1 Introduction

Down the line requests for the advanced flexible and stretchable energy storing devices are expanding day by day. These devices have many potential applications in wearable and convenient electronic gadgets, electronic paper, and wearable frameworks for individual media [1,2,3]. Various carbon-based materials like carbon nanotubes (CNTs), graphene, and their hybrids with other nanomaterials have shown application in various fields [4,5,6]. Especially in the field of conductive flexible materials, these materials have been coated onto a paper substrate by various methods like inkjet printing [6], airbrushing [7], sputtering [8], Meyer rod coating, spray coating, and gravure coating [9, 10]. Among flexible devices, polymer-based supercapacitors (Poly-SCs) are equipped for advantages like high performance, small size, and lightweight [11, 12]. Compared with the other energy storage devices, Poly-SCs have high power density, long life cycle, and wide range of working temperature. Flexible and binder-free electrodes with outstanding stability and conductivity are in demand. Activated carbons are most commonly utilized as electrode materials for the supercapacitor applications.

Carbon nanotubes (CNTs) have great potential to be utilized in flexible electronics due to its outstanding properties. Multi-walled carbon nanotubes (MWCNTs) have cylindrical shape and hexagonal lattice structure and have unique kind of physical [11], chemical, structural, and mechanical properties [12]. The hybrids of carbon nanotubes with graphene showed many advanced applications in flexible electronics and energy storage [13]. However, full potential of carbon nanotubes for many applications is still unharnessed [1]. MWCNTs have high aspect ratio and surface area, which can provide an advantage in loading of other nanomaterials on its surface. MWCNTs have been utilized in the field of energy storage–related applications like solar cells [1, 14], lithium-ion batteries [15], and supercapacitors [16]. Supercapacitors are mainly of two types; one is electrochemical double-layer capacitor (EDLC) and another is pseudocapacitor [1]. The EDLC depends on carbon material like graphite, graphene, SWCNT, and MWCNTs. Carbon-based nanomaterials are used for supercapacitor electrode materials resulting in high specific capacitance value. This is attributed to the high charge separation capacity of MWCNTs between the electrode and electrolyte [17]. The efficiency of a supercapacitor highly depends on the ionic conductivity of the electrolyte used [1]. Specific surface area plays an important role for electrode material to obtain high specific capacitance [18].

The present work demonstrates the fabrication and characterization of the functionalized MWCNT films by using vacuum filtration technique. Functionalization of MWCNTs was done by acid treatment. The fabricated electrodes were characterized by FESEM, X-ray diffraction, FTIR, cyclic voltammetry (supercapacitor performance), and current-voltage measurement system.

2 Materials and methods

2.1 Materials

MWCNTs with a length of 10–30 μm were purchased from Nanoshel, India. HNO3, phosphate-buffered saline (pH -7.2), N-hydroxysuccinimide, and 1-(3-dimethylaminopropyl)-3-ethyl carbodiimide hydrochloride were purchased from SRL, India and H2SO4 was purchased from Merck, India.

2.2 Functionalization of MWCNTs

MWCNT carboxyl functionalization was done by chemical method. One hundred milligrams of MWCNTs was dissolved in180 mL acid solution of H2SO4/HNO3 (3:1, v/v ratio) [19, 20] and allowed to react for 4 h. After the completion of this reaction, the sample was dissolved in deionized water and vacuum filtered through a PVDF filter paper (0.22-μm pore size).

2.3 Preparation of the electrode

Eighty milligrams of acid-oxidized MWCNTs (1 mg/mL) was dissolved in 80 mL of deionized water. Forty milliliters of this solution was filtered by vacuum filtration through PVDF membrane (pore size 0.22 μm). Acid oxidized-MWCNT film on PVDF membrane was treated with 20 mL of EDC (25 mM)/NHS (12.5 mM) solution in filtration funnel without applying vacuum. The filtrate is passed through the membrane 4 times without vacuum. Afterwards, 1 mL of tri-ethylene-tetra-amine was allowed to react with the MWCNT/PVDF film. As a result of this process, amine-functionalized MWCNT film on PVDF (amine-FMWCNT/PVDF film) was obtained. Forty milliliters of remaining acid-oxidized carbon nanotubes was added to the membrane in the same filtration funnel followed by the addition of 20 mL of EDC/NHS. The filtrate was allowed to pass through the membrane repeatedly for four times for complete attachment of carbon nanotubes followed by washing with DI water and hot air drying. Schematic of the process is displayed in Fig. 1.

Fig. 1
figure 1

Schematic for the preparation of CNT/PVDF film: grafting carboxyl, amine group, and their crosslinking using EDC/NHS cross linker

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3 Results and discussion

3.1 X-ray Diffractometry

X-ray diffraction (XRD) was used to study the interplanar dividing and structure of MWCNTs. The diffractograms have been recorded using Rigaku Minifles-600 X-ray diffractometer using Cu Kα as characteristic wavelength (1.54 Å) at an operating voltage of 40 kV. XRD pattern was recorded with 2θ scan range from 20–70°. Figure 2 demonstrated the XRD spectra of amine-functionalized MWCNT/PVDF film. The XRD pattern of amine-functionalized MWCNT/PVDF film prepared by vacuum filtration assembly shows major diffraction peak at 2θ = 24.16° (Fig. 2), which corresponds to interplaner spacing of 0.36 nm. The interplanar spacing has been calculated by Bragg’s law:

$$ n\lambda =2d\sin \theta $$
Fig. 2
figure 2

XRD spectra of MWCNTs showing characteristic peak of CNTs

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where the crystal plane is (002) for corresponding angle of 24.16° as compared with normal graphite, 2θ = 26.5°; this peak shows a downward shift, which indicates the increase in interlayer spacing [21].

3.2 FTIR spectroscopy

FTIR study was carried out on Shimadzu IR affinity 1S at a range from 400 to 4000 cm−1 for both amine-functionalized and acid-oxidized MWCNTs (Fig. 3). Acid-oxidized MWCNT nanotubes show absorption peak at 1732 cm−1 attributing to C=O bond stretching vibrations of –COOH functional groups grafted on the surface of nanotubes, whereas in case of amine-functionalized MWCNTs (amine-FMWCNTs), peaks were observed at 1013, 1632, and 3320 cm−1 corresponding to stretching vibrations of C–O, N–H, and N–H bonds respectively (Fig. 3).

Fig. 3
figure 3

Comparative FTIR spectral study of FMWCNT and amine-FMWCNT: black line shows FTIR spectra of FMWCNT and red line shows FTIR spectra of amine-FMWCNT

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3.3 Microstructural properties

Field emission scanning electron microscope (FESEM) is utilized to examine the morphology of amide bond–interlinked MWCNT film (Fig. 4). The figure clearly indicates the embedment of nanotubes in the pores of PVDF membrane as well as the crosslinking of the MWCNTs.

Fig. 4
figure 4

Microstructural properties of amine FMWCNT/PVDF: FESEM micrographs demonstrate the morphology of the embedded amine-FMWCNTs in PVDF paper

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3.4 Electrochemical performance study

The electrochemical performance of the amine-FMWCNT/PVDF film was studied by cyclic voltammetry and galvanostatic charge-discharge (CV-CD) measurement using Metrohm Autolab potentiostat (Fig. 5). We have used Ag/AgCl as reference electrode and platinum wire as counter electrode, and the prepared film (cross-linked MWCNT/PVDF film) was used as working electrode. 1M H2SO4 solution was used as electrolyte. Figure 5 a shows the CV response curve of the film. Primarily, we studied the electrochemical performance of the prepared film at 10 mV/s scan rate in 1 M H2SO4 (Fig. 5a). Specific capacitance can be calculated by surface area of the curve [22, 23]. Further, we carried out a detailed study of CNT-based electrodes at different scan rates, i.e., 10 mV/s, 20 mV/s, 30 mV/s, 50 mV/s, and 100 mV/s (Fig. 5b). The potential window for CV analysis was kept from − 0.1 V to 0.6 V. It can be clearly seen that a lower scan rate gives high current value. Charge-discharge curve of different scan rates is shown in Fig. 5c. Specific capacitance Cs of the fabricated electrodes was calculated using the following equation (1).

$$ {C}_{\mathrm{s}}=\frac{1}{mv\left({V}_{\mathrm{b}}-{V}_{\mathrm{a}}\right)}{\int}_{V_{\mathrm{b}}}^{V_{\mathrm{a}}} IdV $$
(1)
Fig. 5
figure 5

The electrochemical performance of the amine-FMWCNT/PVDF film. a Cyclic voltammetry (CV) curves of amine-FMWCNT/PVDF film at the scan rate of 10 mV/s. b Cyclic voltammetry curve for various scan rates. c Charge-discharge curve of different scan rates. d Relation between different scan rates and specific capacitance

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I is the current (A) which shows by area under the curve found from cyclic voltammogram, m is the mass of deposited films in grams, and S is the scan rate in millivolts per second.

In this study, it is clearly seen that at various scan rates, Cs values were 24.2 F/g, 23.4 F/g, 21.6 F/g, 19.5 F/g, and 16.4 for the 10 mV/s, 20 mV/s, 30 mV/s, 50 mV/s, and 100 mV/s respectively (Fig. 5d). From the above study, it is clear that the highest specific capacitance (Cs) was calculated by a low scan rate at 10 mV/s. The results were favorable for the electrochemical energy storage using flexible MWCNT/PVDF electrode.

4 Conclusion

In this study, we have prepared a covalently cross-linked network of carbon nanotubes on PVDF membrane. The covalent bonding was confirmed by FTIR spectroscopy. Morphological studies carried out by FESEM microscopy indicated the embedment of CNTs in PVDF membrane pores and crosslinking of the nanotubes. Explicit capacitance of the film was determined utilizing cyclic voltammetry. Higher Cs is obtained at low scan rates.