Materials for Supercapacitor Separators

One of the main technological concerns for the uptake of renewable energy sources and the strive for more efficient and transport systems is the lack of high power and high energy storage capabilities. The Supercapacitor is one of the most promising technologies to fulfil our growing need for electrical energy storage as the power output of supercapacitors can reach up to 10kW kg-1 but lack in energy density compared to that of batteries. Currently, a considerable amount of research is focused on the production of high capacitive electrode materials, to increase the energy density of these devices without sacrificing the innate power density. This has been mainly using high specific surface area carbon material or redox active pseudocapacitive elements.
An important yet often overlooked component of the device is the separator material, which provides a physical barrier between the electrodes to prevent shorting and high porosity to allow the flow of electrolyte for charging and discharging. Traditionally micro-porous membranes such as polyethylene and polypropylene are used as electrode separators due to their chemical stability and their significant mechanical properties. However, these materials suffer from low porosity, resulting in weak ion conductivity which inhibits charge/discharge rates within the device. Polyvinylidene fluoride (PVDF) nanofibers have been seen to exhibit an ionic conductivity of over 1.8 mScm-1 as they provide low interfacial resistance and higher ion diffusion rate due to the controllable porosity when manufacturing via electrospinning [1]. What's also interesting about these PVDF materials is that they are one of the few polymer materials that exhibit piezoelectric properties being able to crystallise into 4 different phases. This has opened opportunities for providing multifunctionality with supercapacitor devices, as replacing the separator material with (beta) phase crystallized PVDF has let to research in the development of enhanced supercapacitor devices with the benefit of self-charging under the influence of mechanical force [2]. Mechanical stress upon the device will induce an internal electric field upon the separator, forcing ions in the electrolyte to separate to the cathode and anode, thus self-charging of the device upon application of a mechanical force.
This project sets out to study the effect of incorporating both piezoelectric energy harvesting and supercapacitor energy storage into one device. Focussing on experiments to optimise the separator component in the aim to maximise ionic conductivity and provide a high piezoelectric coefficient for charge separation and self-charging. Studies will go beyond the current research into the use of PVDF nanofibers, exploring different piezoelectric polymer nanostructures such as Nylon-11, which has been shown to inhibit strong piezoelectric behaviours [3]. Fabrication of device components will be based on electrospinning of polymer nanofibers and carbon fibre electrodes to produce high power density devices with fast self-charging capabilities.