Materials for Supercapacitor Separators

利用可再生能源和争取更有效的运输系统的主要技术问题之一是缺乏高功率和高能量储存能力。超级电容器是满足我们日益增长的电能存储需求的最有前途的技术之一，因为超级电容器的输出功率可以达到10kW kg-1，但与电池相比缺乏能量密度。目前，大量的研究集中在生产高容性电极材料上，以提高这些器件的能量密度而不牺牲固有的功率密度。这主要是采用高比表面积的碳材料或氧化还原活性假电容元件。该装置的一个重要但经常被忽视的组件是隔膜材料，它在电极之间提供物理屏障，以防止短路和高孔隙率，从而允许电解质流动进行充放电。传统上，微孔膜（如聚乙烯和聚丙烯）由于其化学稳定性和显著的机械性能而被用作电极分离器。然而，这些材料的孔隙率低，导致离子电导率低，从而抑制了器件内的充电/放电速率。聚偏氟乙烯（PVDF）纳米纤维的离子电导率超过1.8微米微米-1，因为通过静电纺丝[1]制造时，它们具有可控的孔隙率，从而提供了低的界面电阻和更高的离子扩散速率。PVDF材料的另一个有趣之处在于，它们是少数几种具有压电特性的聚合物材料之一，能够结晶成4种不同的相。这为超级电容器器件提供了提供多功能的机会，因为用（β）相结晶的PVDF取代隔膜材料，使得研究人员可以在机械力[2]的影响下开发具有自充电优势的增强型超级电容器器件。装置上的机械应力会在分离器上产生一个内部电场，迫使电解液中的离子分离到阴极和阳极，从而在施加机械力的情况下使装置自充电。该项目旨在研究将压电能量收集和超级电容器能量存储结合到一个设备中的效果。专注于优化分离器组件的实验，以最大限度地提高离子电导率，并为电荷分离和自充电提供高压电系数。研究将超越目前对PVDF纳米纤维使用的研究，探索不同的压电聚合物纳米结构，如尼龙-11，它已被证明可以抑制强压电行为[3]。器件组件的制造将基于聚合物纳米纤维和碳纤维电极的静电纺丝，以生产具有快速自充电能力的高功率密度器件。

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.