2D Ti3C2 nanosheets, multilayer structures, and conical scrolls produced by room temperature exfoliation of Ti3AlC2 in HF are reported. Since Ti3AlC2 is a member of a 60+ group of layered ternary carbides and nitrides, this discovery opens a door to the synthesis of a large number of other 2D crystals.

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.

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 Ti2 AlC, Ti3 AlC2 , and Ta4 AlC3 . 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.

Herein we report on the synthesis of two-dimensional transition metal carbides and carbonitrides by immersing select MAX phase powders in hydrofluoric acid, HF. The MAX phases represent a large (>60 members) family of ternary, layered, machinable transition metal carbides, nitrides, and carbonitrides. Herein we present evidence for the exfoliation of the following MAX phases: Ti(2)AlC, Ta(4)AlC(3), (Ti(0.5),Nb(0.5))(2)AlC, (V(0.5),Cr(0.5))(3)AlC(2), and Ti(3)AlCN by the simple immersion of their powders, at room temperature, in HF of varying concentrations for times varying between 10 and 72 h followed by sonication. The removal of the "A" group layer from the MAX phases results in 2-D layers that we are labeling MXenes to denote the loss of the A element and emphasize their structural similarities with graphene. The sheet resistances of the MXenes were found to be comparable to multilayer graphene. Contact angle measurements with water on pressed MXene surfaces showed hydrophilic behavior.

OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible. This is an author-deposited version published in : http://oatao.univ-toulouse.fr/ Eprints ID : 13896To link to this article :

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Ultra-high-rate pseudocapacitive energy storage in twodimensional transition metal carbides. (2017) Nature Energy, 2 (8). 1-12.

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