitances of over 300 farads per cubic centimetre. Here we report a method of producing this material using a solution of lithium fluoride and hydrochloric acid. The resulting hydrophilic material swells in volume when hydrated, and can be shaped like clay and dried into a highly conductive solid or rolled into films tens of micrometres thick. Additive-free films of this titanium carbide 'clay' have volumetric capacitances of up to 900 farads per cubic centimetre, with excellent cyclability and rate performances. This capacitance is almost twice that of our previous report, and our synthetic method also offers a much faster route to film production as well as the avoidance of handling hazardous concentrated hydrofluoric acid.

Conductive two-dimensional titanium carbide ‘clay’ with high volumetric capacitance

The MN+1AXN phases: A new class of solids

Since its first isolation in 2004, graphene has become one of the hottest topics in the field of materials science, and its highly appealing properties have led to a plethora of scientific papers. Among the many affected areas of materials science, this 'graphene fever' has influenced particularly the world of electrochemical energy-storage devices. Despite widespread enthusiasm, it is not yet clear whether graphene could really lead to progress in the field. Here we discuss the most recent applications of graphene - both as an active material and as an inactive component - from lithium-ion batteries and electrochemical capacitors to emerging technologies such as metal-air and magnesium-ion batteries. By critically analysing state-of-the-art technologies, we aim to address the benefits and issues of graphene-based materials, as well as outline the most promising results and applications so far.

The role of graphene for electrochemical energy storage

Graphene allotropes: Graphene allotropes

Graphynes and graphdyines

We have performed accurate ab initio total energy calculations using the full-potential linearized augmented plane-wave (FP-LAPW) method with the generalized gradient approximation (GGA) for the exchange–correlation potential to systematically investigate elastic properties of 18 stable, metastable and hypothetical hexagonal (AlB2-like) metal diborides MB2, where M = Na, Be, Mg, Ca, Al, Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Ag and Au. For monocrystalline MB2, the optimized lattice parameters, independent elastic constants (Cij), bulk moduli (B) and shear moduli (G) are obtained and analyzed in comparison with the available theoretical and experimental data. For the first time, numerical estimates of a set of elastic parameters of the polycrystalline MB2 ceramics (in the framework of the Voigt–Reuss–Hill approximation), namely bulk and shear moduli, compressibility (β), Young's modulus (Y), Poisson's ratio (ν) and Lame coefficients (μ, λ), are performed.

http://iopscience.iop.org/article/10.1088/0953-8984/20/41/415218/pdf

Elastic properties of mono- and polycrystalline hexagonal AlB2-like diborides of s, p and d metals from first-principles calculations

Graphene-like titanium carbides and nitrides Tin+1Cn, Tin+1Nn (n = 1, 2, and 3) from de-intercalated MAX phases: First-principles probing of their structural, electronic properties and relative stability

The full-potential linearized augmented plane wave method with the generalized gradient approximation for the exchange and correlation potential (LAPW-GGA) is used to understand the electronic and elastic properties of the first thorium-containing nitride perovskite TaThN3. Total and partial density of states, charge distributions as well as the elastic constants, bulk modulus, compressibility, shear modulus, Young modulus and Poisson ratio are obtained for the first time and analyzed in comparison with cubic ThN. The chemical bonding in TaThN3 is a combination of ionic Th–N and of mixed covalent–ionic Ta–N bonds. The cubic TaThN3 is semiconducting with the direct gap at about 0.65 eV. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Alexander L. Ivanovskii

Papers

Graphene allotropes: Graphene allotropes

Graphynes and graphdyines

Elastic properties of mono- and polycrystalline hexagonal AlB2-like diborides of s, p and d metals from first-principles calculations

Graphene-like titanium carbides and nitrides Tin+1Cn, Tin+1Nn (n = 1, 2, and 3) from de-intercalated MAX phases: First-principles probing of their structural, electronic properties and relative stability

Electronic structure, chemical bonding and elastic properties of the first thorium‐containing nitride perovskite TaThN3