1 Introduction

The escalating environmental pollution and the world’s energy crisis have encouraged the creation of secure and effective energy storage technologies [1]. Supercapacitors (SCs), fuel cells (FCs), and batteries (Bts) are potentially excellent examples of energy storage/conversion that have attracted a lot of interest from both industry and research [2,3,4]. Particularly, SCs, often referred to as electrochemical capacitors or ultracapacitors, are presently being lauded as extremely effective and pollution-free physical energy storage devices [5]. Based on the energy storage techniques, there are two types of SCs: pseudo capacitors (PCs) and electric double-layer capacitors (EDLCs) [6]. Charges are stored on the electrode and electrolytic contact in pseudo capacitors (PCs) using a faradaic mechanism [7]. The metal oxides (NiO, SnO2, Co3O4, MnO2, TiO2, etc.), metal sulfides (NiS, CuS, CoS, MoS2, etc.), and conducting polymers (Polyaniline, Polypyrrole, etc.) have pseudocapacitive behaviors [8,9,10].

Charges from EDLCs build up at the electrode/electrolyte contact as a result of the formation of electric double layers. Graphene, carbon nanotubes (CNT), and activated carbon (AC) are examples of members of the carbon family that can be employed as EDLC materials [11, 12]. Because of the quick, reversible Faradaic redox reaction of the electrode material in the electrolyte solution, the PCs can store more energy than EDLCs. The volumetric energy density of SCs is still far off, which limits their applicability [13].

Moreover, SnO2 is receiving a lot of attention for use in supercapacitors due to its several oxidation states, wide potential window, low price and eco-friendly composition, and high theoretical capacity (782 mA h g−1) [14]. However, due to their poor stability, low electrical conductivity, and quick capacitance decrease, their single metal oxide-based anode materials have demonstrated poor electrochemical performance. To enhance the electrochemical characteristics, these hybrids based on binary or mixed metal oxides are being investigated. For example, Ahmed et al. reported that activated carbon waste (ACW) and the impregnation of a SnO2 nanocomposite electrode exhibited a superior specific capacitance of 30.46 F/g at 0.122 A/g in the neutral Na2SO4 [15]. Recently, Asaithambiet al. reported that the Mn-doped SnO2/MoS2 composite showed a remarkable specific capacitance of 242 F/g at 0.5 A/g and life cycle performance of 83.95% after 5000 continuous GCD cycles [16]. However, there are efficient other methods to produce high-performance cathode materials for supercapacitors, such as metal oxide with carbon material composites.

Herein, we report SnO2/MWCNT hybrids as anode materials synthesized by a one-step hydrothermal method for supercapacitor application. With excellent cycle-life stability (93 percent retention after 1000 cycles at 10 A/g), the calculated specific capacitance was found to be 255 F/g at 2 A/g.

2 Experimental methods

2.1 Chemicals

Tin chloride pentahydrate (SnCl4·5H2O, 99%), sodium hydroxide (NaOH, 99%), and multiwalled carbon nanotubes ((MWCNT) 98.5%, length (1 to 10 μm)). All the chemicals were purchased from Merck (India) and chemicals were used without further purification.

2.2 Synthesis of SnO2 nanostructures

In the typical synthesis of SnO2 nanostructures, 0.1 M of SnCl4·5H2O and 0.2 M of NaOH were subsequently dissolved in 50 mL of mixed solution de-ionized water under magnetic stirring. The final homogenous solution was put into a 100 mL Teflon-lined stainless steel autoclave and heated in an electrical oven for 24 h at 160 °C. The product was then filtrated and dried at 60 °C for more than 24 h.

2.3 Synthesis of SnO2/MWCNT nanocomposites

The SnO2/MWCNT nanocomposites were prepared by the hydrothermal method. As-purchased multiwalled carbon nanotubes (MWCNT) 100 mg was dispersed into 50 mL deionized water with ultrasonication for 2 h. After that, 0.1 M of SnCl4·5H2O and 0.2 M of NaOH were subsequently dissolved in 50 mL of the mixed above solution. After that, the solution was moved into the Teflon-lined stainless steel autoclave and heated at 160 °C for 24 h. The final product was dried at 80 °C in the electrical oven overnight.

2.4 Material characterization

X-ray powder diffractometer (XRD) Rigaku (Cu-Kα radiation), High resolution scanning electron microscope (HR-SEM, Magellan 400 L), HRTEM (JEOL, JEM-2100F, performing at 200 kV), and FT-IR technique (FT-IR, NEXUS 470).

2.5 Electrochemical measurements of a three-electrode cell

The electrochemical energy storage performance of prepared electrodes was studied by a 3 M aqueous KOH electrolyte in three electrodes cell set-up. The standard three-electrode configuration included a nickel-foam-based working electrode, Ag/AgCl as a reference electrode, and graphite rod as a counter electrode. The active material (80 weight percent), activated carbon (10 weight percent), and polyvinylidene fluoride (10 weight percent) were mixed with the help of a mortar and pestle, and three drops of N-Methyl-2-pyrrolidone (NMP), an organic solvent, to make a slurry. The slurry was coated in Ni foam (2 × 1) surfaces and dried vacuum oven at 60 °C for 12 h. The cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) were measured by using an electrochemical workstation (Squidstat Potentiostats- United States).

The GCD plots were used to estimate the specific capacitance (Csp) of working electrodes as follows,

$$C_{sp} = \frac{{I \ast \varDelta t}}{{\varDelta m \ast \varDelta V}}$$
(1)

Where I, Δt, ΔV, and m indicate the applied current (A), discharge time (s), area of discharging time, working potential window, and mass of the active materials (mg) for the three-electrode system.

3 Results and discussion

Figure 1a, shows the XRD patterns of MWCNT were observed at 2θ = 26.2 and 2θ = 44.6 ° corresponding to (002) and (100) planes of hexagonal structures (JCPDS no. 26–1079). The XRD patterns of SnO2 and SnO2/MWCNT nanocomposite were well matched with JCPDS No. 41-1445 in Fig. 1b, c, confirming the presence of the tetrahedral rutile phase. The crystal planes (110), (101), (200), (211), (220), (002), (310), (301), and (110) are represented by the indexed powder XRD patterns with diffraction 2θ = 26.6°, 33.9°, 38.1°, 51.7, 55.12°, 58.32°, 62.90° 65.0 and 67.12°, respectively [17]. The diffraction peaks of the SnO2/MWCNT nanocomposite were similar to the diffraction peaks of SnO2 with the addition of lower intensity diffraction peaks, confirming the presence of graphitic MWCNT in the SnO2/MWCNT nanocomposite [18].

Fig. 1
figure 1

Powder XRD patterns of (a) MWCNTs, (b) SnO2, and (c) SnO2/MWCNT nanocomposite

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The FT-IR spectra of SnO2 and SnO2/MWCNT in the 4000–400 cm−1 range, as measured at room temperature, are shown in Fig. 2. The main absorption bands at 497 cm−1, and 673 cm−1 were assigned to the stretching vibrational modes of the O–Sn–O and Sn–O [19]. In addition, the peaks at 1636 cm−1 and 3421 cm−1 were related to the bending and stretching vibrations of H-O-H from H2O molecule absorbed by the SnO2 surface. Moreover, the absorption peaks of (1640 cm−1), and (1560 cm−1) were as assigned to C = O and C = C in the MWCNT, which confirms the successful formation of the SnO2/MWCNT nanocomposite [17]. The peaks at 1641 cm−1 and 3450 cm−1 corresponding to O-H banding and band stretching vibrations of SnO2/MWCNT nanocomposite.

Fig. 2
figure 2

FT-IR of nanostructured (a) SnO2, and (b) SnO2/MWCNT composites

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FE-SEM techniques provided for the visualization of the surface morphologies of SnO2 and SnO2/MWCNT nanocomposites. The lower and higher resolution FE-SEM images of SnO2 showed a dumbbell-like morphology with an average diameter of 2.5 µm, as shown in Fig. 3a, b. The lower and higher magnification FE-SEM images of the SnO2/MWCNT nanocomposite are also shown in Fig. 3c, d, which also illustrates the irregular rod shape of SnO2 with MWCNTs having an average diameter of 2 µm. Figure 4a–f shows the FE-SEM with EDS mapping images of SnO2/MWCNT nanocomposite with the elements Sn, O, and C, and those results indicate the successful formation of SnO2/MWCNT nanocomposite.

Fig. 3
figure 3

FE-SEM images of (a, b) SnO2, and (c, d) SnO2/MWCNT nanocomposites

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Fig. 4
figure 4

FE-SEM EDS with mapping images of (a–f) SnO2/MWCNTs nanocomposites

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Figure 5a, b shows the TEM and HR-TEM images of SnO2/MWCNTs nanocomposites. The MWCNTs decorated on the SnO2 nanoparticles (small size of 200 nm), which were attached to the surface of the MWCNT in the SnO2/MWCNTs composite in Fig. 5a, b. The HR-TEM image revealed two lattice fringes with a spacing of 0.35 nm and 0.36 nm, corresponding to the (110) and (002) plane of SnO2/MWCNTs nanocomposites.

Fig. 5
figure 5

a TEM and (b) HR-TEM images of SnO2/MWCNT nanocomposites

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Figure 6 shows, a three-electrode cell setup using an aqueous electrolyte (1 M KOH) and the characterized the electrochemical energy storage properties of the prepared SnO2 and SnO2/MWCNTs electrode materials. These electrochemical techniques included cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS). The CV curves of prepared SnO2 and SnO2/MWCNTs composite electrodes are shown in Fig. 6a, b at different scanning rates of 5, 10, 20, 40, 60, and 80 mV/s in a content potential window of 0–0.5 V. Figure 6c shows the comparative CV curves of the SnO2 and SnO2/MWCNT electrodes at a sweep rate of 60 mV/s over the potential range of 0 to 0.5 V. Comparative CV curves for all electrode materials show the different redox couplings that signify the Faradaic redox (battery-type) feature in the alkaline electrolyte [20]. The following equation possible charge storage mechanism for the electrodes using 1 M KOH as an aqueous electrolyte [21].

$$(SnO_2)\,{{{\mathrm{Surface}}}} + K^ + + e^ - \leftrightarrow SnO_2^ - - K^ +$$
(2)
Fig. 6
figure 6

CV curves of (a) SnO2, (b) SnO2/MWCNTs, and (c) comparison CV curves at a constant sweep rate of 60 mV/s

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In addition, the GCD curve of SnO2, and SnO2/MWCNTs electrodes with various current densities from 2 to 12 A/g are shown in Fig. 7a, b. The GCD curves of all the prepared electrodes operate in a potential window from 0 to 0.40 V in the three-electrode cell setup (Fig. 7c). Furthermore, the prepared electrodes’ nonlinear charge/discharge curves show they all behaved like battery electrodes (faradic battery-type redox) [22].

Fig. 7
figure 7

GCD curves of (a) SnO2, (b) SnO2/MWCNTs nanocomposites, and (c) comparison GCD curves of constant current density at 4 A/g

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The calculated specific capacitance of prepared SnO2 and SnO2/MWCNTs composite electrodes with various current densities is shown in Fig. 8a. The SnO2/MWCNT electrodes delivered a maximum specific capacitance of 255 F/g when compared to the SnO2 electrode at 127 F/g at a current density of 2 A/g. The specific capacitance of the SnO2/MWCNT composite electrode was compared to that of other electrode materials (Table 1). The SnO2/MWCNT electrode’s improved energy storage ability is attributable to its increased surface area, increased electrochemical active sites, improved electrical conductivity, and synergistic interaction between SnO2 and MWCNTs in composites.

Fig. 8
figure 8

a Specific capacitance with various current densities, (b) EIS with equivalent circuit (inserted), (c) bode impedance, and (d) Cyclic stability, and columbic efficiency of SnO2 and SnO2/MWCNTs composite at a constant current density of 10 A/g

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Table 1 Comparison of the specific capacitance of the prepared SnO2/MWCNTs with the previously reported literature
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Figure 8b, c shows the N-q and bode EIS plots of SnO2 and SnO2/MWCNT nanocomposite electrodes with equivalent circuits. Table 2 indicates the fitted N-q plot (equivalent circuits) parameters and delivers Rct values of 4.20 Ω and 0.55 Ω corresponding to SnO2 and SnO2/MWCNT nanocomposite, respectively. The SnO2/MWCNT nanocomposite exhibited a lower resistance value compared then pure SnO2. Moreover, the SnO2/MWCNT nanocomposite electrode has a cycle life stability of 1,000 GCD cycles at a constant current density of 10 A/g (Fig. 8d). The estimated capacitance retention and coulumbic efficiency of the SnO2 and SnO2/MWCNTs correspond to 76% and 81% and 93%, 95% after 1000 GCD cycles, respectively [23].

Table 2 the fitted Nyquist plot (N-q) of SnO2 and SnO2/MWCNTs nanocomposites
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4 Conclusion

In summary, a hydrothermal method was used to successfully synthesize SnO2/MWCNTs nanocomposite, which was used as anode material for a supercapacitor application. The phase purity, crystal structure, functional groups, surface morphology, and internal morphology were studied by XRD, FTIR, FE-SEM, and TEM analyses. The SnO2/MWCNTs nanocomposite electrode delivered a maximum specific capacitance of 255 F/g at the current density of 2 A/g with a life cycle performance of 93% after 1000 GCD cycles.