i-MXenes for Energy Storage and Catalysis

Abstract

In 2017, a new family of in-plane, chemically-ordered quaternary MAX phases, coined i-MAX, has been reported since 2017. The first i-MAX phase, (Mo2/3Sc1/3)2AlC, garnered significant research attention due to the presence of chemically ordered Sc within the Mo-dominated M layer, and the facilitated removal of both Al and Sc upon etching, resulting in 2D i-MXene, Mo1.33C, with ordered divacancies. The i-MXene renders an exceptionally low resistivity of 33.2 µΩ m−1 and a high volumetric capacitance of ≈1150 F cm−3. This discovery has been followed by the synthesis of, to date, 32 i-MAX phases and 5 i-MXenes, where the latter have shown potential for applications including, but not limited to, energy storage and catalysis. Herein, fundamental investigations of i-MAX phases and i-MXenes, along with their applicability in supercapacitive and catalytic applications, are reviewed. Moreover, recent results on ion intercalation and post-etching treatment of Mo1.33C are presented. The charge storage performance can also be tuned by forming MXene hydrogel and through inert atmosphere annealing, where the latter renders a superior volumetric capacitance of ≈1635 F cm−3. This report demonstrates the potential of the i-MXene family for catalytic and energy storage applications, and highlights novel research directions for further development and successful employment in practical applications.