3.1 Structure and textural characterization
The XRD diffractogram (Fig. 2) shows a strong peak at 23° in 2θ and two weak peaks at 10° and 44° which reveals a typical pattern for an amorphous and disordered structure. To obtain more insight from the disorder structure a Raman spectroscopy analysis was carried out. Fig. 3 shows the Raman spectra and as can be seen it shows two intense bands, at 1340 cm-1 (D band) and at 1560 cm-1 (G band) the presence of the characteristic G peak arises from the stretching of the C–C bond in graphitic materials and is usual to all sp2 carbon materials [22]. The depicted D peak is due to lattice vibrations of the sp2-sp3 bonds of the amorphous carbon present in interstitial sites and the distorted graphitic lattice. It can be seen that the D peak shows a higher intensity compared to the G peak, with an intensity ratio of R = (ID IG-1) = 1.24, that confirms the presence of a high degree of a disordered carbon material, as observe from the characteristic behavior of these peaks [23].
The surface texture of the AC was investigated by N2 gas adsorption - desorption analysis, specific surface areas were determined via the BET method (SBET). The N2 adsorption and desorption isotherms for the AC are shown in Fig. 4. The analysis reveals a type I (b) isotherm according to IUPAC [24] for a microporous material. The isotherm is concave to the P P0-1 axis and the amount of adsorbed gas approaches without any limiting adsorption at high relative pressure (P P0-1). It is suggested that this type of isotherm could be found in microporous solids having a pore size distribution with a mixture of wide micropores and narrow mesopores instead of type I(a) isotherms that are found in microporous materials having mainly narrow micropores (< 1 nm) [24]. The textural properties of the AC are summarized in Table 1. The apparent BET reveals a high specific surface area value of 1460 m2 g−1 and the total pore volume estimated was 0.69 cm3 g-1, the mean pore width was found about 1.8 nm as shown in Fig. 5. For the QSDFT estimation of textural analysis a slit model pore was applied. It is worth to notice that the presence of micropores and mesopores are important for charge storage since micropores are responsible for ion traps during the energy storage and the mesopores act as channels for ion transport from the electrolyte to the interface between the electrode and electrolyte [25].
3.2 Electrochemical characterization
One way to raise the energy and power density in EDLCs is by increasing the cell voltage above 1.24 V of currently available EDLCs using AC. To analyze performance of the electrodes a symmetric SC device was prepared to carry-out electrochemical tests at three different operating voltages; 1.5, 1.6, and 1.8 V in 1M Li2SO4 solution as electrolyte and selected from previous CV experiments. Instead of using the long cycling charge-discharge performance test we carry out an alternative way known as potentiostatic floating test (or constant voltage hold test) more efficient for determination of the EDLCs stability limits [18-23]. This plot is sketched in figure 1 with 5 GCD cycles and 10 h of floating where for voltage tests and current are plotted. In the floating test, the capacitor is constantly exposed to voltage values above the assumed stability limit. Fig. 6a shows at 1.5 V and 200 h, the behavior of a cell, indicating 25% decay in capacitance at 170 h, with a capacitance of 297 F g-1 calculated at the fifth cycle of every period, the capacitance loss of electrodes, it is possibly ascribed to higher potential in the positive electrode. On the other hand, the cell tested at 1.6 V and 200 h, it is possible to observe a 25% decay at 150 h with a capacitance of 285 F g-1. Finally, for analysis at 1.8 V we can observe the shortest decay capacitance period of 25% at 90 h, indicating a capacitance of 264 F g-1 with a very high voltage and low stability, considering the extensively acceptable criteria for measuring end of life for SC (from 20% to 30% capacitance decay) [23]. The device at 1.5 V showed the greater stability and voltage hold for more hours, capacitance decay of 30% represents the point where SC reach their maximum active lifetime, this test was stopped at 110 h. For comparisons purposes in floating tests and according to other studies, by Bello et al., [7] their results showed capacitance loss of 25% with a constant voltage of 1.2 V for up to 120 h of voltage holding, in other cases the cell voltage increased above 2.5–2.8 V using electrolytes such as ionic liquids, with a decrease of the capacitance observed after 200 h of test at 3.75 V. The capacitance loss of 30% was observed after 340 h [14-16].
Fig. 6b illustrates the resistance according to the floating time, it was possible to notice that the resistance increased a 100% (1.4 Ω) at 150 h of voltage holding at 1.5 V. For the cell at 1.6 V, resistance increased 100% at 140 h and the cell tested at 1.8 V, showed that resistance increased rapidly at 75 h, as resistance increased a moderate degradation in the electrode occurred possibly associated to higher potential in the positive electrode [24-25]
This good electrochemical behavior could be ascribed to the micropore/mesopore distribution within the carbon material which provides the relevant pore sites accessible to electrolyte ions along with short diffusion paths supporting fast ions transport during the lengthy charge-discharge process [26-29]. Floating experiments were used to demonstrate stability in capacitors, as the time elapses, the electrode suffers degradation, and the advantage of an electric double layer capacitor is to increases the lifetime which allows it to be very stable above the nominal cell voltage and can be discharged to zero without harm as discussed previously in Fig. 1. [30]
Electrochemical impedance spectroscopy is a powerful technique that allows measuring both the equivalent resistance and the capacitance at different frequencies and helps to realize the performance of the electrode at open circuit potential or at any alternal current signal. Impedance measurements studies where performed with a voltage amplitude of 10 mV in a frequency range from100 kHz to 0.01 Hz. The Nyquist plot presented in Fig. 6c (red) exhibits a semicircle in the high to mid-frequency area which means the charge transfer resistance, it is also shown a vertical spike (line) in the low frequency area, which indicates an ideal capacitive behavior of carbon electrode material. Equivalent series resistance (ESR) originated from the total resistance of the electrolyte, the resistance of the active materials and the resistance at the interface between the active electrode material and current collector. The Nyquist plot after floating test it is also presented in Fig. 6c (black), showing a little deviation to the initial curve corroborating a minuscule deterioration of the AC electrode material. Before and after floating test in the Nyquist plots was observed comparable internal resistance values (ESR = 0.5 Ω and 1.1 Ω respectively) shown in the figure, indicating a favorable contact between AC material and separators, current collectors and electrolyte. [31]
Fig. 6d presents the CV plots (20 mV) of the cell before and after voltage holding, showing an almost rectangular shape up to 1.5 V indicative of loss in capacitance as mentioned in Fig. 6a previous. The results observed in the CV plots evidently show that floating test does not have a notable degradation effect in the AC electrodes of the symmetric device.
Furthermore, it is acceptable to conclude the performance of the electrode material investigated presents successful and acceptable values, highlighting the promise of this material for electrochemical high power applications.