Dibujo colaborativo

In another set of experiments, we aimed at establishing the size of load the developed device could support and for how long it could support it. Different sizes of load resistors were used in these experiments. The load resistor R, was always connected in parallel to the cell as shown in Figure 3.11. The cell connected was charged for 2hrs and then left to discharge. In the initial stages of this research, the voltage decay measurements were recorded by a camera. The readings were recorded from the voltmeter (multimetre digitool digi 16). The charge - discharge procedure was repeated up to three times on each device that was tested for these experiments. Normally in this we confirmed that the developed devices were rechargeable, and the voltage decay result were more or less the same with no significant change in the profile. However, at this point it was not established how many times the device could be recharged.

Figure 3.11 Circuit connection with load resistor

The load resistor experiments were carried out as a way of quantifying the amount of useful residual charge in our cells. We could not easily calculate the power stored within the cell after charging, due to the voltage decay behaviour of the developed cells. Also for the fact that we were still doubting if the developed device was a battery or a

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capacitor, as it can be seen in our first chapter, the two have different ratings, hence requires different approaches in the efforts to quantify the energy in them.

From the experiments done so far it is obvious that the PEDOT:PSS is self-discharging and the input impedance of the voltage metre is very high. To quantify this self- discharge, a resistor R (Figure 3.11) one at a time of 978 kOhm, 268 kOhm and 100 kOhm was connected in parallel with the PEDOT:PSS cell. After charging for 120 minutes at a constant voltage of 1.5 V, the decaying voltage V was recorded with the resistor connected. Devices with silver coated PBO filament yarn electrodes could not support the load resistors. These results are not shown, because these specific devices could barely support a load resistor. This could be due to the fact that silver coated yarn electrode being a better conductor (lower resistance 3.4 Ω/m) than the stainless steel electrode 9.7 Ω/m), it could release the little stored charges faster (say in milliseconds). This could not be easily observed. Additionally the silver coated yarn electrodes had less amounts of stored charge in them.

The obtained voltage decay curves from pure stainless steel filament yarn electrodes device are shown in Figure 3.12 & Figure 3.13. It can be observed that the decay is faster at the initial phase of the curve as it was without the resistor, but with lower values. It can also be observed that the lower the load resistor R the faster the voltage decay (analogous with ohms law). This means that the device can only power high load resistors which require very little current and therefore can be used for voltage stabilization if the resistor is not too small.

Figure 3.12 Result with load resistors for pure stainless steel filaments yarn electrodes device, voltage decay behaviour for the first 5 000 seconds.

0 0,2 0,4 0,6 0,8 1 1,2 1,4 0 1000 2000 3000 4000 5000 Vo lt a g e (V) Time (s) No resistor 978Kohm resistor 268Kohm resistor 100Kohm resistor

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Figure 3.13 Result with load resistors for pure stainless steel filament yarn electrodes device, voltage decay behaviour for up to 10 000 seconds.

In all experiments done so far, the voltage metre with its input resistance of 10 MOhm was connected to the devices. One may wonder if we were dealing with a self-discharge of PEDOT:PSS cell or a discharge through the 10 MOhm input resistance of the voltage metre. Therefore, a different discharge experiment was done according to the connection circuit. The voltage metre was disconnected regularly for periods of 5 minutes. The metre was only connected for a short time, just enough to measure the voltage. The voltage decay graphs obtained with this experiment turned out to be almost coinciding with the curves shown previously. The conclusion is that we were really measuring the self-discharge of the PEDOT:PSS cells. The voltage metre has a negligible influence, it can be stated that the PEDOT:PSS cell has itself an internal resistance much lower than 10 MOhm.

From the discharge curves obtained with different values of the load resistor, the internal resistance of the PEDOT:PSS cell could be estimated to be around 300 kOhm. From most of the graphs of the voltage decay, one can observe time constants in the order of 1 hour or 3600 s. If the cell would be considered as a capacitor C connected in parallel with a resistance R of 10 MOhm, one has: τ = RC = 3 600 s

Using the capacitance equation 𝑽 = 𝑽𝟎 𝒆−𝒕 𝝉⁄ Equation 6 0 0,2 0,4 0,6 0,8 1 1,2 1,4 0 2000 4000 6000 8000 10000 Vo lta g e (V) Time (s) No resistor 978Kohm resistor 268Kohm resistor 100Kohm resistor

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where V is the voltage at any point within the curve for a perfect capacitor, V0 the initial

charging voltage, t time and RC the time constant, from which we get an estimated value of the capacitance (C) as 360µF.

This is a quite high value taking into account the limited area of the electrodes in contact with the dielectric/electrolyte. A possible conclusion is that only electrolytic phenomena could be responsible for such a high value, i.e. mobile ions (in strong ionic electrolyte) move under the influence of the applied electric field. However, this does not exclude the possibility of the combined electrochemical and capacitive effect as in the case of pseudo capacitors as discussed in chapter one. Also if there is an electrochemical effect it would be a unique one and basically within the electrolyte material, since the yarn electrodes were from the same material and the positive and negative electrodes could be interchanged in the experiments. This can never happen in a conventional battery or supercapacitor, an explosion may occur.

Cyclic voltammetry is one of the methods that could be used in categorizing the electrolytic phenomena, but it was not used to characterize the developed devices due to the enormous time constant involved.