Electroactive Nanofiber materials for high-performance Supercapacitor devices

With the increasing demand for more sustainable energy sources and the decarbonisation of the transport sector, the development of energy storage systems has seen rapid growth in recent years. This growth is only set to continue with several governments all around the globe committing to limit the global temperature increase in this century to 2 degrees Celsius above preindustrial levels through the Paris agreement, further driving the need for the development of a sustainable energy system. Among several existing technologies, the supercapacitor has received increasing interest in the scientific community due to critical characteristics such as high-power densities and long cycle lifetimes, allowing the use of devices with stable performance into the 10 of thousands of cycles. However, supercapacitor devices still lack energy density capabilities compared to competing technologies in this field. An additional drawback of supercapacitor devices is their high self-discharge properties, which diminishes their ability to store energy for periods longer than a few hours; this limits the potential applications of these devices and refrains their broader use in the development of a clean energy infrastructure. In this thesis, the development of high charging power devices with low self-discharge properties was studied through the manufacture of novel separator materials, where the electrospinning of PVDF polymer tailored solutions into directionally-polarised and piezoelectric nanofiber films, consisting of a high proportion of electroactive β crystalline phases reaching levels of 38±0.5% of the total material. The use of these materials as separators in the correct dipole orientation is shown to provide a mechanism that introduces a reverse-diode behaviour within the device, such that it drops from an initial voltage of 1.6 V to 1 V after 10-hr, as opposed to 0.3 V with a non-polarised, commercial separator material. Thus, the energy retained for the polarised separator is 37% and 4% for the non-polarised separator, making supercapacitors a more attractive solution for long term energy storage