Ordered Double Transition Metal MXenes

Since the discovery of MXenes, research has progressed tremendously to explore MXenes with new compositions apart from the conventional Ti₃C₂T_x. Recently, the MXene family further expanded with the discovery of ordered double transition metal MXenes, wherein the metal sites are occupied by two different transition metals, which can be arranged in-plane or out-of-plane forming i-MXenes or o-MXenes, respectively. The feature of optimizing the properties of ordered double transition metal MXenes by precise engineering of the composition, number of metal layers, interlayer spacing, and surface functionalities is distinctive among the existing 2D materials. This review provides a brief overview of the theoretical and experimental studies on the ordered double transition metal MXenes to elucidate its structure and properties. In addition, the recent trends in the synthesis and the effects of fine-tuning the composition, structure, and functional groups on their electrochemical performance are elaborated. The current challenges faced by these emerging MXenes and future research directions are also proposed.