PEDOT:PSS charge storage devices integrated into textiles for smart textile application

Abstract

Smart textile systems enable interaction of the user with his/her environment through sensing and actuation. They find application in sports garment, future fashion with visual light interaction, health and tele monitoring, sound responsive garments for managing autism, and in personal protective clothing etc. Smart textile systems consist of sensors, actuators, power supply unit, data processors and interconnects for transmission of signals and/or data. The energy supply unit can either be energy generated on the spot, or as a form of stored energy in batteries. Currently, the batteries used with smart textile systems, are non-flexible, bulky and weighty, and cannot be compared with the comfort of the textiles themselves. Therefore, this research addresses the fabrication of a suitable charge storage device well integrated into textile and that could provide power to the smart textile system. The developed devices are light weight, flexible and reliable. We start chapter 1 by giving a wide overview of electric energy storage devices (batteries and capacitors), and appreciate the research effort made towards achieving flexible textile-based batteries and capacitors by a number of researchers. However a functional, fully integrated energy storage device is yet to be developed. In this chapter we gave a brief summary on rechargeable textile batteries which was the basis of our research. We developed a similar charge storage device, in a simplified way and with different types of yarn electrodes. We obtained new findings and reported them in our publications. Chapter 2 discusses in detail materials selected and used in the fabrication of the charge storage devices. The materials have been discussed according to the function given in the developed capacitors. The material used as electrolyte was polyethylene dioxythiophene: polystyrene sulphonate (PEDOT:PSS). Three types of conductive yarns (copper coated polybenzoxazole (PBO), silver coated PBO and pure stainless steel filament yarns) were used as yarn electrodes in three different sets of devices. A cotton/polyester blend was selected out of the available fabric variety as the textile substrate. A hot melt adhesive was used to laminate the three layered fabric while the upper surface of the textile substrate was made hydrophobic using thermoplastic polyurethane (TPU). The capacitors were designed and fabricated using different types of yarn electrodes.Chapter 3 discusses the charge - discharge procedure used to characterize the developed devices. From the results, the developed cells experienced a self-discharge. Copper coated yarn electrode devices could barely store any charge. Stainless steel yarn electrode devices performed better than the silver coated yarn electrodes devices. They maintained a charge of at least 0.4 V for a long time, while silver coated yarn electrodes devices had about 0.2 V. The stainless steel yarn electrode devices could also support load resistors. The longer the charging time, the more charge was stored in the devices. The PEDOT:PSS devices had no predefined polarity, both electrodes could be used for positive or negative electrodes and reversed if need be. As a consequence one may not denote the electrodes as cathode or anode, because they were both made from the same material. One may wonder why we are using the term device and/or cell to refer to the developed charge storage devices instead of either a “battery” or a “capacitor”. This was a difficult decision to reach at, bearing in mind that we started from a defined battery principles by another research group. But since we are using two electrodes made from the same material, strictly speaking we were then dealing with a capacitor. On the other hand we could not exclude that some electrochemical reactions could be taking place in the device, because the physical mechanism of charge storage in PEDOT:PSS is still not well understood. In chapter 4, the reliability and stability of the developed devices was tested. The charge storage devices were charged and discharged severally for a number of days until they were worn out. The devices made with stainless steel yarn electrodes show some robustness and could withstand up to 14 cycles of each 7200 seconds charging at 1.5V and discharging for a day. However, the amount of energy stored in the devices after charging is still very low due to the self-discharge. One can roughly say that these capacitors could be used up to 10-15 cycles, with no significant difference in the output voltage level for the first 14 cycles. This shows the limited life time of these developed capacitor compared to the conventional ones which can be charged thousands of times. It was also found that dipping the device in water had an adverse effect on the residual stored charge, therefore the cell cannot be subjected to normal washing with water as it is, unless some covering/packaging is used on it to protect it. Furthermore, the developed devices performed poorly when exposed to temperatures higher than 300C. In chapter 5 different brands (5) of PEDOT:PSS were compared for use in making textile based capacitors. From the analysis, it was clear that the five different types of PEDOT:PSS had different performances in our developed devices. A closer look at the polymer dispersion composition and electrical properties, indicated that these parameters were varying from one brand to the other. We found out that the best electrolyte for our application so far was PEDOT:PSS from Ossila AI 4083 which was drop coated. The performance of pure stainless steel filament yarns in the developed devices dominated the performance of silver coated PBO electrode devices. In chapter 6, three yarn electrode of stainless steel filament yarns with different diameters were used to produce three different PEDOT:PSS capacitors. The performance in terms of voltage decay of the three types of capacitors was studied and investigated. The initial perception was that the voltage decay was related to the yarn linear resistance, but later we found out that this was not true. Therefore it was difficult to clarify the difference in the voltage decay graphs of the thin yarn electrode capacitor from the medium and thick yarn electrodes. With our theoretical model, the yarn electrode diameter was used to calculate the electric field strength around each size of yarn. From this, we could state that the electric field around the yarn is stronger within a thin yarn compared to a thick yarn. This means that in our PEDOT:PSS cell concept we could not achieve a better performing device with thinner yarn of higher resistance compared to the thicker yarns of lower resistance. The aim of chapter 7 was to quantify the amount of useful accumulated energy in the developed charge storage device with stainless steel yarn electrodes, despite their selfdischarge. Flexible capacitors were made using stainless steel yarns as yarn electrodes on textile substrate. The electrolyte material used was a dispersion of polyethylene dioxythiophene: polystyrene sulphonate (PEDOT:PSS) from Ossila company. It was not easy to directly determine the energy stored in these devices, therefore the energy in the cell was estimated from the energy it supplied to the voltmeter. Using the equation relating energy to the capacitance, the capacity of the developed device was estimated to be 180µF. The capacitor was charged normally and used to power a calculator. We stretched the capacitor and charged it at an arbitrary voltage of 3 V and roughly 40 minutes instead of the normal 1.5V for 2hrs. After charging the capacitor for sufficient time at 3 V, the accumulated charge in the device was about 1.2 V, but for a short time. In these experiments too, a sharp voltage drop was observed initially for a few seconds as it has been throughout the other experiments, then the voltage discharge slows down. Despite the self-discharge of the capacitor, a calculator (TOSHIBA LC-810) could run on the developed cell for 37 seconds. This work is concluded by chapter 8 with a list of main achievements presented in this dissertation and recommendations for future work.