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.