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polyaniline nanocomposite film prepared by diffusion driven layer-by-layer assembly for high-performance supercapacitors
Journal of Power Sources 343 (2017) 60e66
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Three-dimensional reduced graphene oxide/polyaniline nanocomposite film prepared by diffusion driven layer-by-layer assembly for high-performance supercapacitors Xiaodong Hong a, Binbin Zhang b, Elizabeth Murphy b, Jianli Zou b, *, Franklin Kim b, ** a b
College of Materials Science and Engineering, Liaoning Technical University, Fuxin 123000, China Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Kyoto 606-8501, Japan
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
3D porous rGO is prepared by diffusion driven layer-by-layer assembly. Polyaniline (PANI) nanoparticles are in-situ grown within the pores of 3D rGO. As-prepared rGO/PANI was used as binder-free electrodes for supercapacitor. The rGO/PANI exhibits capacitance of 438.8 F g1 at 0.5 A g1 in 1 M H2SO4. PANI particles achieve a higher capacitance of 763 F g1 at 0.5 A g1.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 September 2016 Received in revised form 23 December 2016 Accepted 8 January 2017
As a simple and versatile method, diffusion driven Layer-by-Layer assembly (dd-LbL) is developed to assemble graphene oxide (GO) into three-dimensional (3D) structure. The assembled GO macrostructure can be reduced through a hydrothermal treatment and used as a high volumetric capacitance electrode in supercapacitors. In this report we use rGO framework created from dd-LbL as a scaffold for in situ polymerization of aniline within the pores of the framework to form rGO/polyaniline (rGO/PANI) composite. The rGO/PANI composite affords a robust and porous structure, which facilitates electrolyte diffusion and exhibits excellent electrochemical performance as binder-free electrodes in a sandwichconfiguration supercapacitor. Combining electric double layer capacitance and pseudo-capacitance, rGO/PANI electrodes exhibit a specific capacitance of 438.8 F g1 at discharge rate of 5 mA (mass of electrodes were 10.0 mg, 0.5 A g1) in 1 mol L1 H2SO4 electrolyte; furthermore, the generated PANI nanoparticles in rGO template achieve a higher capacitance of 763 F g1. The rGO/PANI composite electrodes also show an improved recyclability, 76.5% of capacitance retains after recycled 2000 times. © 2017 Elsevier B.V. All rights reserved.
Keywords: Diffusion driven layer-by-layer assembly Polyaniline Supercapacitor Graphene oxide
1. Introduction
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Zou), [email protected] (F. Kim). http://dx.doi.org/10.1016/j.jpowsour.2017.01.034 0378-7753/© 2017 Elsevier B.V. All rights reserved.
Advances made in the synthesis of graphene and graphene oxide (GO) have greatly contributed to its wide application, such as in the fields of biomaterials [1], polymer reinforcement [2], membrane [3], and energy storage [4e7]. These days, graphene and GO can be produced through mechanical, chemical, and
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electrochemical exfoliation of graphite on a large scale [7e9]. During the chemical exfoliation of graphite, the oxidation step renders oxygen-containing groups on basal and edge positions of the carbon sheets, giving GO a high stability in various solvents. The conductivity of GO can be recovered by hydrothermal [10], thermal [11], flash [12], or chemical reduction [13]. Its excellent conductivity, mechanical strength, and flexibility, as well as its largesurface area, make rGO an outstanding electrode material for supercapacitors [4,5,7,14]. In general, graphene-related electrodes mainly store energy through electric-double layer capacitance (EDLC) in the range of a few hundred farads per gram. This capacitance is relatively low in comparison with pseudocapacitance materials, which can reach a high capacitance through redox reactions. For example, PANI (polyaniline), a typical conducting polymer, has been applied as a pseudo-capacitive material by undergoing fast and reversible Faradaic redox reactions [15]. Various PANI nanostructures can be synthesized easily and at low cost [16e20]. However, PANI materials are inherently brittle, poorly soluble and unstable, which prevents their wide use as electrode materials. Using carbonaceous materials, such as graphite, carbon nanotubes and GO as a support for PANI has been widely reported [14,21e27]. Evenly distributed PANI nanofibers can be synthesized on the surface of dispersed GO nanosheets via polymerization of aniline [28]. These composites exhibit a combination of electricdouble layer capacitance and pseudo-capacitance, while exhibiting improved PANI stability to some extent. However, in general these hybrid materials are in a powder form and need to be processed into an electrode by casting with a binder and carbon black [14,28e30]. Therefore, it has become crucial to develop straightforward methods to construct binder-free carbon/PANI electrodes. Recently, construction of PANI nanoparticles on 3D rGO framework or aerogel has been directly used as binder-free electrodes for supercapacitors [31,32]. In this field, the CaCO3 particles were applied as a sacrificial template for preparing 3D porous RGO, and PANI nanowire arrays were grown on the 3D-RGO by a dilute polymerization. As electrodes for supercapacitor, the specific capacitance of 385 F g1 was exhibited at 0.5 A g1 [33], and 243 F g1 was achieved at 1 A g1 [34]. Yu et al. [35] prepared 3D hierarchical porous RGO foam (RGO-F) by “dipping and dry” method using nickel foam as the template, PANI nanowire array was in situ polymerized on the RGO-F for symmetric supercapacitor. High specific capacitance of 790 F g1 and volumetric capacitance of 205.4 F cm3 were exhibited at 1 A g1. Yang et al. [36] prepared 3D graphene/PANI hybrid aerogel by hydrothermal method with synthesized PANI nanowires and GO, the hybrid aerogel showed high capacitance of 520.3 F g1 at 0.25 A g1. Although achieving high capacitance, these GO structures are often prepared using rather complicated processes, such as the sacrificial template method, which unavoidably hinders the scale-up of production. A key difficulty in constructing 3D GO structures is how to prevent nanosheets from restacking. This effect is caused by the strong inter-planar van der Waals forces, which greatly decreases the specific surface area and hinders the ion transportation when applied as electrode materials. Li et al. constructed a reduced GO hydrogel film with the nanosheets in a highly corrugated state by a filtration-based evaporation-assisted process [6], which exhibited some of the highest volumetric capacitance reported so far for a graphene capacitor. Although this method is promising, in practice this hydrogel film would hardly endure the polymerization of PANI. Therefore, a 3D GO assembly with high-surface area and high mechanical stability would be a favourable template to host PANI. Diffusion driven layer-by-layer (dd-LbL) assembly was recently reported to build GO sheets into 3D foam-like macrostructure [37], based on strong electrostatic interactions between GO and bPEI
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(branched polyethyleneimine). Dd-LbL provides a simple yet scalable method to produce 3D porous graphene framework, making it promising for practical application in a binder-free electrodes for supercapacitors [38]. Herein, we further demonstrate that this 3D GO framework assembled via dd-LbL is an ideal template for in situ polymerization of aniline. As a binder-free electrode, the resultant rGO/PANI nanocomposites show excellent electrochemical performance in supercapacitors. 2. Experimental 2.1. Materials All the reagents, branched polyethylenimine (bPEI, Mw ¼ 750,000, 50 wt % in H2O, Aldrich), aniline (>98%, Tokyo Chemical Industry Co. Ltd.), ammonia persulfate (APS, >98%, Sigma-Aldrich), perchloric acid (70%, Sigma-Aldrich), and ethanol (>99%, Tokyo Chemical Industry Co. Ltd.) were used as received. 2.2. Sample preparation 2.2.1. Preparation of porous rGO template by dd-LbL assembly and hydrothermal reduction GO was synthesized by a modified Hummers method [39] and the concentration of GO suspension was controlled as 8 mg ml1 in DI water. An assembly GO suspension (4 mg ml1) was prepared by mixing GO suspension with dimethylformamide (DMF) by equal volume. A 25 wt% bPEI aqueous solution (1.4 mL) was used to smear a filter paper within an area of 7.5 10 cm2 by a glass slide, a uniform layer of bPEI solution was formed on the filter paper and set for 5 min. The coated filter paper was then immersed in the above assembly GO suspension and shaken overnight with a speed of 100 rpm. The assembled GO/bPEI film (about several millimeters) was produced on the coated area of the filter paper, the film was rinsed several times with DI water, cut into 2 2 cm2 pieces and detached from the filter paper. The rGO templates were then prepared as following procedures. Several pieces of GO/bPEI film (2 2 cm2) were placed between glass slides separately before putting them in a Teflon vessel (100 mL) with 25 mL DI water. The Teflon vessel was put in an autoclave and kept at 190 C for 24 h. After hydrothermal treatment, the rGO templates were obtained after washing for 5 times by DI water. 2.2.2. Preparation of rGO/PANI film by in situ polymerization The rGO/PANI films were prepared by in situ polymerization [14]. A certain amount of aniline monomer (0.625 mmol, 1.25 mmol, 2.5 mmol, 3.75 mmol, 6.25 mmol and 7.5 mmol) was added into a mixture solution consisting of 20 mL HClO4 (1 M) aqueous solution and 5 mL ethanol. The concentrations of aniline in the mixture solution were 0.025 M, 0.05 M, 0.1 M, 0.15 M, 0.25 M and 0.3 M. The rGO templates (2.4 mg each, 1.4 1.4 cm2) were immersed into the aniline solution for 4 h to allow the adsorption of aniline monomer onto the surface of rGO sheets as nucleation sites. APS (ammonia persulfate) solutions were freshly prepared by dissolving APS (APS: aniline ¼ 2: 3 by molar ratio) in 15 mL of 1 M HClO4 solution and placed in ice bath for at least 10 min. The cooled APS solution was poured into aniline solution containing rGO templates and shaken vigorously for 30 s to mix well, then the reaction vessel was kept in an ice bath (about 1 C) for 24 h to allow the formation of PANI. The resultant rGO/PANI films were taken out and rinsed for 5 times with 1 M H2SO4 to remove physically adsorbed PANI particles on the film surface, as well as unreacted starting materials.
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2.3. Electrochemical performance The rGO/PANI films were immersed in 1 M H2SO4 for 4 h and then assembled into symmetric two-electrode sandwich-configuration supercapacitors with Pt foil as current collector and membrane filter paper (Omnipore ™) as separator. Electrochemical performance of the supercapacitors was tested using cyclic voltammetry (CV) and galvanostatic charge/discharge on an eDAQ potentiostat (Potentiostat 466, eDAQ Pty Ltd). The potential range for CV and galvanostatic charge/discharge tests was from 0.0 to þ0.8 V. 2.4. Materials characterization FTIR spectra were recorded on a JASCO FT/IR-4200 Fourier transform infrared spectrometer by using KBr pellet. XRD were taken on a Rigaku RINT2500 with Cu Ka radiation (l ¼ 0.154 nm). SEM pictures were observed by a JEOL JSM-7001 F4 microscope. Samples were fixed on conductive tape and coated osmium layer (about 5 nm). 3. Results and discussion Fig. 1 illustrates the preparation of rGO/PANI composites. First, a 3D GO macrostructure was prepared by using dd-LBL method [37]. In brief, bPEI solution was coated uniformly on a filter paper. The coated filter paper served as a reservoir of bPEI solution and acted as a substrate for film growth. When the filter paper was immersed in the assembly GO suspension, upon contact, a stable GO/bPEI layer could be formed by the complexation between diffused bPEI and GO. The continuous diffusion of bPEI from the reservoir constructed new GO layers until the GO/bPEI complex developed into a millimetre-thick film, and complexation-and-diffusion process completed. The assembled large GO/bPEI film was cut into 2 2 cm2 pieces and detached from the substrate, the freestanding GO/bPEI film remained intact (Fig. S1). Following this step, GO/bPEI films (2 2 cm2) were reduced through hydrothermal treatment at 190 C for 24 h to recover their conductivity [40], and the resulting material denoted as rGO template. Finally, rGO templates were immersed in a certain concentration of aniline solution (mixture of 20 mL 1 M HClO4 and 5 mL ethanol) for 4 h to allow for the adsorption of aniline monomers onto the rGO sheets as nucleation sites. Freshly prepared APS (ammonia persulfate) solution was added into the aniline solution rapidly to induce polymerization. In this way PANI particles formed on the nanosheets of rGO framework, the resultant samples were denoted as rGO/PANI. Fig. 2a shows the photograph of a piece GO/bPEI film before and after hydrothermal treatment. The size of the GO/bPEI film shrunk
from 2.0 cm to around 1.4 cm after reduction. The shrinkage of the GO/bPEI films was likely due to the decomposition of most bPEI during hydrothermal process, and only a small percentage of bPEI will remain in the macrostructure, therefore the whole structure was pulled together. The degree of shrinkage is affected by temperature, reaction time, concentration of initial GO and bPEI, etc. Despite such shrinkage, the film still retains its well-connected porous structure with micrometre pore size, as shown in Fig. 2b and c. The rGO sheets exhibit flexible and wrinkled features. This highly porous structure is easily accessed by molecules and serves as ideal template for PANI. After polymerization we can clearly observe the formed PANI nanoparticles on the surface of rGO sheets from SEM images (Fig. 2def). The surface of the rGO nanosheets is fully covered by PANI nanoparticles, becoming thick and rough in comparison with rGO. The porous rGO and rGO/PANI-0.1 were characterized by nitrogen adsorption-desorption (Fig. S2), and the corresponding BET specific surface areas are 211.54 and 134.69 m2 g1, respectively. The specific surface area of rGO/PANI0.1 decreases after the PANI nanoparticles generated on graphene nanosheets, which may be ascribed to the higher density of PANI than rGO. Furthermore, the distribution of pore size can be observed from Fig. S2b, the rGO/PANI-0.1 shows a similar pore size distribution (50e120 nm) feature as rGO. The pore volume of rGO/ PANI-0.1 is 0.16 cm3 g1 and less than 0.51 cm3 g1 of rGO framework. These results prove that the PANI nanoparticles on rGO sheets do not change the macroporous structure of rGO, while the PANI nanoparticles dispersing on the graphene sheets inevitably decreases the pore volume of rGO template. The concentrations of aniline were optimized during the preparation of rGO/PANI films. We noticed that the weight of rGO templates increased after polymerization and, accordingly, the weight percentage of PANI in rGO/PANI increased as well (Fig. 3). The weight of one piece of rGO template initially was 2.4 mg and the weight of resultant rGO/PANI composite increased to 3.7 mg when 0.025 M aniline was used, the material denoted as rGO/PANI0.025. The weight of rGO/PANI film kept increasing with increasing concentrations of aniline and reached 15.5 mg when 0.3 M aniline was used, which was the highest concentration in our experiments. When the aniline concentration enhanced from 0.025 M to 0.1 M, the morphology of PANI on rGO sheets noticeably changed (Fig. 3bec and Fig. 2def). The rGO sheet in rGO/PANI-0.025 was covered by a thin layer of PANI; however, when the aniline concentration enhanced to 0.05 M, we can clearly observe the protruding PANI particles on the rGO sheets, and these particle-like PANI became more obvious when 0.1 M aniline was used. At a higher concentration (0.15 M), we observed the aggregation of PANI particles within rGO template along with well-distributed, small PANI particles in the final rGO/PANI film, which we denote as rGO/
Fig. 1. Schematic of preparing rGO/PANI composite film. First, bPEI solution was coated on a filter paper. The coated paper was immersed in assembly GO suspension. GO and bPEI formed a stable complex layer at first. Then, bPEI diffused out of the reservoir and complexed with GO sheets. This complexation-and-diffusion process continued, eventually developed into a thicker GO/bPEI composite film. Some square GO/bPEI films (2 2 cm2) was cut and detached from the substrate, then subjected to a hydrothermal treatment at 190 C. Finally, the hydrothermally-reduced GO films were used as a template for the polymerization of aniline.
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Fig. 2. Photograph and SEM images of rGO template and rGO/PANI film. a) Photograph of the GO/bPEI film, rGO template and rGO/PANI film, b-c) Cross-section SEM images of rGO template. d-f) Cross-section SEM images of rGO/PANI-0.1 clearly show the formation of PANI on the rGO sheets.
PANI-0.15 (Fig. 3dee). The aggregation of PANI causes a decline in the electrochemical performance of rGO/PANI, which will be discussed later. In the case of 0.25 M and 0.3 M aniline, PANI formed only around the periphery of the film and no obvious PANI were found inside rGO template. It is likely that the highly concentrated monomer lead to fast polymerization at the beginning of the reaction, and the large amount of PANI rapidly generated only at the surface of rGO template, blocked the pores of rGO structure. In this case, APS molecules had no time to diffuse into the template, therefore, PANI could not be further generated inside of the rGO (Fig. S3). The FTIR spectrum of rGO/PANI exhibits the characteristic peak of rGO and PANI, which further confirms the formation of PANI on rGO scaffolds (Fig. 4). In rGO/PANI the prominent peaks at 1563 cm1 and 1474 cm1 are assigned to stretching vibration of quinoid rings (C]N) and benzenoid rings (C]C). Two peaks at 1295 cm1 and 1238 cm1 are corresponded to CeN stretching vibrations with aromatic conjugation [41]. PANI formed on GO sheets is also confirmed by XRD (X-ray diffraction) in Fig. S4. In rGO/PANI two peaks are shown at 2q ¼ 19.3 and 25.0 , which are related to PANI, but these peaks are not presented in the XRD curve of rGO [42]. We investigated the electrochemical performance of these rGO/ PANI films in a sandwich-configuration supercapacitor. The rGO/ PANI films were mechanically stable and could be directly used as binder-free electrodes. First, rGO/PANI films were immersed in 1 M H2SO4 aqueous solution for 4 h. The electrochemical performance was measured by using a two-electrode supercapacitor with Pt foil as current collector and Omnipore membrane as separator. Fig. 5a shows the cyclic voltammetry (CV) curves of rGO template and rGO/PANI films at the scan rate of 10 mV s1. Notably, CV curves for rGO template show rectangular shapes at the scan rate ranging
from 10 to 200 mV s1 (Fig. S5), which indicate that the well interconnected structure of rGO makes electrolytes diffuse freely throughout the rGO template. The rGO template shows an ideal double-layer capacitive behaviour. In comparison with rGO template, four rGO/PANI samples clearly exhibit redox peaks in CV curves, which matches well with the characteristic feature of PANI in pseudo-capacitance. These redox peaks can be attributed to the leucoemeraldine base/emeraldine salt and emeraldine salt/pernigraniline base transformation of PANI [43e45]. Since the area enclosed by CV curves is proportional to the specific capacitance, all of four rGO/PANI samples clearly show increased capacitance with regards to rGO template. Galvanostatic charge/discharge curves at a current of 10 mA are shown in Fig. 5b. The charge/discharge curve of rGO template is symmetric, with straight lines, while those of rGO/PANI electrodes exhibit the feature of pseudo-capacitance, with curved charge and discharge lines. The gravimetric specific capacitances of all samples, as presented in Fig. 5c, were calculated by an equation of C ¼ 4·I·Dt/ (V·m), where I is the charge-discharge current, Dt is the discharge time, V is the discharge voltage, and m is the total weight of two electrodes. We use Ct to represent the specific capacitance of the whole electrodes, including rGO and PANI. The Ct of rGO/PANI-0.1 was prominently higher than that of the other three electrodes and reached to a high value of 438.8 F g1 at a charge/discharge current of 5 mA (0.5 A g1), 428.0 F g1 at a charge/discharge current of 20 mA (2 A g1) and 401.5 F g1 at a charge/discharge current of 40 mA (4 A g1). Meanwhile, given the total volume 0.01372 cm3 (1.4 cm 1.4 cm 0.0070 cm) of two electrodes, the volumetric capacity of rGO/PANI-0.1 can be calculated as 319.83 F cm3 at 0.5 A g1, and 311.95 F cm3 at 2 A g1. Comparing with the reported capacitances about relative graphene/PANI composites in Table S1, the capacitances of more than 500 F g1 are
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Fig. 4. The FTIR spectra of rGO, PANI and rGO/PANI-0.1.
Fig. 3. The influence of the concentration of aniline. a) The weight of rGO/PANI film and the percentage of PANI in the whole rGO/PANI film corresponding to different concentrations of aniline during polymerization. SEM images of the resultant rGO/PANI film when the aniline concentration was b) 0.025 M, c) 0.05 M, and d-e) 0.15 M.
almost obtained at lower current density (<0.3 A g1), and the same electrodes show great declining capacitance (<400 F g1) under higher current density (>1 A g1). Even the highest gravimetric capacitance reported as 790 F g1 at 1 A g1, the volumetric capacitance is 205.4 F cm3 [35], which is lower than our work (311.95 F cm3 at 2 A g1). Those results show that our 3D-rGO/ PANI electrodes have a better performance, especially for the higher volumetric capacitance and capacitance stability under higher current density (401.5 F g1 at 4 A g1), which is attributed to the compressible feature of 3D rGO framework prepared by diffusion driven layer-by-layer self-assembly, as reported the higher volumetric capacitance in our previous work [38]. Compared with rGO/PANI-0.025 and rGO/PANI-0.05, the loading of PANI in rGO/PANI-0.1 was much higher, as shown in Fig. 3a, so the increase in Ct was expected. In the case of rGO/PANI-0.15, although the loading of PANI was even higher, Ct decreased to 381.1 F g1 at a charge/discharge current of 5 mA (0.34 A g1). We attribute the sudden drop of Ct to the agglomeration of PANI caused by the high concentration of aniline during the polymerization reaction. These agglomerations of PANI did not fully participate in the redox
reaction due to their large particle size. To further evaluate the capacitance of PANI we deducted the capacitance contributed by rGO template (CGO) from the whole capacitance (Ct) and compared the remaining capacitances, which could be attributed to PANI (CP). As it can be expected, CP of rGO/ PANI-0.15 was the lowest among all samples in Fig. 5d, with only 512.5 F g1 at a discharge current of 5 mA (0.34 A g1). However, it is very interesting that CP of the remaining three electrodes was very close, especially at low discharge rates (763.8 F g1 for rGO/ PANI-0.025, 756.1 F g1 for rGO/PANI-0.05 and 746.7 F g1 of rGO/PANI-0.1 at 5 mA), which indicates that PANI particles in these three electrodes were almost equally active during redox reactions. These CP data also confirmed that the reason Ct of rGO/PANI-0.1 higher than Ct of rGO/PANI-0.15 was indeed because the performance of PANI in the latter electrode was impaired by its agglomeration. Notably, the overall capacitance of PANI materials from these experiments was one of the highest values reported in 1 M H2SO4 electrolyte so far [46]. We attribute the high capacitance to two main features of the electrode: 1) PANI particles are evenly and uniformly distributed throughout the rGO template at the proper concentration of aniline, and 2) porous and wellinterconnected rGO template plays an effective role in improving the diffusion of the electrolyte. The stability of rGO/PANI-0.1 was evaluated by charge/discharge measurements at 40 mA (4 A g1). The decrease of capacitance from rGO template can be neglected during recycling. The rGO/PANI-0.1 electrode retained 76.5% of its initial capacitance after 2000 cycles (Fig. 6). The capacitance of PANI usually decreases dramatically during long-term charge/discharge, which is also one of the main drawbacks of using PANI in energy storage devices. For example, Shi et al. reported a 29% decrease after 800 times of recycling in the case of a PANI-nanowire capacitor [29] and Li et al. also reported a 41% capacity retention of their PANI capacitor after 880 cycles [47].
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Fig. 5. Electrochemical performance of rGO template and rGO/PANI electrodes in 1 M H2SO4 electrolyte. a) The CV curves at a scan rate of 10 mV s1. b) Charge/discharge curves measured at 10 mA. c) Specific capacitances and d) Capacitance of PANI in rGO/PANI electrodes at various charge/discharge currents.
4. Conclusions We demonstrated that the 3D porous GO macrostructure fabricated via dd-LbL served as an ideal template for in situ polymerization of aniline. The well inter-connected structure of the GO not only facilitated the loading of evenly distributed PANI throughout the template, but also played a critical role in improving electrolyte diffusion during charge/discharge process. The resultant graphene based PANI hybrid materials are mechanically stable and highly conductive, which leads to excellent electrochemical performance by combining electric double-layer capacitance of rGO template and pseudo-capacitance of PANI. The set-up of dd-LbL assembly is simple, and the size of the assembled GO film can be scaled up easily, which makes this method highly attractive for practical application.
Acknowledgements Fig. 6. The recycling stability of rGO template and rGO/PANI-0.1 electrode at a charge/ discharge current of 40 mA.
In comparison with the stability of most of PANI supercapacitors, our rGO/PANI electrode exhibited a large improvement while maintaining its high capacitance.
This work was supported by the National Natural Science Foundation of China under grant no. 51403094 and China Scholarship Council (grant no. 201408210052). This work was also supported by the Institute for Integrated Cell-Material Sciences (iCeMS; Kyoto University), JSPS KAKENHI grant no. 24681019 (Wakate-A) and JSPS KAKENHI grant no. 25000007 (specially promoted research).
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Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.01.034.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Three-dimensional reduced graphene oxide/polyaniline nanocomposite film prepared by diffusion driven layer-by-layer assembly for high-performance supercapacitors Xiaodong Hong a, Binbin Zhang b, Elizabeth Murphy b, Jianli Zou b, *, Franklin Kim b, ** a b
College of Materials Science and Engineering, Liaoning Technical University, Fuxin 123000, China Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Kyoto 606-8501, Japan
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
3D porous rGO is prepared by diffusion driven layer-by-layer assembly. Polyaniline (PANI) nanoparticles are in-situ grown within the pores of 3D rGO. As-prepared rGO/PANI was used as binder-free electrodes for supercapacitor. The rGO/PANI exhibits capacitance of 438.8 F g1 at 0.5 A g1 in 1 M H2SO4. PANI particles achieve a higher capacitance of 763 F g1 at 0.5 A g1.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 September 2016 Received in revised form 23 December 2016 Accepted 8 January 2017
As a simple and versatile method, diffusion driven Layer-by-Layer assembly (dd-LbL) is developed to assemble graphene oxide (GO) into three-dimensional (3D) structure. The assembled GO macrostructure can be reduced through a hydrothermal treatment and used as a high volumetric capacitance electrode in supercapacitors. In this report we use rGO framework created from dd-LbL as a scaffold for in situ polymerization of aniline within the pores of the framework to form rGO/polyaniline (rGO/PANI) composite. The rGO/PANI composite affords a robust and porous structure, which facilitates electrolyte diffusion and exhibits excellent electrochemical performance as binder-free electrodes in a sandwichconfiguration supercapacitor. Combining electric double layer capacitance and pseudo-capacitance, rGO/PANI electrodes exhibit a specific capacitance of 438.8 F g1 at discharge rate of 5 mA (mass of electrodes were 10.0 mg, 0.5 A g1) in 1 mol L1 H2SO4 electrolyte; furthermore, the generated PANI nanoparticles in rGO template achieve a higher capacitance of 763 F g1. The rGO/PANI composite electrodes also show an improved recyclability, 76.5% of capacitance retains after recycled 2000 times. © 2017 Elsevier B.V. All rights reserved.
Keywords: Diffusion driven layer-by-layer assembly Polyaniline Supercapacitor Graphene oxide
1. Introduction
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Zou), [email protected] (F. Kim). http://dx.doi.org/10.1016/j.jpowsour.2017.01.034 0378-7753/© 2017 Elsevier B.V. All rights reserved.
Advances made in the synthesis of graphene and graphene oxide (GO) have greatly contributed to its wide application, such as in the fields of biomaterials [1], polymer reinforcement [2], membrane [3], and energy storage [4e7]. These days, graphene and GO can be produced through mechanical, chemical, and
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electrochemical exfoliation of graphite on a large scale [7e9]. During the chemical exfoliation of graphite, the oxidation step renders oxygen-containing groups on basal and edge positions of the carbon sheets, giving GO a high stability in various solvents. The conductivity of GO can be recovered by hydrothermal [10], thermal [11], flash [12], or chemical reduction [13]. Its excellent conductivity, mechanical strength, and flexibility, as well as its largesurface area, make rGO an outstanding electrode material for supercapacitors [4,5,7,14]. In general, graphene-related electrodes mainly store energy through electric-double layer capacitance (EDLC) in the range of a few hundred farads per gram. This capacitance is relatively low in comparison with pseudocapacitance materials, which can reach a high capacitance through redox reactions. For example, PANI (polyaniline), a typical conducting polymer, has been applied as a pseudo-capacitive material by undergoing fast and reversible Faradaic redox reactions [15]. Various PANI nanostructures can be synthesized easily and at low cost [16e20]. However, PANI materials are inherently brittle, poorly soluble and unstable, which prevents their wide use as electrode materials. Using carbonaceous materials, such as graphite, carbon nanotubes and GO as a support for PANI has been widely reported [14,21e27]. Evenly distributed PANI nanofibers can be synthesized on the surface of dispersed GO nanosheets via polymerization of aniline [28]. These composites exhibit a combination of electricdouble layer capacitance and pseudo-capacitance, while exhibiting improved PANI stability to some extent. However, in general these hybrid materials are in a powder form and need to be processed into an electrode by casting with a binder and carbon black [14,28e30]. Therefore, it has become crucial to develop straightforward methods to construct binder-free carbon/PANI electrodes. Recently, construction of PANI nanoparticles on 3D rGO framework or aerogel has been directly used as binder-free electrodes for supercapacitors [31,32]. In this field, the CaCO3 particles were applied as a sacrificial template for preparing 3D porous RGO, and PANI nanowire arrays were grown on the 3D-RGO by a dilute polymerization. As electrodes for supercapacitor, the specific capacitance of 385 F g1 was exhibited at 0.5 A g1 [33], and 243 F g1 was achieved at 1 A g1 [34]. Yu et al. [35] prepared 3D hierarchical porous RGO foam (RGO-F) by “dipping and dry” method using nickel foam as the template, PANI nanowire array was in situ polymerized on the RGO-F for symmetric supercapacitor. High specific capacitance of 790 F g1 and volumetric capacitance of 205.4 F cm3 were exhibited at 1 A g1. Yang et al. [36] prepared 3D graphene/PANI hybrid aerogel by hydrothermal method with synthesized PANI nanowires and GO, the hybrid aerogel showed high capacitance of 520.3 F g1 at 0.25 A g1. Although achieving high capacitance, these GO structures are often prepared using rather complicated processes, such as the sacrificial template method, which unavoidably hinders the scale-up of production. A key difficulty in constructing 3D GO structures is how to prevent nanosheets from restacking. This effect is caused by the strong inter-planar van der Waals forces, which greatly decreases the specific surface area and hinders the ion transportation when applied as electrode materials. Li et al. constructed a reduced GO hydrogel film with the nanosheets in a highly corrugated state by a filtration-based evaporation-assisted process [6], which exhibited some of the highest volumetric capacitance reported so far for a graphene capacitor. Although this method is promising, in practice this hydrogel film would hardly endure the polymerization of PANI. Therefore, a 3D GO assembly with high-surface area and high mechanical stability would be a favourable template to host PANI. Diffusion driven layer-by-layer (dd-LbL) assembly was recently reported to build GO sheets into 3D foam-like macrostructure [37], based on strong electrostatic interactions between GO and bPEI
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(branched polyethyleneimine). Dd-LbL provides a simple yet scalable method to produce 3D porous graphene framework, making it promising for practical application in a binder-free electrodes for supercapacitors [38]. Herein, we further demonstrate that this 3D GO framework assembled via dd-LbL is an ideal template for in situ polymerization of aniline. As a binder-free electrode, the resultant rGO/PANI nanocomposites show excellent electrochemical performance in supercapacitors. 2. Experimental 2.1. Materials All the reagents, branched polyethylenimine (bPEI, Mw ¼ 750,000, 50 wt % in H2O, Aldrich), aniline (>98%, Tokyo Chemical Industry Co. Ltd.), ammonia persulfate (APS, >98%, Sigma-Aldrich), perchloric acid (70%, Sigma-Aldrich), and ethanol (>99%, Tokyo Chemical Industry Co. Ltd.) were used as received. 2.2. Sample preparation 2.2.1. Preparation of porous rGO template by dd-LbL assembly and hydrothermal reduction GO was synthesized by a modified Hummers method [39] and the concentration of GO suspension was controlled as 8 mg ml1 in DI water. An assembly GO suspension (4 mg ml1) was prepared by mixing GO suspension with dimethylformamide (DMF) by equal volume. A 25 wt% bPEI aqueous solution (1.4 mL) was used to smear a filter paper within an area of 7.5 10 cm2 by a glass slide, a uniform layer of bPEI solution was formed on the filter paper and set for 5 min. The coated filter paper was then immersed in the above assembly GO suspension and shaken overnight with a speed of 100 rpm. The assembled GO/bPEI film (about several millimeters) was produced on the coated area of the filter paper, the film was rinsed several times with DI water, cut into 2 2 cm2 pieces and detached from the filter paper. The rGO templates were then prepared as following procedures. Several pieces of GO/bPEI film (2 2 cm2) were placed between glass slides separately before putting them in a Teflon vessel (100 mL) with 25 mL DI water. The Teflon vessel was put in an autoclave and kept at 190 C for 24 h. After hydrothermal treatment, the rGO templates were obtained after washing for 5 times by DI water. 2.2.2. Preparation of rGO/PANI film by in situ polymerization The rGO/PANI films were prepared by in situ polymerization [14]. A certain amount of aniline monomer (0.625 mmol, 1.25 mmol, 2.5 mmol, 3.75 mmol, 6.25 mmol and 7.5 mmol) was added into a mixture solution consisting of 20 mL HClO4 (1 M) aqueous solution and 5 mL ethanol. The concentrations of aniline in the mixture solution were 0.025 M, 0.05 M, 0.1 M, 0.15 M, 0.25 M and 0.3 M. The rGO templates (2.4 mg each, 1.4 1.4 cm2) were immersed into the aniline solution for 4 h to allow the adsorption of aniline monomer onto the surface of rGO sheets as nucleation sites. APS (ammonia persulfate) solutions were freshly prepared by dissolving APS (APS: aniline ¼ 2: 3 by molar ratio) in 15 mL of 1 M HClO4 solution and placed in ice bath for at least 10 min. The cooled APS solution was poured into aniline solution containing rGO templates and shaken vigorously for 30 s to mix well, then the reaction vessel was kept in an ice bath (about 1 C) for 24 h to allow the formation of PANI. The resultant rGO/PANI films were taken out and rinsed for 5 times with 1 M H2SO4 to remove physically adsorbed PANI particles on the film surface, as well as unreacted starting materials.
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2.3. Electrochemical performance The rGO/PANI films were immersed in 1 M H2SO4 for 4 h and then assembled into symmetric two-electrode sandwich-configuration supercapacitors with Pt foil as current collector and membrane filter paper (Omnipore ™) as separator. Electrochemical performance of the supercapacitors was tested using cyclic voltammetry (CV) and galvanostatic charge/discharge on an eDAQ potentiostat (Potentiostat 466, eDAQ Pty Ltd). The potential range for CV and galvanostatic charge/discharge tests was from 0.0 to þ0.8 V. 2.4. Materials characterization FTIR spectra were recorded on a JASCO FT/IR-4200 Fourier transform infrared spectrometer by using KBr pellet. XRD were taken on a Rigaku RINT2500 with Cu Ka radiation (l ¼ 0.154 nm). SEM pictures were observed by a JEOL JSM-7001 F4 microscope. Samples were fixed on conductive tape and coated osmium layer (about 5 nm). 3. Results and discussion Fig. 1 illustrates the preparation of rGO/PANI composites. First, a 3D GO macrostructure was prepared by using dd-LBL method [37]. In brief, bPEI solution was coated uniformly on a filter paper. The coated filter paper served as a reservoir of bPEI solution and acted as a substrate for film growth. When the filter paper was immersed in the assembly GO suspension, upon contact, a stable GO/bPEI layer could be formed by the complexation between diffused bPEI and GO. The continuous diffusion of bPEI from the reservoir constructed new GO layers until the GO/bPEI complex developed into a millimetre-thick film, and complexation-and-diffusion process completed. The assembled large GO/bPEI film was cut into 2 2 cm2 pieces and detached from the substrate, the freestanding GO/bPEI film remained intact (Fig. S1). Following this step, GO/bPEI films (2 2 cm2) were reduced through hydrothermal treatment at 190 C for 24 h to recover their conductivity [40], and the resulting material denoted as rGO template. Finally, rGO templates were immersed in a certain concentration of aniline solution (mixture of 20 mL 1 M HClO4 and 5 mL ethanol) for 4 h to allow for the adsorption of aniline monomers onto the rGO sheets as nucleation sites. Freshly prepared APS (ammonia persulfate) solution was added into the aniline solution rapidly to induce polymerization. In this way PANI particles formed on the nanosheets of rGO framework, the resultant samples were denoted as rGO/PANI. Fig. 2a shows the photograph of a piece GO/bPEI film before and after hydrothermal treatment. The size of the GO/bPEI film shrunk
from 2.0 cm to around 1.4 cm after reduction. The shrinkage of the GO/bPEI films was likely due to the decomposition of most bPEI during hydrothermal process, and only a small percentage of bPEI will remain in the macrostructure, therefore the whole structure was pulled together. The degree of shrinkage is affected by temperature, reaction time, concentration of initial GO and bPEI, etc. Despite such shrinkage, the film still retains its well-connected porous structure with micrometre pore size, as shown in Fig. 2b and c. The rGO sheets exhibit flexible and wrinkled features. This highly porous structure is easily accessed by molecules and serves as ideal template for PANI. After polymerization we can clearly observe the formed PANI nanoparticles on the surface of rGO sheets from SEM images (Fig. 2def). The surface of the rGO nanosheets is fully covered by PANI nanoparticles, becoming thick and rough in comparison with rGO. The porous rGO and rGO/PANI-0.1 were characterized by nitrogen adsorption-desorption (Fig. S2), and the corresponding BET specific surface areas are 211.54 and 134.69 m2 g1, respectively. The specific surface area of rGO/PANI0.1 decreases after the PANI nanoparticles generated on graphene nanosheets, which may be ascribed to the higher density of PANI than rGO. Furthermore, the distribution of pore size can be observed from Fig. S2b, the rGO/PANI-0.1 shows a similar pore size distribution (50e120 nm) feature as rGO. The pore volume of rGO/ PANI-0.1 is 0.16 cm3 g1 and less than 0.51 cm3 g1 of rGO framework. These results prove that the PANI nanoparticles on rGO sheets do not change the macroporous structure of rGO, while the PANI nanoparticles dispersing on the graphene sheets inevitably decreases the pore volume of rGO template. The concentrations of aniline were optimized during the preparation of rGO/PANI films. We noticed that the weight of rGO templates increased after polymerization and, accordingly, the weight percentage of PANI in rGO/PANI increased as well (Fig. 3). The weight of one piece of rGO template initially was 2.4 mg and the weight of resultant rGO/PANI composite increased to 3.7 mg when 0.025 M aniline was used, the material denoted as rGO/PANI0.025. The weight of rGO/PANI film kept increasing with increasing concentrations of aniline and reached 15.5 mg when 0.3 M aniline was used, which was the highest concentration in our experiments. When the aniline concentration enhanced from 0.025 M to 0.1 M, the morphology of PANI on rGO sheets noticeably changed (Fig. 3bec and Fig. 2def). The rGO sheet in rGO/PANI-0.025 was covered by a thin layer of PANI; however, when the aniline concentration enhanced to 0.05 M, we can clearly observe the protruding PANI particles on the rGO sheets, and these particle-like PANI became more obvious when 0.1 M aniline was used. At a higher concentration (0.15 M), we observed the aggregation of PANI particles within rGO template along with well-distributed, small PANI particles in the final rGO/PANI film, which we denote as rGO/
Fig. 1. Schematic of preparing rGO/PANI composite film. First, bPEI solution was coated on a filter paper. The coated paper was immersed in assembly GO suspension. GO and bPEI formed a stable complex layer at first. Then, bPEI diffused out of the reservoir and complexed with GO sheets. This complexation-and-diffusion process continued, eventually developed into a thicker GO/bPEI composite film. Some square GO/bPEI films (2 2 cm2) was cut and detached from the substrate, then subjected to a hydrothermal treatment at 190 C. Finally, the hydrothermally-reduced GO films were used as a template for the polymerization of aniline.
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Fig. 2. Photograph and SEM images of rGO template and rGO/PANI film. a) Photograph of the GO/bPEI film, rGO template and rGO/PANI film, b-c) Cross-section SEM images of rGO template. d-f) Cross-section SEM images of rGO/PANI-0.1 clearly show the formation of PANI on the rGO sheets.
PANI-0.15 (Fig. 3dee). The aggregation of PANI causes a decline in the electrochemical performance of rGO/PANI, which will be discussed later. In the case of 0.25 M and 0.3 M aniline, PANI formed only around the periphery of the film and no obvious PANI were found inside rGO template. It is likely that the highly concentrated monomer lead to fast polymerization at the beginning of the reaction, and the large amount of PANI rapidly generated only at the surface of rGO template, blocked the pores of rGO structure. In this case, APS molecules had no time to diffuse into the template, therefore, PANI could not be further generated inside of the rGO (Fig. S3). The FTIR spectrum of rGO/PANI exhibits the characteristic peak of rGO and PANI, which further confirms the formation of PANI on rGO scaffolds (Fig. 4). In rGO/PANI the prominent peaks at 1563 cm1 and 1474 cm1 are assigned to stretching vibration of quinoid rings (C]N) and benzenoid rings (C]C). Two peaks at 1295 cm1 and 1238 cm1 are corresponded to CeN stretching vibrations with aromatic conjugation [41]. PANI formed on GO sheets is also confirmed by XRD (X-ray diffraction) in Fig. S4. In rGO/PANI two peaks are shown at 2q ¼ 19.3 and 25.0 , which are related to PANI, but these peaks are not presented in the XRD curve of rGO [42]. We investigated the electrochemical performance of these rGO/ PANI films in a sandwich-configuration supercapacitor. The rGO/ PANI films were mechanically stable and could be directly used as binder-free electrodes. First, rGO/PANI films were immersed in 1 M H2SO4 aqueous solution for 4 h. The electrochemical performance was measured by using a two-electrode supercapacitor with Pt foil as current collector and Omnipore membrane as separator. Fig. 5a shows the cyclic voltammetry (CV) curves of rGO template and rGO/PANI films at the scan rate of 10 mV s1. Notably, CV curves for rGO template show rectangular shapes at the scan rate ranging
from 10 to 200 mV s1 (Fig. S5), which indicate that the well interconnected structure of rGO makes electrolytes diffuse freely throughout the rGO template. The rGO template shows an ideal double-layer capacitive behaviour. In comparison with rGO template, four rGO/PANI samples clearly exhibit redox peaks in CV curves, which matches well with the characteristic feature of PANI in pseudo-capacitance. These redox peaks can be attributed to the leucoemeraldine base/emeraldine salt and emeraldine salt/pernigraniline base transformation of PANI [43e45]. Since the area enclosed by CV curves is proportional to the specific capacitance, all of four rGO/PANI samples clearly show increased capacitance with regards to rGO template. Galvanostatic charge/discharge curves at a current of 10 mA are shown in Fig. 5b. The charge/discharge curve of rGO template is symmetric, with straight lines, while those of rGO/PANI electrodes exhibit the feature of pseudo-capacitance, with curved charge and discharge lines. The gravimetric specific capacitances of all samples, as presented in Fig. 5c, were calculated by an equation of C ¼ 4·I·Dt/ (V·m), where I is the charge-discharge current, Dt is the discharge time, V is the discharge voltage, and m is the total weight of two electrodes. We use Ct to represent the specific capacitance of the whole electrodes, including rGO and PANI. The Ct of rGO/PANI-0.1 was prominently higher than that of the other three electrodes and reached to a high value of 438.8 F g1 at a charge/discharge current of 5 mA (0.5 A g1), 428.0 F g1 at a charge/discharge current of 20 mA (2 A g1) and 401.5 F g1 at a charge/discharge current of 40 mA (4 A g1). Meanwhile, given the total volume 0.01372 cm3 (1.4 cm 1.4 cm 0.0070 cm) of two electrodes, the volumetric capacity of rGO/PANI-0.1 can be calculated as 319.83 F cm3 at 0.5 A g1, and 311.95 F cm3 at 2 A g1. Comparing with the reported capacitances about relative graphene/PANI composites in Table S1, the capacitances of more than 500 F g1 are
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Fig. 4. The FTIR spectra of rGO, PANI and rGO/PANI-0.1.
Fig. 3. The influence of the concentration of aniline. a) The weight of rGO/PANI film and the percentage of PANI in the whole rGO/PANI film corresponding to different concentrations of aniline during polymerization. SEM images of the resultant rGO/PANI film when the aniline concentration was b) 0.025 M, c) 0.05 M, and d-e) 0.15 M.
almost obtained at lower current density (<0.3 A g1), and the same electrodes show great declining capacitance (<400 F g1) under higher current density (>1 A g1). Even the highest gravimetric capacitance reported as 790 F g1 at 1 A g1, the volumetric capacitance is 205.4 F cm3 [35], which is lower than our work (311.95 F cm3 at 2 A g1). Those results show that our 3D-rGO/ PANI electrodes have a better performance, especially for the higher volumetric capacitance and capacitance stability under higher current density (401.5 F g1 at 4 A g1), which is attributed to the compressible feature of 3D rGO framework prepared by diffusion driven layer-by-layer self-assembly, as reported the higher volumetric capacitance in our previous work [38]. Compared with rGO/PANI-0.025 and rGO/PANI-0.05, the loading of PANI in rGO/PANI-0.1 was much higher, as shown in Fig. 3a, so the increase in Ct was expected. In the case of rGO/PANI-0.15, although the loading of PANI was even higher, Ct decreased to 381.1 F g1 at a charge/discharge current of 5 mA (0.34 A g1). We attribute the sudden drop of Ct to the agglomeration of PANI caused by the high concentration of aniline during the polymerization reaction. These agglomerations of PANI did not fully participate in the redox
reaction due to their large particle size. To further evaluate the capacitance of PANI we deducted the capacitance contributed by rGO template (CGO) from the whole capacitance (Ct) and compared the remaining capacitances, which could be attributed to PANI (CP). As it can be expected, CP of rGO/ PANI-0.15 was the lowest among all samples in Fig. 5d, with only 512.5 F g1 at a discharge current of 5 mA (0.34 A g1). However, it is very interesting that CP of the remaining three electrodes was very close, especially at low discharge rates (763.8 F g1 for rGO/ PANI-0.025, 756.1 F g1 for rGO/PANI-0.05 and 746.7 F g1 of rGO/PANI-0.1 at 5 mA), which indicates that PANI particles in these three electrodes were almost equally active during redox reactions. These CP data also confirmed that the reason Ct of rGO/PANI-0.1 higher than Ct of rGO/PANI-0.15 was indeed because the performance of PANI in the latter electrode was impaired by its agglomeration. Notably, the overall capacitance of PANI materials from these experiments was one of the highest values reported in 1 M H2SO4 electrolyte so far [46]. We attribute the high capacitance to two main features of the electrode: 1) PANI particles are evenly and uniformly distributed throughout the rGO template at the proper concentration of aniline, and 2) porous and wellinterconnected rGO template plays an effective role in improving the diffusion of the electrolyte. The stability of rGO/PANI-0.1 was evaluated by charge/discharge measurements at 40 mA (4 A g1). The decrease of capacitance from rGO template can be neglected during recycling. The rGO/PANI-0.1 electrode retained 76.5% of its initial capacitance after 2000 cycles (Fig. 6). The capacitance of PANI usually decreases dramatically during long-term charge/discharge, which is also one of the main drawbacks of using PANI in energy storage devices. For example, Shi et al. reported a 29% decrease after 800 times of recycling in the case of a PANI-nanowire capacitor [29] and Li et al. also reported a 41% capacity retention of their PANI capacitor after 880 cycles [47].
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Fig. 5. Electrochemical performance of rGO template and rGO/PANI electrodes in 1 M H2SO4 electrolyte. a) The CV curves at a scan rate of 10 mV s1. b) Charge/discharge curves measured at 10 mA. c) Specific capacitances and d) Capacitance of PANI in rGO/PANI electrodes at various charge/discharge currents.
4. Conclusions We demonstrated that the 3D porous GO macrostructure fabricated via dd-LbL served as an ideal template for in situ polymerization of aniline. The well inter-connected structure of the GO not only facilitated the loading of evenly distributed PANI throughout the template, but also played a critical role in improving electrolyte diffusion during charge/discharge process. The resultant graphene based PANI hybrid materials are mechanically stable and highly conductive, which leads to excellent electrochemical performance by combining electric double-layer capacitance of rGO template and pseudo-capacitance of PANI. The set-up of dd-LbL assembly is simple, and the size of the assembled GO film can be scaled up easily, which makes this method highly attractive for practical application.
Acknowledgements Fig. 6. The recycling stability of rGO template and rGO/PANI-0.1 electrode at a charge/ discharge current of 40 mA.
In comparison with the stability of most of PANI supercapacitors, our rGO/PANI electrode exhibited a large improvement while maintaining its high capacitance.
This work was supported by the National Natural Science Foundation of China under grant no. 51403094 and China Scholarship Council (grant no. 201408210052). This work was also supported by the Institute for Integrated Cell-Material Sciences (iCeMS; Kyoto University), JSPS KAKENHI grant no. 24681019 (Wakate-A) and JSPS KAKENHI grant no. 25000007 (specially promoted research).
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Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.01.034.
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