1 Introduction

Among the various energy storage devices, supercapacitors are drawn the great attention due to its high power density, rapid charge and discharge, and good cycle stability [1,2,3,4,5,6,7,8]. And it has been successfully applied in electronic products, flexible and wearable electronic devices, and hybrid electric vehicles [9, 10]. However, the low energy density of supercapacitors still couldn’t satisfy the people’s demands [11]. To resolve these obstacles, the researcher have devoted to much time and energy to develop the novel supercapacitors. In recent years, they have realized that the good electrode materials and the rational structure of supercapacitor are the key to obtain the high electrochemical performance supercapacitors [12, 13].

As reported, the electrode materials mainly focus on carbon materials, transition metal oxides/hydroxides [14,15,16], transition metal sulfides [17], transition metal nitrides/carbides [18, 19], and conductive polymers [20, 21]. Among these, carbon materials, such as reduced graphene oxide or graphene [22, 23], carbon nanotubes [24], porous carbon [25], active carbon [26], and carbon nanofibers [27], are the most potential candidate for supercapacitor electrode materials [28]. Especially, reduced graphene oxides are regarded as the most promising electrode materials because of their excellent electronic conductivity, good chemical stability, and high oxygen content to enhance the electrolyte infiltration, which is favourable to improve the electrochemical performances. For instance, Cui et al. [29] reported that reduced graphene oxide/carbon nanotube showed a maximum specific capacitance of 272 F g−1 at 5 mV s−1 and a potential windows of − 0.8 to 0 V. Wu et al. [30] investigated the electrochemical performances of ASCs based on rGO and polyaniline in 1 M H2SO4 electrolyte, which showed a specific capacitance of 210 F g−1 at 0.3 A g−1. Li et al. [31] demonstrated rGO as negative electrode for asymmetric supercapacitor and delivered a specific capacitance of 182 F g−1 at 1 A g−1 in 1 M KOH electrolyte.

Generally, the supercapacitors usually be designed by three strategies of building an aqueous, organic electrolyte or ionic liquid, and all-solid-state supercapacitor. Building a organic/ionic liquid symmetric supercapacitor based on rGO as electrode material is an effective approach for advanced supercapacitors owe to the wide operation windows and the excellent chemical stability of rGO, resulting in a considerable benefit for practical application in SCs. Furthermore, Et4NBF4/AN is considered one of the desirable electrolyte because of its low resistance [32]. Hence, it is worthwhile to fabricate a supercapacitor based on rGO electrode material in Et4NBF4/AN electrolyte. Herein, we report a simple strategy to prepare GO/CNTs film electrodes by vacuum filter method and thermal reduced at 300 °C to obtain a freestanding rGO/CNTs hybrid film as electrode materials. In rGO/CNTs films, rGO nanosheets could provide more active sites accessible to charge storage, resulting in high specific capacitance. Moreover, rGO nanosheets intertwined with carbon nanotubes also enhance mechanical stability, increase active surface area and electrode/electrolyte contact area, provide short diffusion length for ions and electrons and high conductivity to improve electrochemical performance [33,34,35,36,37,38]. Firstly, the electrochemical behaviors of rGO/CNTs film electrode is investigated in 1 M KOH electrolyte using a three-electrode systems. The electrochemical results indicate that the the highest specific capacitance of 221 F g−1 is obtained at 1 A g−1, along with long cycles life of 102.9% capacitance retention after 5000 cycles. Moreover, the electrochemical behaviors of rGO/CNTs with 1 M Et4NBF4/AN electrolyte are also evaluated in three and two electrode systems. In three electrode systems, rGO/CNTs hybrid film shows a high specific capacitance of 174 F g−1 at 1 A g−1 and excellent cycle stability. In two electrode systems, a symmetric supercapacitor of rGO/CNTs//rGO/CNTs exhibits a specific capacitance of 24 F g−1 at 1 A g−1, an energy density of 20.8 Wh kg−1 at 1.27 Wh kg−1, and excellent cycle life of 86.1% retention after 5000 cycles. It indicates that the good electrochemical performances of this symmetric supercapacitor has the great potential application value.

2 Experimental

2.1 Preparation of rGO/CNTs hybrid film

The freestanding hybrid films were built using a simple vacuum filtration and thermal reduced method. Firstly, the mixture of GO disperse and CNTs disperse with a mass of 9:1, were sonicated for 10 min. And GO/CNTs films was obtained by vacuum filtration method [39]. Then the GO/CNTs film was naturally peeled from the filter film. Finally, a freestanding rGO/CNTs hybrid film was obtained at 300 °C for 30 min under N2 atmosphere.

2.2 Characterization

X-ray diffraction (XRD), Field-emission scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were used to certify the structure, morphology, and the composites of as-prepared rGO/CNTs hybrid film.

2.3 Electrochemical measurements

The as-prepared rGO/CNTs hybrid film (0.785 mg cm−2) was directly pressured between two pieces of nickel foam at 10 kPa pressure to obtain a working electrode. The electrochemical behaviors of working electrode was tested using 1 M KOH electrolyte, Hg/HgO reference electrode, and Pt flake electrode in three-electrode systems. Moreover, the electrochemical behaviors of rGO/CNTs hybrid film electrode was evaluated using Ag/AgCl reference electrode, Pt flake electrode, and 1 M Et4NBF4/AN electrolyte in three-electrode systems. And a symmetrical supercapacitor based on rGO/CNTs film//Ni foam was also tested in 1 M Et4NBF4/AN electrolyte. All the electrochemical tests were conducted using a CHI 660E. The specific capacitance Cm (F g−1), energy density Em (Wh kg−1), and power density Pm (W kg−1) were determined based on the following equations [40]:

$$C_{m} = I\Delta t/m\Delta V$$
(1)
$$E_{m} = C\Delta V^{2} /2 \times 3.6$$
(2)
$$P_{m} = 3600E_{m} /t$$
(3)

Herein, m (g), I (A), ∆t (s), ∆U (V) mean the mass of active materials, the charging/discharging current, the discharging time, and the operating voltage, respectively.

3 Results and discussion

In this work, rGO/CNTs film is prepared at a temperature of 300 °C under N2 atmosphere, because it can balance specific capacitance and rate capability at this temperature [41]. To better understand the structure, morphology, composite information of rGO/CNTs film, it is characterized using XRD, SEM, TEM, Raman, and XPS, respectively. Furthermore, we also evaluate its electrochemical behaviors in KOH aqueous solution and Et4NBF4/AN electrolyte, respectively.

XRD patterns peaks of the rGO/CNTs film is exhibited in Fig. 1a. And it is observed that only one characterization peak appears at 2θ value of 25°, which attributes to the carbon peak of rGO/CNTs film (JCPDS Card No. 41-1487). This broad characteristic peak indicates the poor crystalline quality. To better understand the surface information of rGO/CNTs, XPS analysis is carried out to acknowledge the weight ratio of C/O and the chemical bonding of the sample, as given in Fig. 1b–d. From Fig. 1b, the peaks of C 1s and O 1s could be apparently observed, and the weight of O element and C element are 19.4% and 80.6% in rGO/CNTs samples, respectively. The C spectra of rGO/CNTs samples is presented in Fig. 1c. Four main peaks at 284.6 eV, 286.4, 288.1, and 289.9 eV are assigned to the groups of C=C, C–O–C, C=O, and C(O)OH, respectively [42]. Figure 1d shows the O spectra of rGO/CNTs films. It is observed that the groups of C=O, C–O–C, and C(O)OH are locked at peaks of 531.7, 533, and 535.1 eV, respectively [42]. This reveals that rGO/CNTs samples mainly contain C=C, C–O–C, C=O, and C(O)OH groups, which can favor the electrolyte infiltration to improve the specific capacitance. Additionally, the as-prepared rGO/CNTs film is also characterized by Raman spectroscopy, because Raman spectrum analysis is regarded as one of most effective strategy to identify carbon-based materials. As shown in Fig. 1e, it demonstrates that Raman peaks at 1346 and 1586 cm−1 attribute to D-band and G-band of rGO/CNTs film. And 1.03 of ID/IG means that rGO/CNTs samples can provide the good electrical conductivity due to GO samples removal the functional groups of carboxyl, hydroxyl, and epoxy at 300 °C [41].

Fig. 1
figure 1

a XRD patterns of rGO/CNTs (JCPDS Card No. 41-1487); b XPS survey spectra for rGO/CNTs; XPS spectra for C 1s (c) and O 1s (d); e Raman spectra of rGO/CNTs

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To acknowledge the detailed morphology, the as-prepared rGO/CNTs films are characterized using SEM and TEM, and the morphology information of these films is exhibited in Fig. 2. From Fig. 2a, it is easily founded that the surface of rGO/CNTs films shows rGO junction with carbon nanotubes, which reveals that rGO/CNTs films could provide good mechanical stability and more active sites for ions. The EDS map** of C and O elements further exhibits the consists of rGO/CNTs films (Fig. 2b, c). After rGO/CNTs films sonicated in ethanol solvent for several minutes, the rGO/CNTs films are further examined by the TEM images in Fig. 2d–f. As shown in Fig. 2d, it is observed that the rGO nanosheets intertwine with carbon nanotubes and the diameter of carbon nanotubes is round 20 nm. In Fig. 2e, it is easily observed that the carbon nanotube junction with the reduced graphene oxide lead to the good mechanical strength and flexibility. We can clearly observe the interface of carbon nanotube junction with reduced graphene oxide in Fig. 2f.

Fig. 2
figure 2

a SEM images of rGO/CNTs samples; b, c EDS map** of C and O element, respectively; d, e TEM images of rGO/CNTs samples; f the magnified TEM images in (e)

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Firstly, cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and cycle stability are conducted in 1 M KOH electrolyte via a three-electrode system to explore electrochemical properties of rGO/CNTs electrode. CV tests for rGO/CNTs electrode are performed at different scan rates of 25–500 mV s−1 and a potential window of − 0.8 to 0 V (Fig. 3a). These CV curves show a rectangular shape at 25, 50, 100, and 200 mV s−1, respectively. And even at a high scan rate of 500 mV s−1, the shape of curve still keep a similar to that at 25 mV s−1, suggesting the good rate capability of rGO/CNTs electrode. Figure 3b shows the GCD curves at current densities of 1, 2, 3, 4, and 5 A g−1, respectively. All the GCD curves are close to symmetric triangle, suggesting an excellent reversible reaction. The specific capacitances of rGO/CNTs electrode are 221, 194, 182, 174 and 170 F g−1 at 1, 2, 3, 4, and 5 A g−1, respectively. Meanwhile, the capacitance of rGO/CNTs electrode can keeps 71% when the current densities increase from 1 to 10 A g−1, indicating a good rate capability (Fig. 3c). Figure 3d reveals the capacitances gradually increase with the increasing cycle numbers for rGO/CNTs electrode, the capacitance retention of 102.9% could be kept after 5000 cycles at 100 mV s−1, indicating the excellent cycle stability. This electrochemical results indicate that rGO/CNTs film could be regarded as an ideal supercapacitors electrode material. Figure 3e gives the electrochemical impedance spectroscopy (EIS) plots of rGO/CNTs films and an equivalent circuit, indicating the low resistance of rGO/CNTs films.

Fig. 3
figure 3

Electrochemical behaviors of rGO/CNTs electrodes based on the three-electrode systems in 1 M KOH electrode. a CV curves at various scan rates; b GCD curves at various current densities; c specific capacitance versus current density; d cycle stability; e EIS plots of rGO/CNTs film in 1 M KOH electrolyte, the inset is equivalent circuit

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The electrochemical properties of rGO/CNTs are also investigated through three-electrode measured systems in 1 M Et4NBF4/AN electrolyte, as depicted in Fig. 4. The suitable potential windows for rGO/CNTs electrode are chosen among − 1 to 0, − 1 to 0.5, − 1 to 1, and − 1 to 1.5 V. It reveals that − 1 to 1.5 V could be chosen as the suitable potential window (Fig. 4a). So all the following of electrochemical tests are performed at the suitable windows of − 1 to 1.5 V. Figure 4b depicts the CV curves at current densities of 10–50 mV s−1. These CV curves show a similar rectangular shape, suggesting a good capacitive property. Figure 4c depicts the GCD curves measured for rGO/CNTs electrode between 2 and 10 A g−1. The shape of GCD curves show a symmetrical triangular with slight curvature at 2–10 A g−1, revealing a good capacitive property. The rate capability for rGO/CNTs electrode at 2–10 A g−1 is shown in Fig. 4d. The highest specific capacitance of 174 F g−1 is obtained at 2 A g−1. These values begin a obvious decrease from 3 to 20 A g−1. And the capacity retention could reach to 73.6% for rGO/CNTs electrode when the current densities vary from 2 to 20 A g−1. It reveals that the rGO/CNTs electrode has the good electrochemical behaviors in Et4NBF4/AN electrolyte. Similarly, Fig. 4e also shows the electrochemical impedance spectroscopy (EIS) plots of rGO/CNTs films in 1 M Et4NBF4/AN electrolyte and an equivalent circuit. It also indicate the low resistance of rGO/CNTs films in 1 M Et4NBF4/AN electrolyte.

Fig. 4
figure 4

Electrochemical behaviors of rGO/CNTs electrodes based on the three-electrode systems in 1 M Et4NBF4/AN electrolyte. a CV curves at various potential windows at 10 mV s−1; b CV curves at various scan rates; c GCD curves at various current densities; d specific capacitance versus current density; e EIS plots of rGO/CNTs film in 1 M Et4NBF4/AN electrolyte, the inset is equivalent circuit

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Additionally, we assemble a symmetric SC (rGO/CNTs//rGO/CNTs) using two pieces of rGO/CNTs films, nickel foams collector, and 1 M Et4NBF4/AN electrolyte. The optimal potential windows is 2.5 V based on CV tests at various potential windows in two-electrode systems. And the total loading mass of rGO/CNTs films is 1.2 mg. As Fig. 5a shown, CV curves of the symmetric SC are tested at various scan rates. We observe that the shape of CV curves are irregular rectangular between 10 and 50 mV s−1, and the area of CV curves gradually increase with the increasing scan rates. Figure 5b exhibits the GCD curves of the symmetric SC, the specific capacitances of 24, 22, 20, 19.2, and 19 F g−1 are obtained at current densities of 1, 2, 4, 6, 8 A g−1, respectively. The specific capacitance of symmetric SC can remain 79% from 1 to 8 A g−1 (Fig. 5c), revealing an excellent rate capability. The EIS plots of the symmetric SC is shown in Fig. 5d. This symmetric SC has the relative low intrinsic resistance due to its low charge-transfer resistance. Figure 5e depicts the Ragone plots of the symmetric SC. The highest energy density of 20.8 Wh kg−1 is obtained at a high power density of 1.27 kW kg−1, and still remains 16.5 Wh kg−1 at 9.85 kW kg−1. The energy density of this symmetric SC is higher than that of the previous reported, such as 5.7 Wh kg−1 of APCN-2 at 10 kW kg−1 [43], 13.4 Wh kg−1 of Ni(OH)2/UGF//a-MEGO at 0.065 kW kg−1 [44], 13.55 Wh kg−1 of N-RC2//N-RC2 symmetric supercapacitor at 0.3998 kW kg−1 [45], 20.3 Wh kg−1 of AC//Ni(OH)2 ASC at 0.0906 kW kg−1 [46]. In Fig. 5f, the symmetric SC shows a good cycle stability of 86.1% capacitance retention after 5000 cycles.

Fig. 5
figure 5

Electrochemical behaviors of rGO/CNTs//rGO/CNTs symmetric SC in 1 M Et4NBF4/AN electrolyte. a CV curves of symmetric SC at various scan rates; b GCD curves of symmetric SC at various current densities; c specific capacitance versus current density; d EIS plots of symmetric SC; e Ragone plots; f cycle performances

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4 Conclusions

In this work, we fabricate the freestanding rGO/CNTs hybrid films via the simple methods of vacuum filtration and thermal reduction. The electrochemical performances of rGO/CNTs film in three-electrode systems exhibit a maximum specific capacitance of 221 F g−1, a 71% capacitance retention, and an excellent cycle life in 1 M KOH electrolyte. We also investigate the electrochemical performances of rGO/CNTs films in Et4NBF4/AN electrolyte under three-electrode systems. The results demonstrate a maximum specific capacitance of 174 F g−1 and good rate capability. Moreover, a symmetric supercapacitor of rGO/CNTs//rGO/CNTs demonstrates a maximum specific capacitance of 24 F g−1 at 2 A g−1, a energy density of 20.8 Wh kg−1 at 1.27 kW kg−1 and an excellent cycle life of 86.1% retention after 5000 cycles. It suggests that the symmetric SC has a great potential in practical application.