Flexible Supercapacitors Based on Polyaniline Arrays Coated Graphene Aerogel Electrodes
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Flexible supercapacitors(SCs) made by reduced graphene oxide (rGO)-based aerogel usually suffer from the low energy density, short cycle life and bad flexibility. In this study, a new, synthetic strategy was developed for enhancing the electrochemical performances of rGO aerogel-based supercapacitor via electrodeposition polyaniline arrays on the prepared ultralight rGO aerogel. The novel hybrid composites with coated polyaniline (PANI) arrays growing on the rGO surface can take full advantage of the rich open-pore and excellent conductivity of the crosslinking framework structure of 3D rGO aerogel and high capacitance contribution from the PANI. The obtained hybrid composites exhibit excellent electrochemical performance with a specific capacitance of 432 F g-1 at the current density of 1 A g-1, robust cycling stability to maintain 85% after 10,000 charge/discharge cycles and high energy density of 25 W h kg-1. Furthermore, the flexible all-solid-state supercapacitor have superior flexibility and outstanding stability under different bending states from the straight state to the 90° status. The high-performance flexible all-solid-state SCs together with the lighting tests demonstrate it possible for applications in portable electronics.
KeywordsSupercapacitor Electrodeposition rGO aerogel Polyaniline array Flexible
The increasing demand for modern electronics such as display panels, light-emitting diodes (LEDs) and various sensors have facilitated the rapid advancement of flexible energy storage devices. Flexible supercapacitors (SCs) as an important member of energy storages family have attracted more and more concentration due to their sensational capacity performance, high power density and energy density compared to traditional capacitor and batteries, respectively [1, 2, 3, 4]. By far, in spite of the obvious advances, flexible SCs utility are greatly limited because of the relatively poor performance of the electrode materials, so the choice of electrode materials is still very important [5, 6, 7, 8, 9].
Until now, electrode materials are mainly divided into three main groups: carbon materials, metal oxides, and conductive polymer. Among them, carbon-based materials for electrical double-layer capacitors (EDLCs) possess the advantages of the large specific surface area, high electroconductivity, and long cycle stability, however, the low specific capacitance has limited them further application [10, 11, 12]. On the contrary, the metal oxides and conductive polymer for pseudocapacitors have the high specific capacitance due to the extra capacitance contribution from faradic reaction in the charge-discharge process, but the short cycle life hinders these material-based SC developments . Therefore, extensive reports have been presented to synthesis the nanocomposites of carbon materials and metal oxides/conductive polymer materials owing to their combining unique properties of individual nanostructures and possibly synergistic effects. For example, He et al.  fabricated 3D graphene-MnO2 composite networks using the method of chemical vapor deposition (CVD) and electrochemical deposition and its specific capacitance is of 465 F g-1 with cycle performance of 81.2% (5000 cycles). Meng et al.  developed 3D rGO-PANI film by template filtration and polymerization which provide a specific capacitance value up to 385 F g-1 at the current density 0.5 A g-1. Xin et al. fig prepared a graphene-based composite by the in-situ growth of a self-supporting graphene on a flexible graphite sheet via electrochemical intercalation and then the electrodeposited the polyaniline on the surface of graphene, the prepared electrode has a specific capacitance 491.3 F g-1. Although those nanocomposites exhibit the excellent electrochemical performance, little attention has been devoted to the mechanical property of the electrodes, which also play a crucial role, especially for flexile SCs.
In this study, novel flexible all-solid-state supercapacitors based on 3D rGO aerogel/polyaniline array hybrid electrodes were fabricated via a mechanical pressing and followed by electrodeposition process. The ultralight 3D rGO aerogel with excellent mechanical property, which could sustain a 4000 times of its original weight and stand on the stamen of flower, can be used as the ideal framework for growth of the PANI array, facilitating the enhanced mechanical stability of flexible all-solid-state electrode. The hybrid composites were further demonstrated with advantages of high specific capacitance of 432 F g-1, excellent rate capability (81.4% after the current density increases 20 times), and good energy density (25 W h kg-1 at the power density of 681 W kg-1). More importantly, the developed all-solid-state SCs have superior flexibility and outstanding stability under different bending states status with long time measurements.
Synthesis of 3D rGO Aerogel
The 3D rGO aerogel was synthesized by one-step self-assembled hydrothermal process . A 60 mL of 2 mg mL-1 homogeneous GO aqueous dispersion was sealed in a 100 mL Teflon-lined autoclave and maintained at 180 °C for 12 h. Then the autoclave was naturally cooled to room temperature and the as-prepared rGO hydrogels were taken out with a filter paper to remove surface water. Subsequently, the as-prepared rGO hydrogels were cut into small slices with a diameter of about 10 mm and thickness of about 1 mm and undergo freeze-drying under −83 °C for 48 h. Then, with the assistance of roller press the 3D-rGO slice was pressed directly onto the stainless steel wire mesh (the size of active material was 1 × 1 cm) and the 3D-rGO-based aerogel was obtained.
Electrodeposition Process for Growth of Flexible Hybrid Composites
The electrodeposition experiments were carried out in a three-electrode configuration with the as-prepared 3D-rGO film as the working electrode, a Pt plate as the counter electrode, and Hg/Hg2SO4 (sat. K2SO4) electrode as the reference electrode. The electrolyte was mixed with 0.05 M aniline and 1 M H2SO4 solution. The electrodeposition was performed at a current density of 2 mA · cm-2 for 7000 s at room temperature. The area of 3D-rGO employed for electrodeposition PANI was 1 × 1 cm. After washed with water, absolute ethyl alcohol, and dried at room temperature in vacuum oven for 24 h, the hybrid composites were prepared. For comparison, the aniline arrays prepared by electro-polymerization were directly growth on the stainless steel wire in the same way.
The surface morphology and microstructure of the samples were investigated by scanning electron microscopy (SEM, MAGELLIAN-400) and transmission electron microscope (TEM, JEOL JSM-2010 F), respectively. X-Ray Diffraction (XRD) was recorded on a Japan Rigaku 2550 X-ray powder diffractometer system with Cu Kα radiation (λ = 1.54056 Å) operating at 40 kV, 250 mA and the scanning angle from 10° to 70°. The Raman spectra were collected by Raman spectroscopy (Renishaw), using a 514 nm laser to identify the molecular structure of the samples. The X-ray photoelectron spectroscopy tests (XPS) were measured with a VG ESCALAB MK II electron spectrometer to characterize the surface chemical states of the samples. Electrochemical experiments of the samples were carried out by using a CHI 760E electrochemical workstation (Shanghai Chenhua Instrument Company Instruments, China) and an electrochemical workstation (IVIUM, Netherlands) at ambient temperature (about 20 °C).
Where E is the energy density (W h kg-1), P is the power density (W kg-1), C presents the total capacitance of the flexible all-solid-state SCs, ∆V is the potential drop during discharge process, and t is the discharge time .
Results and Discussion
XPS were used to monitor the surface composition of hybrid composites which was shown in Fig. 3c. Figure 3d exhibits the N1s spectrum, several new types nitrogen-containing functionalities attributed to PANI appeared in the spectrum of hybrid composites. The new group include the quinoid amine groups (=N-), the benzenoid amine nitrogen (–NH–) and positively nitrogen cationic radical (N+) with a binding energy centered at 398.8, 399.4, and 401 eV, respectively [25, 26]. The high ratio of N+ also illustrates that nitrogen protons are successfully doped in hybrid composites and it can improve the electrical conductivity. Simultaneously, a well peak at 285.6 eV can be assigned to the chemical bond C-N in C1s spectrum, found in Fig. 3e, indicates that PANI and 3D rGO are well connected too . Figure 3f provides the O1s spectrum, three peaks at 531.1, 532.1, and 533.4 eV corresponding to the bond of C = O, C-O and H-O-H appeared because of the presence of water or other oxygen molecules groups . All above analysis results prove the PANI were tightly deposited on the surface of 3D rGO, which is beneficial for flexible and tough self-supported structure.
In conclusion, a flexible all-solid-state SC based on 3D rGO/polyaniline array hybrid composites has been fabricated. The obtained hybrid composites have a specific capacitance of 432 F g-1 at the current density of 1A g-1, and robust cycling stability with a capacitance retention of 85% after 10,000 charge/discharge cycles. Ulteriorly, the all-solid-state supercapacitor showed a good energy density of 25 W h kg-1 and power density of 681 W kg-1. The excellent performance of hybrid composites based SCs can be attributed to the special 3D structure and synergistic effect of 3D rGO aerogel and PANI arrays. In addition, the fabricated SCs have superior flexibility and outstanding stability under different bending states. Considering the combined high mechanical and electrochemical properties, the hybrid composite-based flexible all-solid-state SC are particularly promising for the wearable electronics.
The authors sincerely acknowledge financial support from National Natural Science Foundation of China (NSFC Grant Nos. 21571080), and the Special program of international science and technology cooperation projects of China (No.2014DFR61140).
YY carried out the sample fabrication and the measurements of the materials and devices as well as drafting the manuscript. WH analyzed the data and helped modify the manuscript. YLX, GDW, and NIK participated in the discussion of results. JZL helped to prepare the SEM and TEM sample. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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- 39.Li L, Lou Z, Han W, Chen D, Jiang K, Shen G (2017) Highly stretchable micro-supercapacitor arrays with hybrid MWCNT/PANI electrodes. Adv Mater Tech. doi: 10.1002/admt.201600282
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