Double transition metal MXenes for enhanced electrochemical applications: Challenges and opportunities

Double transition metal (DTM) MXenes are a recently discovered class of two-dimensional composite nanomaterials with excellent potential in energy storage applications. Since their emergence in 2015, DTM MXenes have expanded their composition boundary beyond traditional single-metal carbide and nitride MXenes. DTM MXenes offer tunable structures and properties through variations in the constituent transition metals and positioning within the layered lattice. These MXenes can exist in two primary forms: ordered DTMs and solid solutions. The compositional versatility of DTM MXenes offers opportunities to enhance their performance in electrochemical energy storage applications. However, the quality, stability, and surface chemistry of DTM MXenes are influenced by several factors, including the etching process, etchant type, and synthesis route. Currently, limited literature is available on experimentally synthesized DTM MXenes, with most studies focusing on carbide-based MXenes. Most of the articles have dedicated their efforts only to generalized synthesis strategies. Although extensive theoretical studies have explored the suitability of etchants, synthesis parameters, and methods for producing high-quality MXene with selective terminal functional groups, their stability issues have not been thoroughly examined. This review addresses various types of DTM MXenes, their synthesis techniques, and the impact of these methods on their physicochemical properties and electrochemical performance. Additionally, it provides a critical analysis of the causes of instability in MXenes, particularly DTMs, from synthesis to application. The challenges associated with these materials are discussed, along with opportunities and prospects for enhancing synthesis, structural tuning, surface modification, and applications in electrochemical energy storage.