Tailoring Structure, Composition, and Energy Storage Properties of MXenes from Selective Etching of In-Plane, Chemically Ordered MAX Phases.

The exploration of 2D solids is one of our time's generators of materials discoveries. A recent addition to the 2D world is MXenes that possses a rich chemistry due to the large parent family of MAX phases. Recently, a new type of atomic laminated phases (coined i-MAX) is reported, in which two different transition metal atoms are ordered in the basal planes. Herein, these i-MAX phases are used in a new route for tailoriong the MXene structure and composition. By employing different etching protocols to the parent i-MAX phase (Mo2/3 Y1/3 )2 AlC, the resulting MXene can be either: i) (Mo2/3 Y1/3 )2 C with in-plane elemental order through selective removal of Al atoms or ii) Mo1.33 C with ordered vacancies through selective removal of both Al and Y atoms. When (Mo2/3 Y1/3 )2 C (ideal stoichiometry) is used as an electrode in a supercapacitor-with KOH electrolyte-a volumetric capacitance exceeding 1500 F cm-3 is obtained, which is 40% higher than that of its Mo1.33 C counterpart. With H2 SO4 , the trend is reversed, with the latter exhibiting the higher capacitance (≈1200 F cm-3 ). This additional ability for structural tailoring will indubitably prove to be a powerful tool in property-tailoring of 2D materials, as exemplified here for supercapacitors.