Two-dimensional (2D) Metal Carbides as Efficient Energy Storage Electrodes for Supercapacitor Applications

Abstract

The increasing global requirements of energy, coupled with the constraint to mitigate the harmful environmental effects of fossil fuels, have stimulated engineers, scientists, and researchers to explore alternative renewable energies. Solar and wind power systems have emerged as major competitors in the field, offering efficient, clean, and environment-friendly solutions to meet future energy needs. However, a significant challenge lies in developing efficient energy storage devices that can be deployed at both, domestic and industrial scales to complement these renewable energy sources. Twodimensional (2D) MXenes and their atomically thin heterostructures have been extensively researched because of their outstanding characteristics and vast applications in different fields including advanced energy storage systems. Herein, a particular focus is on supercapacitors (SCs) due to their rapid chargedischarge rates, long cyclic life, high power density, and reliability as compared to traditional Li-ion batteries. Moreover, the challenge is to improve the energy density of SCs while retaining their high-power capabilities which requires the establishment of hybrid supercapacitors (HSCs) that combine capacitor-type and battery-type materials in a single device. Meanwhile, the performance of HSCs also relies deeply on advanced electrode materials with promising properties, e.g. high surface, good electronic conductivity, better thermal stability, and fast redox reversibility. Discovered in 2011, a new class of 2D material called MXene, based on single or double transition metals bonded with carbon in layered form, offer special properties including high electronic conductivity, excellent mechanical structure, hydrophilicity, and improved electrochemical performance superior to other 2D materials making them a potential candidate for SCs, K-ion, Li-S, Liion, and Na-ion batteries owing to their low ion diffusion barriers on the electrode surface. However, a significant challenge lies in addressing the natural restacking tendency of xv MXenes during charging and discharging processes, like other 2D materials, which hinder their use in practical applications. In this thesis, less studied MXenes (other than Ti3C2) such as Nb2C and Mo2TiC2 were employed to explore their electrochemical energy storage properties for SCs application. To improve the electrochemical characteristics of these MXenes and to overcome self-restacking issue among the MXene sheets, novel heterostructures were designed. For that purpose, in the first project, the surface group functionalized pristine Nb2CTx MXene was prepared via HF etching method and fabricated its nanocomposite with cetyltrimethylammonium bromide capped silver nanoparticles (AgNPs-CTAB). Furthermore, the nanocomposite (Nb2CTx/AgNPs-CTAB) prepared through an electrostatically self-assembled method was successfully investigated as a proficient energy storage electrode in asymmetric supercapacitor (ASC) in both neutral and basic electrolytes. In the second project, pristine double transition metal (DTM) Mo2TiC2 MXene and its hybrid electrode material with MnO2 nanowires (NWs) were synthesized. The hybrid electrode material (MnO2 NWs@Mo2TiC2) showed enhanced electrochemical energy storage properties as compared to pristine Mo2TiC2 MXene. In the third project, pristine Mo2TiC2 MXene was successfully intercalated with Sn+2 ions that enhanced the electrochemical properties of pristine MXene. The composite Sn@ Mo2TiC2 proved itself an efficient energy storage electrode with improved electrical conductivity, large surface area, and good ion diffusion characteristics. This work on Nb2C and DTM carbide-based (Mo2TiC2) MXenes could pave the way for exploring interesting properties of various MXenes beyond Ti3C2 which have not received extensive attention yet suitable for a broad spectrum of energy storage devices.