Preparation of vertically aligned carbon nanotube/polyaniline composite membranes and the flash welding effect on their supercapacitor properties

Abstract

A vertically aligned carbon nanotube (CNT)/polyaniline (PANi) composite membrane was prepared by a simple filtration and electrical synergy method. Considering the high alternating electric field and strong shear forces during suction, vertically aligned single-walled nanotubes (SWNTs) were found in the SWNT/PANi composite membrane. Scanning electron microscopy and polarised Raman scattering demonstrated that the degree of alignment of the SWNTs improved with increasing electric voltage. Nanoindentation technology indicated that the modulus and hardness of the SWNT/PANi composite membrane increased in the vertically aligned CNTs. Owing to the special photo-thermal property of the PANi nanofibres, a SWNT/PANi composite film with an improved crosslinking structure and strong π–π interactions between PANi and SWNTs was obtained by intensity flash irradiation. The surface density, water contact angle and thermal stability of the composite film increased after flash welding. Nanoindentation technology indicated that the SWNT/PANi composite film significantly increased in modulus and hardness after flash welding. The SWNT/PANi film fabricated by filtration and electrical synergy method also demonstrated an improved specific capacitance of 146.0 F g⁻¹ in 6 M KOH at 0.5 A g⁻¹ after flash welding while maintaining good cycling stability. The prepared SWNT/PANi composite film is suitable for portable and wearable electronic applications.

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1. Introduction

Carbon nanotubes (CNTs),¹ which consist of graphene carbon atoms curling into a hollow tubular structure, possess an ultra-high aspect ratio, atomically smooth nanoscale pores for gas transport,^2,3 high mechanical strength⁴ and unique electronic properties.^5–7

Recent studies have investigated nanocomposites of CNTs and conducting polymers because of the special synergistic effect resulting from the excellent properties of both components.^8–11 Polyaniline (PANi) is an important structural conducting polymer because of its easy preparation and simple doping and de-doping mechanisms, which enable easy control, good environmental stability and outstanding electrical properties.^12,13 However, PANi suffers from large volumetric swelling and shrinking during doping/dedoping because of the repeated insertion/de-insertion of electrolyte ions.^14–16 Fast capacitance decay of PANi-based pseudocapacitors occurs because of their poor mechanical stability, which often leads to cracking and breaking. Given their favourable mechanical property and conductivity, CNTs can restrain this PANi deficiency in CNT/PANi composites.

The status of CNTs considerably affects the supercapacitive property of CNT/PANi composites. Lin et al.¹⁷ synthesised PANi composite films with aligned multi-walled carbon nanotubes (MWNTs) through electrodeposition, owing to the superior ionic motions of aligned CNT arrays, the prepared composite films were flexible with a high specific capacitance. Khalid et al.¹⁸ deposited PANi thin film onto PET sheet with the assistance of MWNT, the formation of nanostructured PANi framework which provides better accessibility for supercapacitive behaviour. Li et al.¹⁹ compared the electrochemical behaviour of single-walled carbon nanotube (SWNT)/PANi and MWNT/PANi composites. They found that the former composite presents higher specific capacitance and better cycling stability than the latter composite. Souza et al.²⁰ synthesised nanocomposite thin films of SWNT/PANi in a liquid–liquid interface and subsequently constructed a flexible all-solid supercapacitor. The pseudo-capacitive behaviour of the thin films was characterised by a volumetric specific capacitance under mechanical deformation, indicating that this nanocomposite has considerable application potential in new-generation energy storage devices. Xiang et al.²¹ engraved CNT film by laser drilling to form organized holes on it, and attached PANi hydrogel onto the porous CNT film by dip-coating process, the prepared flexible all-solid-state supercapacitor showed good electrochemical performance, meanwhile, displayed a rapid and reversible chromatic transition between different working stages.

Vertical aligned CNT (VACNT) membranes demonstrate numerous promising applications.^22–24 Infiltrating conducting polymer into a large surface-area VACNT membrane has attracted considerable attention because this process produces a novel composite membrane with synergic properties and improved performance. Several routes, such as chemical vapour deposition²⁵ (CVD) and spin coating,²⁶ can be used to deposit a conducting polymer into the space between vertically aligned and dense carbon-packed CNTs. Schnoor et al.²⁷ synthesised aligned MWNT/polypyrrole composite films by employing a simple in situ pyrrole monomer electropolymerisation process on a MWNT array. Meanwhile, VACNT/PANi composite membranes were successfully fabricated through microwave-assisted in situ polymerisation in our previous study,²⁸ in which the uniform filling of PANi between the interspaces of VACNTs was realised. However, the method is difficult to operate, entails a high cost and is time consuming. Furthermore, only a small device can be obtained.

Over the last decade, with the goal of further improving the performance, lot of efforts have been made to synthesize PANi with different nanostructures.^29–33 PANi nanofibres demonstrate a unique photothermal property when exposed to flash irradiation. Light absorption by a material generates heat, but heat transfer to neighbouring nanostructures and the environment is slow, resulting in chemical crosslinking between PANi nanofibres.^34,35 Henderson et al.³⁶ discussed the effect of substrates, partial masking and flash light wavelength on the flash welding efficiency of PANi. We previously reported that flash welding can be applied to tune the morphology, porosity, conductivity, permeability and nanoparticle selectivity of SWNT/PANi asymmetric ultrafiltration membranes.³⁷ In addition, as reported in our previous work, simple filtration yields MWNT/PANi composite membranes with limited vertical MWNT alignment, and subsequent flash welding improves the surface density and smoothness, as well as increases the water contact angle of the membranes.³⁸

In the present study, a vertically aligned SWNT/PANi nanocomposite membrane was fabricated by electric field–mechanical force field coupling, which is a simple and versatile technique that combines the use of an alternating current (AC) electric field with a liquid shear force. Vertically aligned SWNT/PANi nanocomposite membranes were prepared on a large scale in a short time, and then flash irritation was applied to the SWNT/PANi nanocomposite membranes. The effects of flash welding on the morphology, chemical structure, thermostability, modulus and hardness of the SWNT/PANi nanocomposite were analysed, and the supercapacitive properties of the nanocomposite were investigated.

2. Experiment

2.1 Materials

Raw SWNTs (>90 wt%) with inner diameters of 0.8–1.6 nm and lengths of 1–3 μm were purchased from Chengdu Organic Chemicals Co., Ltd., Chinese Academy of Sciences and subjected to CVD. Other analytical reagents, including aniline, ammonium persulphate (APS), nitric acid (HNO₃), sulphuric acid (H₂SO₄), hydrochloric acid (HCl), 4-aminodiphenylamine, ethanol, triethylenetetramine, acetylene black and KOH, were purchased from Sinopharm Chemical Reagent Co., Ltd. and used as received. Poly(vinylidene fluoride) (PVDF), N-methyl-2-pyrrolidone (NMP) and epoxy resin 828 were purchased from Solvay (Shanghai) Co., Ltd., Aladdin Shanghai Biochemical Technology Co., Ltd. and Royal Dutch Shell, respectively, and used as received.

2.2 SWNT/PANi membrane preparation

2.2.1 Acidification of SWNTs. To obtain short SWNTs, 400 mg of the raw SWNTs was sonicated in the HNO₃ and H₂SO₄ mixture (1/3, v/v, 100 mL) at 40 °C for 24 h. The resulting SWNTs were washed with deionised water and then treated with H₂SO₄ and hydrogen peroxide (4 : 1, v/v, 50 mL) under ultrasonication for 30 min. The acids were removed by centrifugation until the pH of the suspension was approximately 7. Finally, 200 mL of SWNTs as a deionised water suspension (0.94 mg mL⁻¹) was obtained.

2.2.2 Preparation of PANi. PANi was previously synthesised by chemical oxidation polymerisation in our previous experiments.³⁸ Aniline is the monomer, APS is the oxidant and 4-aminodiphenylamine is the initiator. A 400 mg portion of aniline (4.295 × 10⁻³ mol) was dissolved in 100 mL of HCl (0.1 mol L⁻¹), and then the initiator (1.7087 × 10⁻⁴ mol) was added to the monomer solution and stirred with a glass rod. The oxidant solution comprised 245.2 mg of APS (1.075 × 10⁻³ mol) dissolved in 100 mL of HCl (0.1 mol L⁻¹). The obtained oxidant solution was rapidly poured into the monomer solution and vigorously shaken for 15 s; then, the solution was left undisturbed for 24 h. After polymerisation, the product was washed with deionised water. Finally, 200 mL of PANi nanofibres was obtained as a deionised water suspension (0.28 mg mL⁻¹).

2.2.3 Fabrication of membrane. The mass ratio of SWNTs and PANi was 3 : 2. Therefore, acid-treated SWNTs (19.1 mL) and the PANi suspension (42.9 mL) were mixed and subsequently diluted to 300 mL with deionised water in an ultrasonic bath for 3 h. The mixture was then placed into a homemade filtration/electric device, which is shown in the schematic in Fig. 1. Two pieces of copper sheets were fixed parallel to each other as electrodes in a cylindrical vessel composed of polytetrafluoroethylene. A piece of PVDF filter (0.45 μm pore size) and a core were placed between the electrodes. The electrodes were set at a distance of approximately 12 mm (the minimum safe distance based on our previous research was selected), and the AC electrical source with virtual voltages ranged from 0 V to 450 V. The bottom of the cylindrical vessel was connected to the vacuum pump (180 W, Gongyi City Yuhua Instrument Co., Ltd., China). The duration of the electric field synergy filtration was set as 24 h. The obtained membrane was freeze-dried for 24 h at −40 °C and 80 Pa (Freeze dryer, LGJ-10D, Beijing Sihuan Science Instrument Co., Ltd.). Subsequently, the membrane was peeled off from the PVDF filter paper, and the SWNT/PANi composite membrane was prepared. Depending on the different AC voltages used, the obtained membranes were named as SWNT/PANi-0 V, SWNT/PANi-250 V, SWNT/PANi-350 V and SWNT/PANi-450 V. The SWNT/PANi membrane was thereafter flash-welded (Studio Flash Light, A8-600 Oubao Co., Ltd.) at a distance of 15 mm by a flashlight within the power range of 0–600 W s.

Fig. 1 Schematic illustration for the fabrication process of the SWNT/PANi composite film and product photos.

2.3 Characterisation

Cross-sections of the SWNT/PANi membranes before flash welding and the surfaces of the composite film after flash welding were observed by scanning electron microscopy (SEM, Quanta FEG450 FEI Co., Ltd., USA) at 20 kV. All samples scanned by SEM were coated with a thin layer of gold. The cross-section samples were prepared as follows: epoxy was used as an embedding medium, and triethylenetetramine as a curing agent. The mass ratio of triethylenetetramine to epoxy was 13.16%. The mixture was stirred using a glass rod and then placed in an embedding template. A strip of the membrane sample was placed in the embedding medium for curing at 60 °C for 3 h. The cured strips were then quenched in liquid nitrogen to obtain brittle fracture cross-sections. Polarised Raman spectra were measured to ascertain the degree of CNT alignment with a laser excitation of 532 nm (XploRA-ONE; Horiba Ltd., Kyoto, Japan). Water contact angle images measured in a static regime (contact angle measuring instrument, DSA30 KRUSS GMBH, Germany) were obtained from the surfaces of the SWNT/PANi membrane and the flash-welded composite films with different powers. The angle was read at 10 s after the water drop came in contact with the sample. The chemical structures of SWNT/PANi before and after flash welding were tested by Fourier transform infrared spectroscopy (FTIR, Spectrum 100, PerkinElmer Co., Ltd., USA), using pellets of potassium bromide in a frequency range of 450–4000 cm⁻¹ and Raman spectroscopy with a laser excitation of 785 nm (400F Raman Spectrometer, Perkin Elmer Co., Ltd., USA) at a frequency range of 200–3000 cm⁻¹. Thermal stability was characterised by thermogravimetry (TG, Pyris 1, Perkin Elmer Co., Ltd., USA) in nitrogen atmosphere at a heating rate of 10 °C min⁻¹ from 50 °C to 700 °C. Nanoindentation (Agilent Nano G200) was performed to investigate the nanomechanical properties of the samples. With the depth of impression set at approximately 100 nm, the values of elastic modulus and hardness were the average of 16 points. For the preparation of supercapacitor electrodes, the SWNT/PANi membranes before and after flash welding were mixed with an acetylene black conducting agent and a PVDF binder (dissolved in NMP, 2 mg/0.1 mL) at the mass ratio of 8 : 1 : 1, spread onto nickel foam and then pressed at 10 MPa (Tablet machine, YLJ-60T, Manufacturing Technology, Inc.). Electrochemical analyses of the samples were performed with a CHI760D three-electrode electrochemical system (Shanghai Chen Hua Instrument Co., Ltd.), with Hg/HgO electrode and a platinum plate used as the reference and counter electrodes, respectively. The electrolyte used for the electrochemical tests was 6 M KOH.

3. Results and discussion

3.1 Voltage effect on SWNT alignment

3.1.1 Morphology and SWNT alignment. Fig. 2 shows the cross-section SEM and polarised Raman spectrograms of SWNT/PANi membranes prepared by simply filtrating and filtration/electric field synergy method under different voltages. As shown in Fig. 2a, SWNTs were partly aligned vertically in the SWNT/PANi composite membrane prepared solely by the filtration method. A higher degree of vertically aligned SWNTs can be observed with the increase in electrical voltage up to 450 V.

Fig. 2 Cross-section morphologies and polarised Raman spectrograms of SWNT/PANi composite membranes prepared at different voltages: (a) 0 V, (b) 250 V, (c) 350 V and (d) 450 V; (e) polarised Raman spectrograms of SWNT/PANi composite membranes prepared at different voltages at 0° and 90° (0° corresponds to a configuration where the polarization direction of the laser light is parallel to the CNT alignment direction, whereas 90° corresponds to a configuration in which the laser light polarization direction is perpendicular to the CNT alignment direction).

The alignment of SWNTs by the synergistic effect of filtration and electric field can also be examined using polarised Raman spectroscopy.³⁹ For SWNTs, the peak at 1590 cm⁻¹ is referred to as the G-band, which is attributed to elongations of the carbon–carbon bonds along the longitudinal axis of the CNTs. The high alignment of CNTs produces a high G-band intensity ratio because Raman scattering is intense when the polarisation of the incident light is parallel to the CNT axis. Polarised Raman measured at two directions, parallel (0°) and perpendicular (90°) to the SWNTs array in SWNT/PANi composite membranes prepared at different voltages. The ratio of G-band intensity in the parallel configuration to that in the perpendicular configuration (R = I_G∥/I_G⊥) was used to characterise the degree of SWNT alignment.^40,41

The G-band intensity ratio R of the SWNT/PANi composite membranes prepared without an electric field was 1.57 (Fig. 2e). As the intensity of the electric field markedly increased with an electric voltage of 450 V, the R value of the SWNT/PANi composite membranes was enhanced to 4.13 (Fig. 2e). SWNTs can only be partly aligned by the shearing force during filtration suction, whereas the alignment can be markedly improved in the direction of the electric field because of the strong dielectrophoresis effect of SWNTs. Therefore, the filtration/electric field synergy method can yield vertically aligned SWNT/PANi composite membranes by combining the advantages of an electric field method.

3.1.2 Nanoindentation measurement for different voltages. The prepared SWNT/PANi membranes were characterised using the Nano Scratch Tester. The direction of the applied force was vertical to the surface of the composite membranes, which was parallel to the alignment of SWNTs. Fig. 3 shows the indentation load–depth curves for different voltages, and the values of elasticity modulus and hardness are shown in the inset. Applying higher electric voltages significantly improved the elasticity modulus and hardness of SWNT/PANi-450 V to 4.785 ± 0.510 and 0.277 ± 0.020 GPa, respectively, which are 1.5 times those of the SWNT/PANi prepared without an electrical field. The enhancement tendency is consistent with the SWNT vertical alignment trend R. As the electric field intensity increased, higher load can be transferred to SWNT along the axial direction, which exhibits higher strength and modulus.

Fig. 3 Nanoindentation for SWNT/PANi composite membranes at different voltages.

3.2 Flash welding effect on the SWNT/PANi composite membrane

According to the above vertical alignment of SWNTs, flash welding was applied on the SWNT/PANi-450 V composite membranes, and flashlight with a power of 150, 300 and 600 W was used. The effects of flash welding on the morphology, thermal stability, elasticity modulus, hardness and supercapacitor property of the composite were discussed.

3.2.1 Surface morphology and water contact angle. The surface morphology and water contact angle of SWNT/PANi-450 V before and after flash welding at different powers are shown in Fig. 4. The unwelded SWNT/PANi-450 V membrane was relatively rough, with a number of aggregative PANi particles. With improved flash welding intensity, these aggregative particles gradually reduced and even disappeared. The surface of the composite membrane became smoother and denser with the increase in flash welding intensity. Flash welding results in dehydrogenated benzene rings, which produce a crosslinking structure as previously verified.²⁹ Meanwhile, uniform SWNT/PANi film with sufficient flash welding thickness can be realised owing to the favourable thermal transfer properties of SWNTs. A dense morphology of the composite film for the flash irritation between PANi particles can also be realised. The unwelded SWNT/PANi-450 V membrane displayed a similar porous structure to a fibrous filter paper that easily absorbs water droplets. Water contact angles also increased from 45.1°, 51.4°, 59.5° to 61.9° with the increase in flash intensity (inset of Fig. 4). The improved contact angle can also be attributed to a less porous surface, and the lower polarity of the membranes can be ascribed to the PANi crosslinking structure after flash welding. In the following experiments, 600 W s flash irritation was selected for the SWNT/PANi-450 V membranes.

Fig. 4 Surface morphologies of processed SWNT/PANi-450 V composite membranes: (a) unwelded, (b) 150 W s, (c) 300 W s, (d) 600 W s flash welding. The insets on the top right corner are water contact angle photos.

3.2.2 Chemical structure. The FTIR spectroscopy of SWNT/PANi-450 V before and after flash welding are shown in Fig. 5a. Band at 1639 cm⁻¹ for the unwelded SWNT/PANi-450 V can be attributed to C=C stretching of the polyaromatic backbone of the nanotubes. Bands at 1559, 1349 and 1118 cm⁻¹ indicated the quinoid, C–N aromatic amine and –N=quinoid=N– stretching of the PANi. After flash welding, the band at 1559 cm⁻¹ red-shifted to 1547 cm⁻¹, implying a chemical conversion of quinoid rings into benzenoid rings, the chemical cross-linking was established through a link of the imine nitrogen with its neighboring quinoid ring.⁴² After flash welding, the carboxylic OH (1416 cm⁻¹) on the acid treated SWNTs was nearly eliminated, which may facilitate a chemical interaction between the two components.⁴³

Fig. 5 (a) FTIR and (b) Raman spectra for SWNT/PANi-450 V before and after flash welding.

Raman spectroscopy was used to measure the chemical changes in SWNT/PANi-450 V after flash welding (Fig. 5b). For the unwelded SWNT/PANi-450 V, SWNTs exhibited a sharp and strong G-band (1590 cm⁻¹) and a weak D-band, whereas PANi was identified with typical peaks of 1512 cm⁻¹ (C=N quinoid), 1386 cm⁻¹ (Ar–N benzenoid), 1326 cm⁻¹ (C–N benzenoid) and 1170 cm⁻¹ (C–H). After flash irritation, the band at 1512 cm⁻¹ red-shifted to 1498 cm⁻¹, and C–N stretching of benzenoid (1386 and 1326 cm⁻¹) became weaker, revealing newly generated bonds, such as tertiary amine nitrogen and tri-substituted benzene groups. Quinoid rings were chemically converted into benzenoid rings during the flash irritation occurred, as explained in our previous work,^37,38 and the result corresponded with the FTIR analysis. The G-band of SWNTs remained sharp and strong after flash welding, indicating that the chemical structure of SWNTs was retained. Meanwhile, the G-band red-shifted from 1598 cm⁻¹ to 1588 cm⁻¹, and the hidden C=C stretching (1598 cm⁻¹) of the quinoid of PANi appeared, indicating a strong π–π interaction between the more planar PANi conformation and the hexagonal surface lattice of the SWNTs after flash welding.¹⁹

3.2.3 Nanoindentation and TG measurement. Nanoindentation experiments of SWNT/PANi-450 V after flash welding were obtained (Fig. 6a). Fig. 6b shows the TG curves. After flash welding, the elasticity modulus improved from 4.785 ± 0.495 GPa to 6.694 ± 0.230 GPa, whereas the hardness improved from 0.227 ± 0.020 GPa to 0.412 ± 0.040 GPa. The increase level was significantly higher than that of the vertical alignment SWNT effect. On the basis of flash irradiation, the chemical crosslinking of PANi nanofibres may seamlessly infiltrate between the vertically aligned SWNTs and bond them together to form a compact composite membrane. Uniform and seamless crosslinked PANi between SWNT gaps can provide strong support and stress transfer effect. As shown in Fig. 6b, the thermal decomposition temperature of SWNT/PANi-450 V apparently increased after flash welding, and the amount of carbon residues at 700 °C increased from 27% to 71%. The significant enhancement in thermal stability can be attributed to the chemical crosslinking between PANi nanofibres during flash welding. The result correlates well with the surface morphology, water contact angle and nanoindentation experiments.

Fig. 6 (a) Nanoindentation and (b) TG analysis for SWNT/PANi-450 V before and after flash welding.

3.3 Supercapacitor properties

Cyclic voltammetry (CV) at scan rates ranging from 20 mV s⁻¹ to 100 mV s⁻¹ was used to analyse the electrochemical behaviour of SWNT/PANi-0 V and SWNT/PANi-450 V before and after flash welding, as shown in Fig. 7. Two peaks arising from oxidation and reduction (potential difference from −0.6 V to 0.4 V) appeared in the CVs, suggesting a pseudocapacitance property. After flash welding, the envelope area of the SWNT/PANi-0 V decreased from 0.0158 W s⁻² to 0.0097 W s⁻² (100 mV s⁻¹), while the SWNT/PANi-450 V composite film presented a larger envelope area after flash welding, which increased from 0.0149 W s⁻² to 0.0579 W s⁻² (100 mV s⁻¹); meanwhile, evidenced by a broad peak at negative voltage, more charges were accumulated on the SWNT/PANi-450 V nanocomposite after flash welding, which were due to the formation of a donor–acceptor pair between SWNT and PANi. According to the filtration and electrical synergy method, more uniform PANi may seamlessly infiltrate between the vertical aligned SWNTs, a compact composite membrane with hierarchical structure can be constructed after flash welding. The good synergistic effect between the two components can reduce energy loss during charge transfer and storage, thereby improving the capacitive performance of the SWNT/PANi composite as the electrode material in supercapacitors.

Fig. 7 CV curves of (a) SWNT/PANi-0 V, (b) welded SWNT/PANi-0 V, (c) SWNT/PANi-450 V and (d) welded SWNT/PANi-450 V.

The charge–discharge curves of the samples were measured using a three-electrode system at discharge currents of 0.5, 1, 2.5 and 5 A g⁻¹ (Fig. 8). The specific capacitance (capacitance per unit mass of a single electrode) of the four samples was also calculated. The specific capacitance at 0.5 A g⁻¹ of SWNT/PANi-0 V was 48 F g⁻¹, higher than that of pure PANi and CNT (approximately 10.2 F g⁻¹ (ref. 17) and 40.0 F g⁻¹ (ref. 44)). After flash welding, the specific capacitance of the welded SWNT/PANi-0 V decreased to 28.5 F g⁻¹, and the discharging time reduced from 96 s to 57 s. The specific capacitance of SWNT/PANi-450 V improved from 58.5 F g⁻¹ to 146.0 F g⁻¹ after one simple flash irritation, and the discharging time significantly increased from 113 s to 276 s. According to the uniform dispersion of PANi nanofibers between the vertical aligned SWNTs, the PANi and SWNT with hierarchical interpenetrating framework was constructed in the SWNT/PANi-450 V composite film after flash irritation, showing an enhanced charge storage capacity.

Fig. 8 Charge discharge curves of (a) SWNT/PANi-0 V, (b) welded SWNT/PANi-0 V, (c) SWNT/PANi-450 V and (d) welded SWNT/PANi-450 V.

Good cycling stability is crucial for high-performance supercapacitors. The welded SWNT/PANi-450 V composite was tested at a scan rate of 50 mV s⁻¹ for 500 cycles, and the discharge and charge capacity difference fading along with increasing cycle number can be observed in Fig. 9. The total capacitance drop was 17% after 500 cycles. The initial capacity loss for the first 25 cycles was approximately 5%, which can be attributed to the electrolyte and some aniline oligomers, whereas the capacity loss for the following cycles gradually decreased. The crosslinking between PANi and the strong π–π interactions between PANi and SWNTs after flash welding can decrease the energy loss during charge transfer and storage. Meanwhile, the good synergistic effect between the two components can restrict the volumetric swelling and shrinking during insertion/de-insertion of electrolyte ions. With their improved thermostability, elastic modulus and hardness, the prepared SWNT/PANi composite films can be used to design and validate the performance of the SWNT/PANi film and its structure for supercapacitance applications.

Fig. 9 Cycling stability of welded SWNT/PANi-450 V composite for 500 cycles.

4. Conclusions

A simple filtration and electrical synergy method was used to prepare vertically aligned SWNT/PANi membranes, and the effect of flash welding on the properties of the membranes was studied. On the basis of the high AC electric field, vertically aligned SWNTs could be formed because of their electrophoresis and dielectrophoresis properties. The degree of SWNT alignment improved with the increase in electric voltages, as demonstrated by SEM and polarised Raman scattering measurements. Meanwhile, the nanoindentation modulus and hardness of the SWNT/PANi membrane increased for higher vertically ordered SWNTs. SWNT/PANi composite films with improved crosslinking structure and strong π–π interactions between PANi and SWNTs could be obtained by flash irritation. These films showed improved surface density, water contact angle, thermal stability, elastic modulus and hardness. The SWNT/PANi nanocomposite films also demonstrated improved specific capacitance from 58.52 F g⁻¹ to 145.99 F g⁻¹ while maintaining good cycling stability (≈83% retention after 500 cycles at 50 mV s⁻¹) after 600 W s flash welding. The simple preparation method presents a considerable prospect for large-scale fabrication of the material for flexible solid capacitors.

Conflict of interests

The authors declare no competing interests.

Authors' contributions

Xiaoyan Li and Xia Wang designed the experiments. Jinrui Zhang carried out all of the experiments, performed the analysis of the results and drafted the manuscript. Biwei Qiu, Zhoujin Li and Jie Ding assisted in the experiments. Xiaoyan Li and Xia Wang also revised the manuscript. All authors read and approved the final manuscript.

Acknowledgements

The authors are grateful to the National Natural Science Foundation of China (51373100 and 51403128), the Natural Science Foundation of Shanghai (12ZR1446700) and the Innovation Project of Shanghai Municipal Education Commission (13YZ074).

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