Synthese van vrijstaande flexibele rGO/MWCNT-films voor symmetrische supercondensatortoepassing

Schematic diagram of hydrated ionic radii of the ions associated with different electrolytes used for GP10C electrode measurement

Electrochemical Performance of rGO/MWCNT Films

We also investigated the effect of MWCNT addition on the electrochemical performance of rGO/MWCNT films in a three-electrode setup with 2 M KOH electrolyte. Figure 8a depicts the CV curves of as-synthesized rGO, GP5C, GP10C, and GP15C film electrodes recorded at a scan rate of 50 mVs ⁻¹ in the potential range − 0.9 to 0.0 V. Evidentially, in comparison to GP, GP5C, and GP15C, the CV curve of GP10C occupies the larger area, and it belongs to nearly rectangular shape, implying the electrical double-layer (EDL) capacitive behavior of this electrode with higher SC value [52]. Figure 8b represents the GCD curves of all the films recorded at 1 Ag ⁻¹ in the potential range − 0.9 to 0.0 V. Furthermore, similar to CV results, the charge/discharge curves being nearly triangular in shape also verify the electrical double-layer capacitor (EDLC) behavior of the film electrodes. Here, it is clear that the GP10C has significantly longer discharge time (∆t ), and hence higher SC among the synthesized films. The values of CVs calculated from the GCD curves using Eq. (2) as function of discharge current densities are shown in Fig. 8c. The GP10C exhibits specific capacitances of 200, 161, 147, 137, 134, 123, 120, and 114 Fg ⁻¹ at 0.25, 0.5, 1, 2, 3, 5, 7, and 10 Ag ⁻¹ , respectively, and it is able to maintain ~ 57% of its initial capacitance value (200 Fg ⁻¹ ) from 0.25 to 10 Ag ⁻¹ . The specific capacitance of rGO increases significantly after the addition of MWCNTs, which is obvious from the electrochemical performances of GP5C and GP10C samples. The improved electrochemical performances of the composite can be ascribed to the fact that CNTs prevent the restacking of rGO sheets and hence facilitate the electrolytic ions to move deeper into the film samples. As the amount of CNTs is increased beyond the optimum value, specific capacitance decreases, which can be ascribed to the limited dispersibility and poor specific capacitance (~ 20 F/g) of MWCNTs [53, 54].

Electrochemical performance of rGO, GP5C, GP10C, and GP15C electrodes in 2 M KOH electrolyte, a CV curves at the scan rate of 50 mVs ⁻¹ , b GCD curves at the current density 1 Ag ⁻¹ , c CV as determined from GCD curves, and d Nyquist plots comparison of all the papers

Moreover, the specific capacitance of all the synthesized films decreases with an increase in the current density because the diffusion of electrolytic ions into the film electrodes becomes slower at higher current density values. Figure 8d shows the Nyquist plots of all the electrodes, indicating that with an increase of MWCNT content, internal resistance starts to decrease. The internal resistance is the Ohmic resistance, which consists of ionic resistance of electrolyte, inherent resistance of substrate and active electrode material, and contact resistance at the active electrode material and substrate interface. GP10C film electrode demonstrates the smallest internal resistance (1.14 Ω), while the internal resistances for rGO, GP5C, and GP15C are found to be about 2.2, 1.41, and 1.19 Ω, respectively. The smaller value of internal resistance for GP10C film can be ascribed to the better contact and its higher electrical conductivity. The “knee” frequency is defined as the highest frequency value at which impedance of the system is dominated by the capacitive nature [55]. It is related to the diffusion coefficient and effective diffusion length of the active electrode material. Further, at the frequencies higher than knee frequency, the electrolytic ions come across semi-infinite diffusion and finite diffusion at the frequencies lower than this [56, 57]. The knee frequency values for GP5C, GP10C, and GP15C are 1.37, 1.49, and 1.10 Hz, respectively. The higher knee frequency value for GP10C implies that lesser time is required by the charge species to accumulation at the interface for this sample. Further, it is well documented that larger semicircle at higher-to-medium frequency region corresponds to the larger charge-transfer resistance (Rct) [31, 58]. The Rct for GP15C film seems to be quite higher than that of GP10C, that may be due to its lower electrical conductivity and higher contact resistance with aqueous electrolyte [59].

Further, EIS data can be used to find out the relaxation time constant (τ ₀ ) of the devices like supercapacitors in terms of complex power with the help of Eqs. (8) and (9). Relaxation time constant (τ ₀ ) is an important parameter and considered as a factor of merit for a supercapacitor. To determine the relaxation time constant, normalized imaginary factor (|Q |/|S |) and real factor (|P |/|S |) of power are plotted vs. frequency (in logarithmic scale) (Fig. 9). Both these two curves cross each other at a point called resonance frequency (f _° ), which is utilized to calculate the relaxation time of a supercapacitor using the following formula:τ ₀  = 1/2πf ₀ [49]. From the graphs, we observe that at a higher frequency, |P |/|S | attains maximum value, which implies maximum power dissipates in the system, i.e., supercapacitor behaves similar to pure resistor. As the frequency decreases, |P |/|S | decreases up to a point at which |Q |/|S | attains the highest value. At this point, supercapacitor works similar to a pure capacitor. Evidently, for all the tested films GP(rGO), GP5C, GP10C, and GP15C, both the |P|/|S| and |Q|/|S| curves act contrarily with frequency variation and cross each other at resonance frequency (f _° ). The relaxation time constant values for GP, GP5C, GP10C, and GP15C as calculated using resonance frequencies are 1.3 s, 196 ms, 194 ms, and 378 ms, respectively. After adding MWCNTs in the rGO film, relaxation time decreases remarkably. This may be due to the fact that CNTs prevent the restacking of rGO sheets and hence allow the electrolytic ions to move faster into the film. As the amount of MWCNTs increases further (15 wt%) in the rGO film, increment in the relaxation time constant is observed. This can be ascribed to the smaller diameter of MWCNTs (10–20 nm) that offers higher ionic diffusion resistance, which become significant as the amount of MWCNTs is increased beyond optimum value [60, 61]. EIS results can also be used to determine the diffusion coefficients of the synthesized films for electrolytic ions (Fig. 9d). The calculated diffusion coefficients (D _een ) of electrolytic ions at the interfacial region using Eq. (7) come out to be 1.0112 × 10 ⁻¹³ , 8.0286 × 10 ⁻⁹ , 7.8457 × 10 ⁻⁹ , and 2.1919 × 10 ⁻⁹ for GP, GP5C, GP10C, and GP15C, respectively, in 2 M KOH. It can be seen that the relaxation time constant and diffusion coefficient of GP5C and GP10C are almost the same, but the Cs and rate capability of GP10C is much better than those of GP5C. The small relaxation time constant and high diffusion coefficient of GP10C film electrode, allow it to deliver stored energy quickly, and high specific capacitance make it desirable for engineering high-power capacitors.

een –c are the normalized real part |P|/|S| and imaginary part |Q|/|S| of the complex power as a function of frequency for GP, GP5C, and GP10C, respectively, and d Randles plots of all the synthesized electrodes

From the above results, GP10C film-based supercapacitor electrode exhibits the best electrochemical properties among the synthesized films. Therefore, we investigate its electrochemical performance in detail. Figure 10a indicates the CV curves of GP10C at 5, 10, 25, 50, and 100 mVs ⁻¹ in the potential range − 0.9 V to 0.0 V vs Ag/AgCl reference electrode. It is shown that all the CV curves possess almost rectangular and symmetric shape, indicating the perfect EDL capacitive behavior and fast charging/discharging characteristics. The inset in Fig. 10a shows nearly a linear relationship between average peak current and the square root of the scanning rate with correlation coefficient R ²  = 0.98878. This phenomenon indicates that the electrochemical process in the film is a diffusion-controlled process [62]. Figure 10b represents the GCD curves of GP10C evaluated at 0.25 to 10 Ag ⁻¹ in − 0.9 to 0.0 V. During the charge/discharge process, the corresponding curves also verify that the charging curve of GP10C is almost symmetric to its corresponding discharging curve. To evaluate the durability of the GP10C, the long cycle test was carried out in 2 M KOH electrolyte at 2 Ag ⁻¹ . Figure 10c depicts the long cycle stability, which is another important parameter to examine the electrochemical performance of an electrode material. After 15,000 cycles, GP10C electrode exhibits excellent retention of 92.5%. The inset in Fig. 10c shows first and last 5 successive cycles. It demonstrates that even after 15,000 cycles, the electrode maintains good symmetric charge/discharge characteristic features, which verify its excellent electrochemical durability. Figure 10d represents the Nyquist plots of the GP10C electrode recorded during long cycle test. It can be observed that the value of internal resistance goes higher during cycling process from the first cycle to 15,000 cycles. GP10C electrode shows lowest internal resistance (1.12 Ω) during the first cycle and after 10,000 and 15,000 cycles, as the electrochemical active sites in the electrode are slowly consumed, the values of internal resistance increases from 2.64 to 3.04 Ω, respectively. As a consequence of it, CV value decreases slowly and repeatedly during electrochemical cycling (Fig. 10c). Furthermore, to find out any morphological changes in the GP10C film electrode after long cycle test, we performed ex situ studies (FESEM and TEM), and the results are shown in Fig. 11. Figure 11a shows the TEM images of GP10C electrode before the long cycle test, while Figs. 11b and c represent the FESEM and TEM images of the GP10C after 15,000 cycles. We can see that the morphology of the GP10C electrode does not change even after 15,000 cycles, which reveals the sustained chemical stability of the film. The observed capacitance of GP10C film electrode is higher than those of several recently reported free-standing graphene-based supercapacitor electrodes as shown in Table 1.

Electrochemical performance of GP10C in 2 M KOH electrolyte a CV curves at the scan rate of 5, 10, 25, 50, and 100 mVs ⁻¹ ; b GCD curves at the current densities of 0.25, 0.5, 1.0, 2.0, 3.0, 5.0, 7.0, and 10 Ag ⁻¹ ; c cyclic stability performance for GP10C electrode at 2 Ag ⁻¹ and inset shows the GCD curves of first and last 5 cycles; en d Nyquist plot for the GP10C and inset shows the EIS performance during 1st, 10,000 and 15,000 cycles

een TEM images of the CP10C electrode before long cycle test and b FESEM and c TEM images of the CP10C after 15,000 cycles

Electrochemical Performance of Symmetrical Supercapacitor

Further, to investigate the practical application of the GP10C film, we made a symmetric coin cell supercapacitor using two GP10C electrodes of identical weight separated by a separator in 2 M KOH aqueous electrolyte. Figures 12a and b show the CV profiles of the device at the scan rates of 2, 5, 10, 15, 25, 50, 75, and 100 mVs ⁻¹ . We can observe nearly identical rectangular shape, which implies the perfect EDLC behavior of the supercapacitor. Figure 12c represents the linear GCD curves at all current densities demonstrating the high rate response of the device. Moreover, the smaller internal resistance (0.4 Ω) of the coin cell indicates better charge transportation in the supercapacitor (Fig. 12d). The calculated specific capacitances from CVs of the device (Fig. 12e) are 53, 51, 49.8, 48, 46.7, and 45 Fg ⁻¹ at 0.1, 0.2, 0.3, 0.5, 0.7, and 1.0 Ag ⁻¹ , respectievelijk. From the capacitance profile (Fig. 12e), it is clearly shown that the device retains 85% of its initial capacitance value at current density 0.1 Ag ⁻¹ up to 1 Ag ⁻¹ , i.e., good rate capability. Additionally, we calculate the energy density (Whkg ⁻¹ ) and power density (Wkg ⁻¹ ) of the device using equations given below [8, 9]:

Electrochemical performance of GP10C/KOH/GP10C symmetrical coin supercapacitor cell a , b CV curves of GP10C/KOH/GP10C coin cell at 2, 5, 10, 15, 25, 50, 75, and 100 mVs ⁻¹ , c Nyquist plot, d GCD curves of the device at different current densities, e SC at different current densities, f Ragone plot

where Cs is the SC calculated from the GDC curves, ∆V is the potential window, t is the discharge time (s).

The device exhibits maximum and minimum energy densities of 29.4 and 25.0 Whkg ⁻¹ at power densities of 439 and 4500 Wkg ⁻¹ , respectively (Fig. 12f).

This symmetric device shows excellent retention of ~ 85% and columbic efficiency of 92% after 10,000 successive cycles at 0.3 Ag ⁻¹ (Fig. 13a). The excellent cyclability of the device can be ascribed to the electrochemical stability of the active electrode material. In the GP10C nanocomposite film, the optimum amount of MWCNTs mainly prevents the restacking of rGO sheets and thus offers a more exposed area to the electrolytic ions for surface adsorption. This also strengthens the material structure to resist the structural deformation upon cycling. The ex situ TEM and FESEM micrographs of the tested electrode after 15,000 cycles (Fig. 11a–c) verify the behavior that the morphology of GP10C electrode remains the same even after 15,000 cycles, which reveals the sustained chemical stability of the synthesized composite film. The inset in Fig. 13a shows the GCD profiles of 1st, 5000th, and 10,000th charge-discharge cycles, indicating the symmetric charge/discharge characteristic features of the device. The high retention at even after 10,000 continuous long cycles verifies its outstanding electrochemical durability. Figure 13b depicts the Nyquist plots of the device during long cycle test, implies that with repeated cycles, the Warburg region in the middle frequency region is increasing. It can be attributed to the consumption of active sites presented in the active material of the supercapacitor electrodes during a long cyclic test, which results in an increase of the internal resistance of the device. The inset (Fig. 13b) shows that our symmetric coin cell can light up a red LED. Further, our designed FSSSD using GP10C flexible film electrodes and gel electrolyte depicts no significant changes in the shape of CV curves when bending the device at angles from 0 to 180° at a scan rate of 20 mVs ⁻¹ (Fig. 13c). Digital photographs of the device under the bending angles 0°, 60°, 90°, and 180° are shown in Fig. 13d–g, respectively.

The long cycle performance of GP10C/KOH/GP10C symmetrical coin cell. een Cyclic stability and columbic efficiency recorded at 0.3 Ag ⁻¹ for 10,000 successive cycles, and inset shows the GCD profiles of 1st, 5000th and 10,000th GCD cycles. b Nyquist plots recorded just after 1st, 5000th and 10,000th cycles, and inset shows a red LED light up by single coin cell. c The CV curves at a scan rate of 20 mVs ⁻¹ of symmetrical solid state flexible device using gel polymer electrolyte under different bending angles. Digital photographs of the device under different bending angles, d 0°, e 60°, f 90°,and g 180°, respectively

The above results prove the potential applications of our synthesized GP10C film for the supercapacitors. Moreover, this facile approach may open future prospects for energy storage devices application.