# 2D in-Plane Ordered MXene Nanosheets Derived from (Mo2/3Er1/3)2AlC Rare-Earth i‑MAX for Energy Storage Applications

## TL;DR

Researchers synthesized a new type of 2D MXene material from a rare-earth compound and studied its potential for energy storage.

## Contribution

A new quaternary rare-earth-based i-MXene was synthesized and its electrochemical properties and impurity challenges were analyzed.

## Key findings

- The synthesized Mo1.33C@Er i-MXene showed a 24-fold increase in specific capacitance compared to its parent phase.
- RE fluoride impurities formed during synthesis persisted and increased charge-transfer resistance.
- Theoretical calculations confirmed the metallic nature of Mo1.33C even after functionalization.

## Abstract

MXenes have become
one of the most versatile families
of two-dimensional
(2D) materials due to their high conductivity, hydrophilicity, and
remarkable electrochemical performance. This has stimulated intense
efforts to design and synthesize MXenes, including structurally unique
in-plane ordered 2D MXenes called i-MXenes. Here, we have synthesized
the quaternary rare earth (RE)-based i-MAX phase (Mo2/3Er1/3)2AlC using an arc melting method, and
the corresponding 2D i-MXene was then obtained through a LiF/HCl soft
etching process. Literature studies have shown that Al and the RE
element are etched out during the etching process, leading to the
formation of pure vacancy-ordered Mo1.33C 2D 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, resulting in impure Mo1.33C@Er i-MXene. These
results were confirmed by various characterizations such as X-ray
diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy,
and scanning transmission electron microscopy. Although the Mo1.33C@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 2D Mo1.33C@Er i-MXenes. This study contributes
to identifying pathways for the preparation of pure 2D i-MXenes from
RE-based i-MAX phases and developing improved synthesis methods. With
additional process optimization, the 2D i-MXene holds a strong potential
for electrochemical energy storage applications. Additionally, the
electronic structures of Mo1.33C were theoretically studied
using first-principles density functional theory calculations, which
revealed that pristine Mo1.33C is metallic, and this metallic
nature is preserved even with –O, –F, and mixed functionalization.