Electrochemical Properties of Two-Dimensional (2D) MXenes and Their Hybrid/Hetero-Structures

Abstract

Two-dimensional (2D) transition metal carbides and nitrides (MXenes) have gained huge interest over the past few years for electrochemical energy storage applications because of their exceptional physical and (electro)chemical properties. Since their discovery in 2011, many researchers around the world have investigated the electrochemical properties of MXenes in various energy storage systems. Despite the substantial work done on these materials, the work on MXenes is still in its infancy. For instance, electrochemical investigations of MXenes have been largely limited to aqueous supercapacitors and monovalent ion batteries (lithium or sodium ion batteries), and their performances in multivalent battery systems have been barely studied. Batteries based on multivalent ion chemistry, such as aluminum batteries, are emerging as potential alternative technologies to current Li-ion batteries because of their higher safety and (theoretically achievable) energy densities. Lack of high-performance cathode materials for these systems, however, is the main challenge facing their development. The high potential of MXenes as electrode materials for multivalent ion batteries have been suggested by theoretical studies. Therefore, the experimental investigation of MXenes in these systems is important and needs to be carried out. On the other hand, although, to date around 30 different MXene materials have been experimentally synthesized, most of the researches have only focused on one MXene composition, Ti3C2Tx. The lack of systematic and controlled methods for the synthesis of many MXenes in their 2D (delaminated) form, as well as their chemical instabilities, are some of the underlying reasons that have hampered their applications, particularly in aqueous systems where they are more prone to oxidation and degradation. Moreover, establishing practical forms of 2D MXenes can enable fabrication of 2D vertical heterostructures based on different compositions of them with new and improved properties; an emerging field that has received enormous attention among researchers. My Ph.D. research aimed to shed light on some of these challenges (to the extent possible) and lay down a scientific understanding about them, provide solutions, and finally demonstrate the high-performance of MXenes and their hybrid/hetero-layered structures in different electrochemical energy storage systems. The outcomes of my Ph.D. research will directly impact the ongoing research on 2D MXenes in complex electrochemical systems and fabrication of advanced electrode materials based on them.