Progress of 2D MXene as an Electrode Architecture for Advanced Supercapacitors: A Comprehensive Review

Abstract

Supercapacitors, designed to store more energy and be proficient in accumulating more energy than conventional batteries with numerous charge–discharge cycles, have been developed in response to the growing demand for energy. Transition metal carbides/nitrides called MXenes have been the focus of researchers’ cutting-edge research in energy storage. The 2D-layered MXenes are a hopeful contender for the electrode material due to their unique properties, such as high conductivity, hydrophilicity, tunable surface functional groups, better mechanical properties, and outstanding electrochemical performance. This newly developed pseudocapacitive substance benefits electrochemical energy storage because it is rich in interlayer ion diffusion pathways and ion storage sites. Making MXene involves etching the MAX phase precursor with suitable etchants, but different etching methods have distinct effects on the morphology and electrochemical properties. It is an overview of the recent progress of MXene and its structure, synthesis, and unique properties. There is a strong emphasis on the effects of shape, size, electrode design, electrolyte behavior, and other variables on the charge storage mechanism and electrochemical performance of MXene-based supercapacitors. The electrochemical application of MXene and the remarkable research achievements in MXene-based composites are an intense focus. Finally, in light of further research and potential applications, the challenges and future perspectives that MXenes face and the prospects that MXenes present have been highlighted.

Structure, Properties, and Preparation of MXene and the Application of Its Composites in Supercapacitors

“…The morphology of MXene also has a great impact on its conductivity; single-layer and large-sized flakes have better interaction than multi-layered and small-sized flakes, usually improving conductivity [14,50,51]. In addition, compared with other 2D materials such as metal sulfides/hydroxides or graphene, MXene has plenty of chemical functional groups, for example, hydroxyl and fluorine atoms, and by changing these functional groups, the electronic structure and charge transfer properties of MXene can be controlled, affecting its conductivity performance [52]. Overall, the improvement of MXene conductivity is mainly due to its unique structure, containing transition metal elements and abundant chemical functional groups, which together make MXene an excellent conductive material with a multitude of potential applications.…”

“…Typically, through the selective removal of atomic layers, MXenes can be synthesized (for example, Al, Si, Ga, or Sn) from the MAX phase using different methods. When the metal-metallic M-A bonds in MAX materials are less powerful than the M-X covalent bonds, selective etching of the A layer atoms from MAX can be achieved, and it will not break the M-X bonds [18,33,52,59]. Currently, MXenes are produced by wet-chemical etching using HF or HF-containing or HF-generating etchants, resulting in the presence of -F surface terminations [45,[72][73][74].…”