Design of MXene-based 3D structures for capacitive energy storage

Abstract

MXene, a type of two-dimensional (2D) transition metal carbides or nitrides, has attracted immense interest in recent times due to its distinctive properties and potential applicability across an array of fields, particularly in energy storage. In particular, the potential of MXene as a material for supercapacitors, a form of energy storage device known for high power and energy density, has been demonstrated through recent studies. The large surface area and excellent electrical conductivity of MXene make it an attractive choice for supercapacitor electrodes. MXene can also be easily functionalized and tailored to meet specific requirements for supercapacitor applications, making it a highly versatile material. However, the electrochemical performance is hindered by the inherent problem of layered stacking in two-dimensional materials, which restricts the full utilization of active surface sites and results in sluggish ion and electron transport. As a consequence, strategies need to be developed to overcome these limitations and improve MXene's overall electrochemical performance. In order to mitigate these challenges, considerable efforts have been allocated towards the development of design and preparation of hybrid nanocomposites with different hierarchical structures to enhance conductivity and thus improve electrochemical performance. This thesis focuses on the study of Ti3C2Tx, aiming to provide guidance for high-performance MXene and promoting its practical application progress. The synthesized material properties were characterized by testing X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), and others. The electrochemical performance was evaluated under three-electrode and two-electrode systems. The main conclusions of two main work in this thesis obtained are: (1)Traditional three-dimensional porous structures frequently trade off density for high porosity, which is unsuitable for energy storage in constrained spaces. It should be to find a facile approach that ingeniously balances the density and porosity of porous materials. Herein, using a hydrazine-induced foaming method, we successfully use a high-concentration MXene slurry to fabricate a 3D structure clay without any binder. The 3D porous structure in MXene clay promotes fast ion transport while keeping a high packing density. After hydrazine treatment, the large flakes of clay show high electrochemical performance, which is conducted as electrode materials for symmetrical capacitors. The material demonstrates a notable volumetric capacitance of 298.3 F g-1, accompanied by a favorable capacitance retention rate of 98.18% even after undergoing 10,000 cycles. Moreover, the device delivers a high volumetric energy density of 86 Wh L-1 with PVA/H2SO4 sol-gel electrolyte. A high energy density has been achieved by bridging the gap between conventional supercapacitors and batteries, which might lead to intriguing new possibilities for mobile power supply in various uses. (2)The integration of pseudo-capacitance nanomaterials into MXene-based 3D aerogel networks has demonstrated promising potential for high-rate energy storage. However, traditional thermal treatment processing is impeded by the ease with which MXene surfaces oxidize. To address this challenge, we report the successful synthesis of Ti3C2Tx-Ga hybrids via room temperature casting and fast freezing to produce self-assembling electrodes. Our results reveal an ultrahigh gravimetric capacitance of 80.78 F g-1 at 0.5 A g-1 in a 1M Li2SO4 electrolyte and provide a deeper understanding of gallium's contribution to the process. Furthermore, we achieved a superior gravimetric capacitance of 125.62 F g-1 and a high energy density of 44.67 Wh kg-1 in all-solid-state supercapacitors with neutral PVA/Li2SO4 gel electrolytes. This study offers practical guidelines for synthesizing high-capacitive Ti3C2Tx-Ga hybrids for use in neutral electrolytes and may contribute to the development of flexible, safe, and high-performance energy storage solutions.