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, (Mo 2/3 Sc 1/3 ) 2 AlC, 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, Mo 1.33 C, 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 Mo 1.33 C 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.