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Title: Synthesis and characterization of organophosphazene and porous carbon materials for energy storage applications
Author: Pappas, George S.
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2017
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The developments in energy storage systems in the last two decades, resulted in a series of interdependent events in consumer’s habits; the modern commercial devices became thinner, smaller and lighter with enhanced portability. Hence, the way in which users relate with electronic gadgets changed and this has derived in the invention of new applications. These developments within the electronic industry have demanded new materials and formulations in order to fulfil the growing requirements of new technologies and, especially in the last years, the electrification of transport. The need to pursue research to develop safe, environmental friendly, recyclable and low-cost materials is, in this context, a pressing issue. While hybrid materials offer unique properties for the development of advanced energy storage systems such as hydrogen and solar cells, lithium-ion batteries and capacitors, these are usually expensive and entail serious environmental hazards. This thesis advances scientific analyses on the cyclomatrix organophosphazenes synthesis that could be applied for the development of low-cost and eco-friendly industrial processes for the production of electrode materials. In Chapter 1, a brief introduction on the fundamental chemistry of phosphazene materials is presented followed by an outline description of the Li-ion batteries and dielectric materials and their current trends for the improvement of their efficiency. The research aim and objectives of this thesis are then presented. Chapter 2 deals with the synthesis of cyclomatrix organophosphazene (OPZ) nanospheres. The effects of the solvent, organic base, and organic co-monomer on the morphology of the nanoparticles are studied. The morphology of the nanospheres was highly dependent on the polarity of the solvent, pKa of the base and solubility of the produced salt during the reaction. A new design for the scale up synthesis of OPZs was developed based on distillation and reuse of the solvent and organic base. The dependence of the morphology on the polarity of the solvent is further investigated in detail in Chapter 3 where a self-template-direct formation of hollow OPZs is described. The presence of water in the reaction mixture promoted the formation of nanospheres with a single hole on their surface while the reactions at higher water contents resulted in the formation of hollow nanospheres and semi-shells. The subject of heteroatom-doped carbon nanospheres (CNS) as anode electrodes in Li-ion batteries is described in Chapter 4. A detailed analysis and discussion of the microporous structure and the heteroatom-doping carbon is presented in order to understand the structure-to-electrochemical properties of the CNS as anode materials. The ternary heteroatom doping had significant impact on the microporous structure of the CNS and their electrochemical performance. CNS with high pyridinic-N content and abundant micropores, showed impressive charge/discharge cycling stability for over 1100 cycles, delivering 130 mA h g−1. The facile OPZ chemistry is further applied in the presence of BaTiO3 nanoparticles in order to produce electron insulating OPZ@BaTiO3 and electron conducting C@BaTiO3 core-shell hybrid nanoparticles. The successful growth of OPZ as a thin layer/coating is based on the substrate-independency of the method. The morphology and structural characteristics of obtained core-shell particles is discussed in detail, along with the results from the impedance spectroscopy characterisation. A stable relative permittivity (εr ~ 35) and low dielectric loss over a wide range of frequency was achieved by adjusting the OPZ shell thickness. The initial insulating OPZ shell was transformed to e- conducting carbon shell, by a pyrolysis step at 700 °C and the resulted C@BaTiO3 showed a relatively high ac conductivity and specific surface area. The research outcome of this project together with suggestions for future directions are summarised in Chapter 6.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: TK Electrical engineering. Electronics Nuclear engineering