The reduction from GO to rGO is confirmed by the FT-IR and XPS spectra as shown in Fig. 1. The FT-IR spectra of Fig. 1(a) represent that the intensities of peaks relating to carbon-oxygen bonds shown in GO decrease in rGO; carboxyl/carbonyl stretching (C=O; 1737 cm−1), epoxy (C-O; 1368 cm−1) and alkoxy (C-O; 1215 cm−1) stretching. On the other hand, the intensity of peak corresponding to aromatic C=C stretching (1659 cm−1) increases in rGO, which demonstrates the occurrence of reduction from GO to rGO. Figure 1(b) shows the XPS spectra of the GO and the rGO, indicating the peaks associated with C1s and O1s. Compared with the peak intensities of C1s and O1s for the GO and the rGO, the atomic ratio of C1s to O1s increases from 1.8 for the GO to 6.2 for the rGO, which means the deoxygenation of GO by the thermal reduction process. In the C1s XPS spectra of the GO as shown in Fig. 1(c), we found four components to account for the overlapping C1s features: C-C (sp3 C, peak curve 1; 284.5 eV), C-O (epoxy C, peak curve 2; 286.8 eV), C=O (carbonyl C, peak curve 3; 287.8 eV) and -COOH (carboxyl C, peak curve 4; 289.0 eV)12. On the other hand, low-intensity peaks related with carbon-oxygen bonds appear in the C1s XPS spectra of the rGO as exhibited in Fig. 1(d). In addition, the proportion of the sp3 carbon peak increases from 40.5 to 72.1%, indicating that the GO was sufficiently deoxygenated.

Figure 1 (a) FT-IR spectra of the GO and rGO. (b) XPS spectra of the GO and the rGO and C1s XPS spectra of (c) the GO and (d) the rGO. Full size image

Figure 2 shows the electrochemical performance of the rGO SC and the rGO-CB SC. The CV analysis of the rGO SC and rGO-CB SC were conducted in a voltage range of 0 to 1.0 V at scan rates of 10, 20, 50, and 100 mV/s, as shown in Fig. 2(a,b). The CV curves of the rGO-CB SC show the larger areas and more quasi-rectangular shapes with smaller distortions for four different scan rates than those of the rGO SC since the added CB improves the conductivity of the electrodes. Considering that the relationship between the electrical properties of SCs and the electrical conductivity of electrodes13,14, the rGO-CB SC has the better capacitive behavior and higher mobility of charge carriers in the electrodes, compared to the rGO SC. Figure 2(c,d) exhibit the GCD curves of the rGO SC and rGO-CB SC, respectively, and all curves show the symmetric triangular shapes indicating a high faradaic efficiency during electrochemical reactions15. The specific capacitance is calculated by the equation C = (I/m)·Δt/ΔV, where I is the applied current, m is the total mass of active electrode materials, Δt is the discharge time, and ΔV is the cell voltage after subtracting the IR drop from discharging voltage16. Herein, the IR drop means an instantaneous voltage drop at the beginning of the discharging state and is attributed to the equivalent series resistance (ESR) that is combined by the resistances of the electrolyte, the active electrode materials, the current collectors and the contacts of the SC17. Figure 2(e) exhibits the specific capacitances of the rGO SC and the rGO-CB SC, and at a current density of 0.2 A/g, the specific capacitances of rGO SC and rGO-CB SC are 115 F/g and 160 F/g, respectively. As a current density increases, the specific capacitances of both SCs slightly decrease. Nevertheless, the rGO-CB SC shows 40% larger specific capacitance than the rGO SC. Figure 2(f) represents the IR drops of the rGO and the rGO-CB SCs, indicating that the IR drops of the rGO-CB SC are smaller than those of the rGO SC in all cases.

Figure 2 Cyclic voltammetry curves of (a) the rGO SC and (b) the rGO-CB SC. Galvanostatic charge/discharge curves of (c) the rGO SC and (d) the rGO-CB SC. Specific capacitance (e) plots and IR drops (f) of the rGO SC and the rGO-CB SC at various current densities. Full size image

The effects of CB on the ESR of SCs are examined by the impedance spectroscopy analysis. Figure 3 shows the Nyquist plots (a) of the rGO SC and the rGO-CB SC in a frequency range from 0.1 Hz to 0.1 MHz and the magnified Nyquist plots (b, c) in a higher frequency region; in the Nyquist plots, the Z′ axis and the Z″ axis are the real and imaginary parts of complex impedance, respectively. The Nyquist plots of both SCs match up with Randle’s equivalent circuit (in the inset of Fig. 3(a)) and consist of semicircles, diffusion lines and capacitive lines. In the semicircle of Nyquist plot, the left intersection on the Z′ axis indicates electrolyte resistance (R s ) and the diameter of the semicircle represents the charge transfer resistance (R ct ) including both the electronic and ionic resistances. As shown in Fig. 3(b,c), there is no significant difference between the R s values of the rGO SC and the rGO-CB SC because the same electrolyte and separator were used for both the rGO SC and the rGO-CB SC. The electronic resistance depends on the electrical conductivity of electrode materials (rGO or rGO-CB) and the electrical contact at the interface between the electrode material and the current collector. In the low frequency region, the x-intercept of the Nyquist plot indicates the internal resistance (R int ) of the SC18. Compared with the rGO SC (R ct = 0.4 Ω and R int = 42 Ω), the rGO-CB SC has relatively low values of R ct (0.2 Ω) and R int (18 Ω), indicating that the addition of conductive CB improves electrical contacts between rGO sheets and as a result, lowers the internal resistance of the SC. According to the previous study19, the conductive CB provides good electrical connects between rGO sheets since the CB serves as a linker between rGO sheets, which accords with our results. Figure 3(d) exhibits the Bode plots of the rGO SC and rGO-CB SC. The red dash lines indicate the knee frequency where the (−) phase angle reaches 45° and the resistance and the reactance have the same magnitude at that point. From the reciprocal of the knee frequency, the relaxation time constants (τ) of the rGO SC and rGO-CB SC are calculated to be 34 and 25 ms, respectively. Low τ indicates the fast frequency response capability. Considering that the frequency response capability is related to the internal resistance and the electrical conductivity of the SC electrodes20, it is clear that the addition of CB lowers the internal resistance and improves the electrical conductivity of SC electrodes. The Randle’s circuit elements and τ of the rGO SC and rGO-CB SC are summarized in Table 1.

Figure 3 (a–c) Nyquist plots of the rGO SC and rGO-CB SC for a frequency range from 0.1 MHz to 0.1 Hz, and (d) Bode plot of the rGO SC and rGO-CB SC. Full size image

Table 1 Randle’s equivalent circuit elements of rGO SC and rGO-CB SC. Full size table

In order to investigate the effect of CB on thermal stability of SCs, the GCD curves of the rGO SC and rGO-CB SC are obtained in a temperature range from 30 to 90 °C at a current density of 2 A/g, as shown in Fig. 4(a,b). As the temperature increases, the rGO SC shows larger variations in charging/discharging times than the rGO-CB SC, which is directly related to the variations in specific capacitances of the rGO SC and the rGO-CB SC. Figure 4(c) represents the variations in specific capacitances of the rGO SC and the rGO-CB SC as a function of temperature. The rGO-CB SC shows a smaller variation in specific capacitance (12%) than that of rGO SC (30%) as the temperature increases from 30 to 90 °C. For the rGO-CB SC, the smaller variation in the specific capacitance originates from the lowering of the thermal conductivity of the electrodes and thereby from the reduction of the amount of heat transferred to the electrolyte; note that the thermal conductivity of CB (0.02 W/m∙K) is lower than that of rGO (0.14~2.87 W/m∙K) and that the added CB induces the phonon scattering on the interface in contact with the rGO sheets21,22. Consequently, the thermal stability of the rGO-CB SC is superior to that of the rGO SC. The beneficial effect of the addition of CB into the electrodes is attested by capacitance retention test done at 90 °C. The rGO-CB SC exhibits the good capacitance retention performance during 1000 charge/discharge cycles, whereas the function of the rGO SC fails after the 166th charge/discharge cycle as shown in Fig. 4(d).

Figure 4 Galvanostatic curves of (a) the rGO SC and (b) the rGO-CB SC as a function of temperature. (c) Variations in specific capacitance of the rGO SC and the rGO-CB SC as a function of temperature. (d) Capacitance retention test of rGO SC and rGO-CB SC for 1000 charge/discharge cycles at 90 °C. Full size image

Furthermore, the effect of CB on thermal stability of SCs is examined with the impedance spectroscopy. Figure 5 show the Nyquist plots of the rGO SC and the rGO-CB SC in a 0.1 Hz to 0.1 MHz frequency range at temperatures of 30~90 °C. As the temperature increases, the curves in the Nyquist plots of the rGO SC and the rGO-CB SC are shifted to the higher frequency region in the Z’ direction because of the decrease in the ESR of the rGO SC and the rGO-CB SC shown in Fig. 5(c). As the temperature increases up to 90 °C, the variations in ESR of the rGO SC and the rGO-CB SC are 51% and 30%, respectively. This reveals that the addition of CB with relatively lower thermal conductivity and relatively higher electrical conductivity into the rGO electrodes improves the thermal stability and capacitive performances of SCs. To further investigate the changes in the charge transfer characteristics, we analyzed the Bode plots of the rGO SC and rGO-CB SC as shown in Fig. 5(d,e). As temperature increases from 30 to 90 °C, the phase versus frequency curves of the rGO SC and rGO-CB SC are shifted toward the lower frequency region, and the knee frequency values of both SCs decrease. The amount of the change in τ of the rGO SC and rGO-CB SC, derived from the reciprocal of the knee frequency, are plotted in Fig. 5(f), indicating that the changes in τ of the rGO SC and rGO-CB SC increase up to 21% and 40% as the temperature increase from 30 to 90 °C. Owing to the temperature-dependent charge transfer characteristics of the SCs, the addition of CB being a heat-resistant additive helps improve the performance of thermal stable SCs.