Abstract
High-entropy (HE) ceramics, by analogy with HE metallic alloys, are an emerging family of multielemental solid solutions. These materials offer a large compositional space, with a corresponding large range of properties. Here, we report the experimental realization of a 3D HE MAX phase, Ti1.0V0.7Cr0.05Nb1.0Ta1.0AlC3, and a corresponding 2D HE MXene in the form of freestanding flakes of average composition Ti1.1V0.7CrxNb1.0Ta0.6C3Tz (Tz = −F, −O, −OH), as produced by selective removal of Al from the HE MAX phase in aqueous hydrofluoric acid (HF). Initial tests on HE MXene “paper” electrodes show their high potential as electrode materials in supercapacitors through volumetric and gravimetric capacitances of 1688 F/cm3 and 490 F/g, respectively, originating from a combination of diffusion- and surface-controlled charge storage processes. The introduction of the HE concept into the field of 2D materials suggests a wealth of future 2D materials and applications.
Highlights
High-entropy alloys (HEAs) are a material class with multiconstitutive elements, developed from the concept of a configurational entropy contribution to the total free energy overcoming the enthalpy contribution and, stabilizing the material and optimizing its properties.[1]
The detailed materials synthesis process can be found in the Experimental section
HAADF is a mass-contrast imaging mode where the detected intensity is proportional to the atomic number (∼Z1.6).[28]
Summary
High-entropy alloys (HEAs) are a material class with multiconstitutive elements, developed from the concept of a configurational entropy contribution to the total free energy overcoming the enthalpy contribution and, stabilizing the material and optimizing its properties.[1]. In pursuit of novel electrode materials, two-dimensional (2D) layered structures are of special interest due to their abundant electrochemically active sites, high specific surface areas, shortened electronic transfer paths, and good structural stabilities during the charge/discharge process.[10] In particular, 2D pseudocapacitive materials deliver a high capacitance due to the occurrence of double-layer and intercalation capacitances One such example is the family of MXenes, which have demonstrated high charge storage performance in supercapacitors (300−1150 F/cm3) and secondary-ion batteries (400 mAh/g) owing to the metallic conductivities, redox-active surfaces, and 2D layered structures with large and tunable interlayer spacing.[11−15] Generally, MXenes are described by the formula Mn+1XnTz, where M is an early transition metal, X is C −O, and/or N, −F, −OH, and Tz denotes −Cl, −Br, −Se, surface terminations, −S, and Te.[14,16−19] such as MXenes are produced by the selective etching of relatively weakly bonded A-group elements from the parent 3D laminated MAX phases, the latter being a large group composed of more than 150 members.[14,20] There are ∼30 different MXenes to date, and their abundant composition, excellent conductivities and stabilities, and tunable surface chemistries have rendered them quite promising in applications ranging from energy storage, conductive transparent electrodes, and catalysis to electromagnetic interference shielding.[14]. The introduction of the HE concept into the field of 2D materials, here exemplified by MXenes, is important for the further expansion of the MXene family in particular and the 2D material family in general and attainable properties
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