Chapter 1 Introduction
6.3 Results and Discussion
6.3.1 Capacitive behavior of oxidized CNT in KOH
while for experiments in 1 M Et4NBF4/PC electrolyte, it is limited to 2.5V to prevent a faradaic reaction with CNT array electrodes and titanium current collectors. The entire cyclic voltammetry characterization is conducted at a scan rate of 100mV /s.
The specific capacitance of CNT array electrodes with various degrees of oxidation is measured using galvanostatic charge-discharge cycles in a two electrodes configu- ration. The galvanostatic charge-discharge cycles are performed using a potentiostat (Biologic SP-200) at a constant current density, and the response potential is recorded and plotted against time. The specific capacitance measurement is conducted at a constant current density of 5A/g. Note that the current density and specific capaci- tance are determined and measured based on the mass of CNT array electrodes only.
The mass of current collectors, separators, electrolytes, and packaging are not taken into consideration. The cyclic lifetime of CNT based EDLC is also measured using galvanostatic charge-discharge cycles at a constant current density of 5A/g. The per- formance of CNT based EDLC in terms of their energy density and power density is calculated from the specific capacitance and IR drop obtained by galvanostatic charge-discharge cycles analysis.
tric constant of the electrolyte, while charge transfer resistance is influenced by the compatibility between electrodes and the electrolyte. Contact resistance is the resis- tance between electrodes and current collectors, and has to be minimized during the fabrication process. Because of the presence of Faradaic processes and the nonzero ESR, the shape of cyclic voltammogram of an EDLC is typically trapezoidal with non-constant current during the linear potential sweep, especially at the beginning and the end of the scan.
The cyclic voltammograms of CNT arrays in 6M KOH in two electrodes configura- tion show a smooth and symmetrical shape over a potential range of 0-1 V. In general, this behavior can be observed from CNT arrays with low oxygen content. The cyclic voltammograms for CNT arrays with a very low O/C ratio of 3% are featureless (Fig- ure 6.4.a). Such featureless voltammograms indicate the absence of Faradaic reaction during the charge and discharge cycles. A linear increase of response current during the linear potential sweep indicates the nonzero finite ESR of the CNT arrays. Note that the response current recorded from these CNT arrays is extremely low, in the order of 10−5 mA. Such low response current can be associated with the extremely low capacitance and tremendously high ESR of these CNT arrays. Since these CNT arrays are actually superhydrophobic, there exists a thin layer of air at the interface between their surface and the electrolyte. Such air layer inhibits electrons transfer from the arrays and blocks charged ions to approach the surface of the CNT arrays from the electrolyte.
Similar behavior is observed for CNT arrays with a slightly higher O/C ratio of 6%. The cyclic voltammograms for these CNT arrays are almost featureless, although a small peak at 0.6 V can be observed (Figure 6.4.b). Compared to those of CNT arrays with an O/C ratio of 3%, these cyclic voltammograms exhibit much higher response currents. A higher response current can be attributed to the presence of higher concentration of oxygenated groups on the surface of these CNT arrays. As discussed in Chapter 3, the presence of polar oxygenated groups on the surface of CNT improves its wettability in highly polar electrolytes. Interestingly, an increase of O/C ratio by 3% is enough to increase the response current four orders of magnitude from
Figure 6.4. Cyclic voltammogram of hydrophobic CNT arrays in 6 M KOH. These CNT arrays have an O/C ratio of 3% (a) and 6% (b).
Figure 6.5. Cyclic voltammogram of hydrophilic CNT arrays in KOH. These CNT arrays have an O/C ratio of 12% (a) and 15% (b).
10−5 mA to 10−1 mA.
Also note that, compared to those of CNT arrays with an O/C ratio of 3%, the cyclic voltammograms of CNT arrays with an O/C ratio of 6% are less symmetric.
The response current recorded during charging periods has a steeper slope than that during discharging periods. This suggests that the ESR of these CNT arrays is higher during the charging periods than during the discharging ones. This phenomena is typically caused by the change in electroactive species on the surface of CNT arrays.
Such change can also be observed at a potential higher than 0.8 V. A dramatic increase in the response current suggests the occurrence of potential-dependent redox reactions where the relative quantity of electroactive species is dependent on the potential. These electroactive species may be related to the presence of oxygenated groups on the surface of CNT arrays.
The cyclic voltammograms of CNT arrays with a much higher O/C ratio of 13%
also exhibit a smooth and almost symmetrical shape over a potential range of 0-1 V.
Unlike those of CNT arrays with O/C ratio of 6%, the cyclic voltammograms for these CNT arrays are featureless (Figure 6.5.a). A very sharp transient response when the potential sweep changes signs indicates a rapid charge storage and delivery kinetic of these CNT arrays. A rapid charge storage and delivery kinetic can be attributed to the good wettability of these CNT arrays by the electrolyte. A better wettability also results in the significant increase of the recorded response currents, which are now in the order of 1 mA. Consequently, the capacitance of these CNT arrays is expected to be significantly higher than that of CNT arrays with an O/C ratio of 6%.
A totally different behavior is observed for CNT arrays with an even higher O/C ratio of 21%. The cyclic voltammograms of these CNT arrays are no longer smooth and symmetrical (Figure 6.5.b). In fact, the recorded response current increases almost linearly as the increase of potential with a very steep slope. The large re- sponse current hysteresis typically observed between charge and discharge cycles is also diminished. These voltammograms suggest that the double layer capacitance of these CNT arrays is lower than that of CNT arrays with lower O/C ratio. They also imply that the ESR of these CNT arrays is significantly higher than that of
CNT arrays with lower O/C ratio. This behavior may be attributed to the occur- rence of potential-dependent redox reactions where the relative quantity of electroac- tive species is dependent on the potential. As mentioned earlier, these electroactive species may be related to the presence of oxygenated groups on the surface of CNT arrays. Since a higher O/C ratio means a higher concentration of such oxygenated groups, it is expected that the quantity of electroactive species involves during the charging-discharging cycles is also higher.