Zheng, Sheng Qiang
(2024)
Solvothermal synthesis of nickel-based metal-organic framework (Ni-MOF) and its derivative for advanced symmetric supercapacitor applications.
PhD thesis, University of Nottingham.
Abstract
Nanomaterials have successfully attracted enormous interest in energy storage systems, such as batteries and supercapacitors (SCs), by virtue of the enhanced electronic and ionic conductivity as compared to traditional electrochemical energy storage materials. Meanwhile, small-sized materials are favorable for improving electrolyte ion diffusion and specific capacity due to the possibility of occupying all available sites in the particle volume. Among a wide range of electrode materials, high-crystalline conductive materials (such as nickel-based metal-organic frameworks (Ni-MOFs)), have been utilized as promising electrode active materials for SCs on the strength of their high porosities, ultra-large specific surface areas and structural tunability. More significantly, these materials are not only able to be utilized as electrode active materials, but also function as precursors to synthesize transition metal oxides and carbonaceous nanomaterials for energy storage systems. Nevertheless, there are still some challenges associated in the application of Ni-MOFs in high-performance SCs, such as inferior electrical conductivity, low energy density and durability. Hence, tremendous efforts have been made to study novel MOF-based nanostructures with enhanced electrochemical performance for their widespread implementation.
In this study, the impact of processing parameters, including synthesis temperature and time, on morphological and electrochemical properties of Ni-MOF nanostructures have been extensively studied. The results annotated that the average size of Ni-MOF nanosheets trended to decrease with increasing reaction temperature, whereas the crystallinity of these nanostructures was shown to have a direct correlation with solvothermal temperature. The symmetric supercapacitor device fabricated from nanostructures synthesized at 160°C exhibited an excellent energy density of 22.44 Wh kg-1 and a superior cyclic durability of 118% over 10,000 cycles. Moreover, the effect of different solvent systems on the morphologies and electrochemical properties of Ni-MOFs, which were synthesized via a solvothermal approach, were systematically investigated as well. It was found that binary solvents (Ethylene glycol + DMF) with high polarity and viscosity were favorable for the formation of microspheres, resulting from the self-assembly of ultrathin 2D nanosheets. Besides, the successful incorporation of highly conductive graphene nanosheets with Ni-MOF nanostructures could further enhance electrochemical performance, and a higher specific capacitance of 420.4 F g-1 was obtained in 2M KOH electrolyte. To further improve electrochemical performance of Ni-MOF-based SCs, 2D layered molybdenum disulphide (MoS2) with large surface area and flexibility in the atomic scale dimension, was incorporated onto porous Ni-MOFs via a facile hydrothermal method. Meanwhile, sodium ions were introduced to further improve electronic conductivity and overall electrochemical performance. The results indicated that the as-synthesized porous nanocomposites delivered an excellent supercapacitive performance, in which the delivered maximum energy density was 33.33 Wh kg-1, and the delivered capacitance retention and coulombic efficiency were more than 97% after 10,000 cycles. Additionally, highly porous nanostructured NiO/C yolk-shell nanocomposites via a simple calcination of Ni-MOFs. The porosity and electrochemical property were further enhanced by incorporating Na+ and hierarchical MoS2 nanostructures via a facile hydrothermal method. The delivered electrochemical capacitance of NiO/C yolk-shell structure was 1779.50 F g-1 in 2M KOH, and a superior specific capacitance of 2540.63 F g-1 was acquired after the incorporation of Na+ and 2D layered MoS2 onto the nanocomposite. When a symmetric supercapacitor was fabricated, a remarkable energy density of 36.93 Wh kg-1 was recorded in an environmental-friendly aqueous-based electrolyte. More significantly, the outstanding delivered capacitance retention and coulombic efficiency were recorded at 111.92% and 97.6%, respectively after 4,000 continuous GCD cycles.
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