i-MXene Derived from (Mo_2/3Er_1/3)₂AlC Rare-Earth i-MAX: Structural, Electronic, and Electrochemical Perspectives

Owing to the exceptional properties of MXenes, there has been a growing research focus on synthesizing new MXenes, including in-plane ordered i-MXenes. Here, we have synthesized the quaternary RE-based i-MAX phase (Mo_2/3Er_1/3)₂AlC using a pressureless sintering method, and the corresponding i-MXene, was then obtained through a LiF/HCl soft etching process. Previous studies have shown that Al and the RE element are etched out during the etching process, leading to the formation of pure vacancy-ordered Mo_1.33C i-MXene. However, our investigation reveals that upon exposure to a fluorine solution, the i-MAX phase forms RE fluoride impurities, which are challenging to remove through HCl-DI water washing and persist in the final product which resulting in impure Mo_1.33C@Er i-MXene. These results were confirmed by various characterizations such as XRD, Raman, XPS, and STEM. Although the Mo_1.33C@Er electrode showed a 24-fold increase in specific capacitance compared to its parent i-MAX phase, it still exhibited a high charge-transfer resistance arising from the insulating nature of RE fluoride byproducts, which adversely influence the overall capacitance behavior of the synthesized i-MXenes. This study contributes to identifying pathways for the preparation of pure i-MXenes from RE-based i-MAX phases and to developing improved synthesis methods. Additionally, the electronic structures of Mo_1.33C were theoretically studied using first-principles density functional theory calculations, which revealed pristine Mo_1.33C is metallic, and this metallic nature preserved even with -O, -F, and mixed functionalization.