Figure 6.1 shows cyclic voltammograms measured on cells at each operating temperature from -30 °C to 60 °C for the two electrolytes. The CV curves in Figure 6.1 (a), obtained from the aqueous electrolyte (3 M H2SO4) demonstrate a saturated area of the current responses in the temperature range from -20 °C to 25 °C, which represents the steady charge/discharge capacity. However, the current response was de-saturated at -30 °C. This behavior is
150
attributed to the water-based electrolytes characteristics which experience phase-change at close to their freezing points. For example, the freezing point of sulfuric acid solution (96 %) is about -20 °C [183], which leads to a low current response. At low temperatures in comparison to the aqueous electrolyte, the organic electrolyte (1 M Et4NBF4/PC) current responses are more distorted from the ideal rectangular shape, as shown in Figure 6.1 (b).
(a)
(b)
Figure 6.1 Cyclic voltammetry of (a) 3M H2SO4 and (b) 1M Et4NBF4/PC electrolyte at operating temperature from -30 to 60 °C at a scan rate of 20 mV/s , respectively
30 30
151
In general, there is a slow rise in current at the beginning of each charge/discharge process, which is caused by the internal resistance (ESR) of a cell. When it comes to the effect of temperature, a slower current rise in current and two rounded corners of the rectangle were observed at the beginning of the charge and discharge processes at lower temperatures. This indicates a significant increase in internal resistance, especially for the non-aqueous electrolyte, as the operating temperature goes down. The decreasing rates were found to be dependent on temperature. This observation agrees with previous studies [184]. On the other hand, both electrolytes displayed the slightly sharp tails of CV curves near to 1 V in H2SO4 and 2.7 V in Et4NBF4/PC when the temperature is at 60 °C. This is caused by the decomposition of electrolytes [185].
One important issue of temperature change on an EDLC is the ability to retain specified performance levels, including capacitance, internal resistance and power/energy density. These characteristics for each electrolyte were evaluated by the constant current charge/discharge measurements.
(a) (b)
Figure 6.2 Discharging process at constant current (30 C rate) and ohmic drop (ΔV) for (a) 3M H2SO4 and (b) 1M Et4NBF4/PC electrolyte at various temperatures, respectively
Ohmic drop (ΔV)
152
Figure 6.2 shows discharge curves of a cell at different temperatures operating at a
constant current density of 0.13 A/g in H2SO4 and 0.28 A/g in Et4NBF4/PC electrolyte, respectively. In this figure, the IR drop was observed at the beginning of discharging process. The voltage drop (Ohmic drop) was used to estimate the internal resistance (ESR) deviation with temperature change. As a result, Figure 6.3 shows the plots of the specific capacitance retention of EDLCs and ESR variation calculated from the discharging curves in Figure 6.2. The aqueous electrolyte was studied over the temperature range of -20 – 60 °C, and its specific capacitance was found to change from110F/g to 130 F/g, which indicates a
maximum loss of 15 % of its capacitance. The capacitance decreases to around 100 F/g at the temperature of -30 °C. On the other hand, the organic electrolyte showed a loss of 32 % of its capacitance, from 80 F/gat 25 °C to 52 F/g at -30 °C. For the temperature effect on the internal resistance, the aqueous electrolyte showed an increase of 10 % in ESR, while the organic electrolyte displayed a more significant increase of 60 % over the tested temperature range. Both electrolytes measured their highest resistance at -30 °C. This is attributed to the instability of electrolytes near their freezing temperatures. Moreover, it should be noticed that the variations of ESR and the capacitance can result in lowering of energy/power density as shown below:
(57) (58)
where m is the mass of an electrode and V is the operating voltage. In general, the organic electrolyte can provide larger energy/power densities than the aqueous ones due to the greater operating voltage.
153 (a)
(b)
Figure 6.3 Capacitance retention and internal resistance (ESR) deviation for (a) 3M H2SO4 and (b) 1M Et4NBF4/PC electrolyte depending on temperatures, respectively
In contrast, in Figure 6.4 it is can be seen that as the temperatures decreases, the organic electrolyte showed a decrease in both energy and power densities while no significant change is observed in the aqueous electrolyte. The organic electrolyte achieved a power density of 470 W k/g at -30 °C, 580 W/kg at 60 °C and an energy density of 1.8 Wh/kg (-30 °C), 6.3 Wh/kg (60 °C), respectively. Whereas, a power density of around 100 W/kg and an energy density of 1.7 Wh/kg over the entire temperature range were observed in the case of the
154
aqueous electrolyte. This characteristic is associated with the variation of their capacitance and ESR with change in temperature as shown in Figure 6.3.
Figure 6.4 Ragone chart with specific power (W/kg)/energy density (Wh/kg) for 3M H2SO4 and 1M Et4NBF4/PC electrolytes operating from Vmin to Vmax at different temperatures
Figure 6.5 depicts the Nyquist plots, where x-axis is the real impedance and y-axis is the imaginary impedance, measured from 10 mHz to 100 kHz at different temperatures for each electrolyte. For both electrolytes, the curves were right-shifted with decreasing temperature, which means increasing resistances. In comparing the two electrolytes the organic electrolyte shows more right-shifted in its curves, shown in Figure 6.5 (a) and (b), indicating a larger deviation in resistance over the given temperature change. In the Nyquist plot, the first intersection of the x-axis in the high-frequency region relates to the ohmic resistance of the electrolyte while the diameter of following semicircle in the mid-frequency region provides
1M Et4NBF4/PC
(1.2 ~ 2.4 V)
155
information on the interfacial resistance of the EDLC device. Compared to the aqueous electrolyte resistances shown in Figure 6.5 (a), the organic electrolyte resistances, shown in Figure 6.5 (b), both display greater dependency on temperature.
(a)
(b)
Figure 6.5 Nyquist plots for cells in (a) 3M H2SO4 and (b) 1M Et4NBF4/PC electrolytes, including enlarged images at right side
This phenomenon is in agreement with the result from the constant current charge/discharge study. The capacitive behavior of the electrolytes is investigated by studying the slopes of corresponding vertical curves (seen in the enlarged images in Figure 6.5). At low frequencies,
156
an ideal capacitor would provide a straight line with frequency decreasing. It can be obviously observed that the aqueous electrolyte shows the nearly linear slopes while the slopes from the organic electrolyte decline as decreasing temperature. This indicates that the aqueous electrolyte produces stable capacitances while the values of the organic electrolyte significantly decrease with a lower temperature. The same tendency was found in the Bode plots, see Figure 6.6 and Figure 6.7.
(a)
(b)
Figure 6.6 Bode plots of real capacitance for (a) 3M H2SO4 and (b) 1M Et4NBF4/PC electrolytes
157 (a)
(b)
Figure 6.7 Bode plots of real impedance for (a) 3M H2SO4 and (b) 1M Et4NBF4/PC electrolytes
Figure 6.6 demonstrates that the capacitance in 10 mHz was reduced by decreasing the operating temperature. This variation is more considerable than in that of the organic electrolyte. Similarly, for the real resistance shown in Figure 6.7, the aqueous electrolyte’s resistances increase slightly with decreasing temperature, while the organic electrolyte shows a larger temperature dependency.
158