Chapter 1 Introduction
6.3 Results and Discussion
6.3.2 Capacitive behavior of oxidized CNT in Et4NBF4 / PCPC
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.
6.3.2 Capacitive behavior of oxidized CNT in Et4NBF4 /
Figure 6.6. Cyclic voltammogram of hydrophobic CNT arrays in Et4NBF4.
Figure 6.7. Cyclic voltammogram of hydrophilic CNT arrays in Et4NBF4.
lar CNT arrays, even at room temperature. Although these CNT are not completely wetted by propylene carbonate, the absence of liquid-vapor-solid interface improves the response current by several orders of magnitude.
Similar behavior is observed for CNT arrays with a slightly higher O/C ratio of 6%. The cyclic voltammograms for these CNT arrays are also featureless (Fig- ure 6.6.b). Compared to those of CNT arrays with an O/C ratio of 3%, these cyclic voltammograms exhibit slightly higher response currents. A higher response current can be attributed to the presence of a 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. A higher response current can also be attributed to the increase of the number of mesopores and macropores of CNT arrays due to the oxidation process.
An increase of the number of mesopores and macropores leads to an increase of ef- fective interface surface area between the CNT and the electrolyte. The correlation between oxidation process and the surface area of CNT arrays can be read in Chap- ter 4. Note that CNT arrays with an O/C ratio of 6% exhibit a sharper transient response when the potential sweep changes sign than the CNT arrays with an O/C ratio of 3%. This sharper transient response indicates a faster charge storage and delivery kinetic, which can be attributed to a better wettability of these CNT arrays by the electrolyte. Also note that, compared to those CNT arrays with an O/C ra- tio of 3%, the cyclic voltammograms of these CNT arrays are less symmetric. The response current recorded during discharging periods has a steeper slope than that during charging periods. This suggests that the ESR of these CNT arrays is higher during the discharging periods than during the charging ones. This phenomena is typically caused by the discharge of electroactive species from 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 featureless shape over a potential range of 0-2.5 V. Similar to those of CNT arrays with O/C ratio of 6%, the cyclic voltammograms for these CNT arrays are also not perfectly symmetric (Figure 6.7.a). 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 charging ones. This phenomena is typically caused by significant changes in the quantity of electroactive species on the surface of CNT arrays during the charging period. A very sharp transient response when the potential sweep changes sign 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 a slight increase of the recorded response currents. An increase of response current with negligible increase of slope implies that the electrochemical characteristic of CNT arrays is dominated by their capacitive behavior (Lufrano and Staiti, 2004; Chen et al., 2002). Consequently, the capacitance of these CNT arrays is expected to be slightly 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 17%. The cyclic voltammograms of these CNT arrays are no longer smooth and symmetrical (Figure 6.7.b). In fact, the recorded response current increases almost exponentially as the increase of potential with a very steep slope, suggesting the occurrence of potential-dependent redox reactions where the relative quantity of active redox species is dependent on the potential. The large response 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 caused by the occurrence of strong potential-dependent redox reactions during charging and discharging cycles. As mentioned earlier, it is very likely that the oxygenated groups adsorbed on the surface of CNT are involved in these Faradaic redox reactions.
Figure 6.8. Galvanostatic charge discharge cycle of CNT arrays with different oxida- tion levels in KOH electrolyte.