i‐MXenes for Energy Storage and Catalysis

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. A novel family of in‐plane, chemically‐ordered quaternary MAX phases (i‐MAX), possessing ordered Sc within a Mo‐dominated M layer, renders a novel MXene family (i.e., i‐MXenes), with unique surface structure and ordered divacancies. Herein, fundamental investigations of i‐MAX and i‐MXene phases are summarized to demonstrate the potential of i‐MXenes in energy storage and catalytic applications.