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Polyaniline nanocone arrays synthesized on threedimensional graphene network by electrodeposition for supercapacitor electrodes Mei Yu, Yuxiao Ma, Jianhua Liu *, Songmei Li School of Materials Science and Engineering, Beihang University, No. 37 Xueyuan Road, Beijing 100191, China
A R T I C L E I N F O
A B S T R A C T
Highly ordered polyaniline nanocone arrays were synthesized on three-dimensional graph-
Received 19 September 2014
ene network by template-free electrodeposition method. The morphology of the material
Accepted 2 February 2015
was characterized by scanning electron microscopy. The structural features were analyzed
Available online 7 February 2015
by using Raman spectroscopy. Polyaniline nanocones align vertically on the surface of three-dimensional graphene network. Such morphology provides unblocked diffusion path for electrolyte ions, and increases the specific area of the material. The material possesses specific capacitance of 751.3 F g1 in 1 M HClO4 within the potential window of 0–0.7 V, and has high rate capability as well as good cycling stability. At a current density of 10 A g1, its capacitance is 88.5% of that at a current density of 1 A g1. Furthermore, the material remains 93.2% of initial capacitance after 1000 cycles of charging–discharging test. 2015 Published by Elsevier Ltd.
Supercapacitor is a promising type of energy storage devices due to its ability to store and release energy rapidly and reversibly. Generally, high-conductive carbonaceous materials and pseudocapacitance materials are used comprehensively to get high rate capability and good cycling performance . Graphene has attracted great attention due to its excellent electronic conductivity caused by p–p conjunction system . Usually, graphene is used as the carrier of pseudocapacitance materials and current collector in supercapacitor electrode because of its high specific area [3,4]. Conductive polymer, such as polyaniline (PANI), polypyrrole, and polyporphyrin, undergo fast and highly-reversible electrochemical reactions, during which the basic structure of conductive polymer remains intact. Thus, its stability during charging–discharging cycles is high [5,6]. The comprehensive utilization of
* Corresponding author: Fax: +86 10 82317103. E-mail address: [email protected]
(J. Liu). http://dx.doi.org/10.1016/j.carbon.2015.02.017 0008-6223/ 2015 Published by Elsevier Ltd.
graphene and PANI in supercapacitor has been widely researched since 2011 [7,8]. Traditionally, graphene applied in supercapacitor electrodes are two-dimensional graphene sheets with the size of micrometers. The existence of inter-sheet junction resistance restricts the application of graphene as current collector . The synthesis of three-dimensional (3D) graphene network and its application in energy devices have been reported since 2012 [10–15]. The network is an intact, interconnected, and highly conductive network instead of separated sheets. Therefore, it may avoid the inter-sheet junction resistance existed in two dimensional graphene sheet based electrode materials. Furthermore, the porous structure of 3D graphene and the absence of sheet stacking improve the specific surface area, enhancing the diffusion of electrolyte  and making 3D graphene network an ideal carrier for pseudocapacitance materials [17,18].
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3D graphene network is doped by pseudocapacitance materials such as PANI, manganese oxide, and NiO to improve its capacitance [19–21]. Jun et al.  studied the improved supercapacitive performance of chemically grown cobalt–nickel hydroxides on 3D graphene foam. The structure has a morphology of needle-shaped nanocone, and its superiority was confirmed. He et al.  synthesized freestanding 3D graphene/MnO2 composite networks by electrodeposition, and the intimate attachment of pseudocapacitance material was proved. PANI has already been widely researched as supercapacitor electrodes due to its excellent pseudocapacitance properties, light weight, high electrochemical stability, and comparatively high conductivity. Jun et al.  prepared supercapacitor electrode based on a PANI nanofibers/3D graphene framework with chemically growth method, and obtained excellent supercapacitance performance. Jin et al.  prepared graphene–polyaniline nanofiber composite with in situ one-pot method. The nanorod array structure was also reported in some literatures. Using in situ polymerization, Chen et al.  synthesized graphene/polyaniline nanorod arrays as electromagnetic absorption material. Highly ordered structural polyaniline–graphene bulk hybrid materials were fabricated with chemical oxidative polymerization by Xu et al. . However, in the previous literatures, PANI nanorod materials are grown on two-dimensional graphene sheets. To our best knowledge, the electrodeposition of PANI with vertically-aligned nanocone arrays structure on 3D graphene network has not been studied yet. Thus, we aim to synthesize 3D graphene network–PANI nanocone arrays by a template-free electrodeposition method for supercapacitor electrode material. As shown in Fig. 1, charges generated by pseudocapacitive reaction of PANI can be easily conducted out by high-conductive graphene network. 3D Graphene network also plays a role of spacer to prevent the agglomeration of PANI nanostructure. As the pseudocapacitance of PANI comes from the reversible reaction between its different oxidation states, it requires hydrogen ions from electrolyte for protonation. Vertically aligned nanocone morphology offers large expose area to electrolyte, contributing to charge transfer between electrode
and electrolyte. Space between nanocones in the array provides unblocked diffusion path for electrolyte ions. Thus, the protonation and deprotonation process of PANI are facilitated. These factors upsurge the capacitance performance and charging-discharging rate of the material. It is very interesting to find out a template-free preparation method for vertically aligned PANI nanocone arrays on 3D graphene network, as well as to control the length and shape of PANI nanocones precisely. In order to prepare such material, it is vital to control the polymerization condition of aniline to make it polymerize along one dimension rather than forming new nuclei. By controlling the parameters of electrodeposition process, such morphology can be obtained. Herein, we report the synthesis of vertically aligned PANI nanocone arrays on 3D graphene network by a template-free electrodeposition method, and explore its potential application as supercapacitor electrode.
The fabrication of 3D graphene network–PANI nanocone arrays requires two steps. In the first step, 3D graphene network was prepared by chemical vapor deposition (CVD) of graphene on nickel foam, which was followed by the etching of nickel framework . In the second step, vertically aligned PANI nanocone arrays were electrodeposited on 3D graphene network. The CVD preparation of 3D graphene was carried out with an OTF-1200X tube furnace. Nickel foam was purchased from Lyrun Co., Ltd. with areal density of 0.038 g cm1 and porosity of 97.3%. First, nickel foam with the size of 2 cm · 4 cm · 1 mm was cleaned by being soaked in 1 M HCl solution for 10 min, and then washed by deionized water and acetone for 15 min with sonication, respectively. Second, the nickel foam was heated in the furnace from room temperature to 1050 C in 50 min under H2 (20 sccm) and Ar (50 sccm) gas flows. The nicked foam was annealed at 1050 C for 30 min to eliminate the oxide layer. CH4 (10 sccm) gas flow was then introduced into the tube for 120 min growth. Third,
Fig. 1 – Structural schematic illustration of 3D graphene network–PANI nanocone arrays. (A color version of this figure can be viewed online.)
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the graphene/Ni foam structure was etched in 2 M Fe(NO3)3 solution at 80 C for 12 h to dissolve the nickel framework to obtain 3D graphene network. Finally, the network was washed with deionized (D.I.) water and alcohol. In the electrodeposition process, 1 M HClO4 and 0.1 M aniline aqueous solution were used as electrolyte. Working electrode was as-prepared 3D graphene network sample with the size of 2 cm · 4 cm · 1 mm. A rectangular area (2 cm · 1 cm · 1 mm) of the sample was immersed in the electrolyte, and PANI was doped on the area. The mass of the immersed area was 13 mg. A platinum plate and a Ag/AgCl electrode were used as counter and reference electrodes, respectively. The galvanostatic electrodeposition was carried out at an anodic current density of 0.1 A g1. Afterwards, the working electrode was washed with D.I. water and alcohol, and dried at room temperature. The samples prepared with electrodeposition time of 2500, 5000, 7500 and 10,000 s are denoted as GPN1, GPN2, GPN3 and GPN4, respectively. Bare 3D graphene network without PANI nanocone arrays is denoted as GPN0.
Morphology and structure characterization
As-prepared material was characterized by scanning electron microscopy (SEM) and Raman spectroscopy. SEM images were taken by FEI-XL30S scanning electron microscope. Raman spectra was recorded at room temperature on a Nanophoton RAMAN-11 Raman spectrometer with 532 nm wavelength laser.
Electrochemical tests were conducted on a CHI-660e electrochemical workstation using a three-electrode system. A platinum plate was used as the counter electrode and a AgCl/Ag electrode was used as the reference electrode. The doped section of 3D graphene network was immersed in electrolyte of 1 M HClO4 aqueous solution as working electrode. The potential windows of cyclic voltammetry (CV) tests and galvanostatic charge–discharge tests were 0.2–0.8 V and 0–0.7 V, respectively.
Results and discussion
Morphology and structure
The morphologies of PANI nanocone arrays are illustrated in Fig. 2. Shown in low magnitude SEM images of GPN0 and GPN3 in Fig. 2(a–f), the surface of GPN3 is much rougher than that of GPN0, demonstrating the existence of PANI nanocone arrays. In the top-view image (Fig. 2(g)), it is clear that nanocones distribute evenly on almost the whole surface of 3D graphene. This suggests a good uniformity of the material. The tiled-view image (Fig. 2(h)) illustrates that verticallyaligned PANI nanocone arrays are formed, and nanocones separate well from each other. The diameter of nanocones is around 50 nm. Shown in Fig. 2(i), the lengths of PANI nanocones are 137, 207, 303, and 355 nm for GPN1–4, respectively. It is shown that nanocones grow higher as the deposition time increasing.
The forming of PANI nanocone arrays is attributed to the electronegativity of PANI molecules and low current density. Anodic current is introduced to the 3D graphene network, and the surface of 3D graphene network is positively charged. Due to the acidity of electrolyte, aniline monomers exist in the form of Ph-NH+3 cation, and the electrostatic repulsion prevents them from approaching 3D graphene network surface, as shown in Fig. 3. As a result, the monomers of aniline concentrate on the top of already-formed PANI molecular, being oxidized there. The low current density also contributes to the formation of nanocone arrays as it prevents the forming of a uniform PANI film on the surface. During the electrodeposition process, aniline polymerizes along one dimension rather than forming new nuclei . The mechanism has been studied by Epstein et al. . The relevance between deposition time, the mass loading of PANI, and the length of nanocones were studied and shown in Fig. 4. Both mass and length increase with deposition time. However, an obvious rise of mass can be observed in GPN4 without simultaneous increase of nanocones’ length. As shown in the last image of Fig. 2(i), there appears a thicker film under PANI nanocone arrays, which cause the rise of mass without growing of nanocone. Such phenomenon is attributed to potential barriers which influenced the position of electrodeposition. As the length of nanocones increases, it is more difficult for electrons to be conducted from the top of nanocone. When the potential barrier caused by electron conductive resistance exceeds that caused by electrostatic repulsion force (introduced in the analysis of Fig. 3), aniline monomer would be oxidized on the root segment of PANI nanocones instead of top segment, and a thicker film underneath is formed . Raman spectroscopy was applied to characterize the structure of 3D graphene network–PANI nanocone arrays, and the Raman spectra of the samples are shown in Fig. 5. In the spectrum of GPN0, two peaks are exhibited at 1581 and 2723 cm1, corresponding to the G and 2D bands of graphene, respectively . The low intensity of D band at 1355 cm1 suggests good structural integrity and thus high conductivity of 3D graphene network. In the spectrum of GPN1–4, the high intensity of characteristic peaks of PANI at 1168, 1330, 1497 and 1622 cm1 confirms the existence of PANI nanocone on the surface of 3D graphene network . The intensity ratio between characteristics peaks of PANI and graphene (e.g. I1622/I1581) obviously rises as the length of PANI nanocones increases. It suggests a more integrated coverage of PANI on 3D graphene network. The intensity of 2D bands of graphene also weakens.
The CV curves of GPN0–4 are shown in Fig. 6. The curve of GPN0 in the inset plots confirms the electric double layer property of pure 3D graphene as there is no obvious anodic nor cathodic peaks in that curve. The curves of GPN1–4 contains typical anodic and cathodic peaks of PANI around 0.24 and 0.04 V, respectively. The curves are in agreement with previous literature . It can also be observed that as the length of nanocones increases, the anodic peaks gradually move to higher
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Fig. 2 – SEM images of (a–c) GPN0 with low magnification, (d–f) GPN3 with low magnification, (g and h) vertically aligned PANI nanocone arrays on the surface of 3D graphene, and (i) length of PANI nanocones of GPN1–4, respectively.
Fig. 3 – Electrodeposition process of PANI on the surface of 3D graphene network. (A color version of this figure can be viewed online.)
potential and the cathodic peaks to lower potential. Such results can be explained with the mechanism shown in Fig. 1. As the length of nanocones increases, it is more and more difficult for electrons to be conducted to or from the top section of nanocone as a result of the comparatively poor conductivity of PANI. In the oxidation process, such delay would cause an anodic polarization, and result in positive shift of the peak. And vice versa, negative shift of peak would occur in reduction process. In order to evaluate the supercapacitance performance of PANI nanocone arrays on 3D graphene network, charging-dis-
charging tests were carried out. The superiority of electrodeposited PANI nanocone arrays is clearly shown in Fig. 7(a). As listed in Table 1, at a current of 2 mA, the capacitance ratio of GPN4 to bare 3D graphene network (GPN0) is 627.8, while the mass ratio of PANI only increases from 0% to 26.6%. The capacitance ratio is much higher than the ratio of mass (126.6%). The increase of capacitance is attributed to both higher mass loading of PANI and longer nanocones. Pseudocapacitance materials such as PANI obtain capacitance by redox reaction, so capacitance increases as mass of PANI rises. Fur-
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Fig. 4 – Mass loading of PANI and length of nanocones vs. electrodeposition time. (A color version of this figure can be viewed online.)
Fig. 6 – Cyclic voltammetry curves of GPN0–4. The inset plot is the magnified curve of GPN0. (A color version of this figure can be viewed online.)
472.6 F g1, respectively. This result is in agreement with previous analysis of peak shifts in Fig. 6. It is obvious that longer nanocones hinder the electron conduction in PANI. The exterior part of nanocone becomes less active since the pseudocapacitance performance of PANI requires electrons to be conducted in or out rapidly. The rate capability of PANI-deposited 3D graphene network is shown in Fig. 8. The high-rate capacitance percentage (HRCP) is defined as follows, HRCPð%Þ ¼ ðC10 =C1 Þ 100
Fig. 5 – Raman spectra of GPN0–4. (A color version of this figure can be viewed online.)
thermore, as shown in Fig. 1, space between nanocones provides diffusion path for electrolyte ions. Thus, the expose area of PANI rises as the length of nanocones increases, promoting charge transfer on the interface between PANI and electrolyte. The average diameter of the pores in 3D graphene network and 3D graphene network–PANI nanocone arrays is about 500 lm, which can be seen in Fig. 2(a) and (d). The specific capacitance of PANI nanocone arrays on 3D graphene network were calculated with the data provided by Fig. 7(b) with the formula as follows, Cm ¼ ði DtÞ=ðDV mÞ
where i is the current (A), Dt is the time span of discharging process (s), DV is the potential window (V), and m is the mass loading of PANI (g). At the current density of 1 A g1, the specific capacitance values of GPN1–4 are 751.3, 561.7, 536.6, and
where C1 and C10 are the specific capacitance at current densities of 1 and 10 A g1, respectively. At the current density of 10 A g1, GPN1 remains 88.4% of the capacitance at the current density of 1 A g1, much higher than those of the previous reported works [9,20]. This is attributed to two factors. First, the structure has nice electron conductive property between PANI nanocones and 3D graphene network. Second, the large surface area of PANI nanocone arrays as well as the diffusion path between the nanocones enhance unblocked electrolyte ion diffusion. As shown in Fig. 8, GPN1–3 remain more than 85% of the capacitance value at large current density, proving good rate capability of the materials. The obvious performance degradation of GPN4 is caused by the thicker film in root segment of the nanocone array (discussed in Fig. 4), which blocks both electron conduction and electrolyte ion diffusion. The electrochemical stability of 3D graphene network– PANI nanocone arrays was studied with long-term charge– discharge cycle tests at a current density of 4 mA cm2. The results are shown in Fig. 9. The capacitance retention of GPN2 after 1000 cycles is 93.2% of its initial capacitance. It is attributed to the large expose area of PANI nanocones to electrolyte solution. Good charge transfer between PANI and electrolyte prevents the charges from accumulating on PANI nanocones. Thus, performance degradation caused by electrostatic force can be minimized .
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Fig. 7 – Galvanostatic charging–discharging curves of (a) GPN0–4 at a current of 2 mA (the inset plots is the curve of GPN0) and (b) GPN1–4 at a current density of 1 A g1. (A color version of this figure can be viewed online.)
Table 1 – Capacitance performance of GPN0–4. Sample
Mass ratio of PANI Discharging time (s) Capacitance ratio
0% 1.18 1
7.1% 258.5 219.1
12.2% 306.0 259.3
17.2% 508.9 431.3
26.6% 740.8 627.8
Fig. 9 – Charge–discharge cycles of 3D graphene network– PANI nanocone arrays. (A color version of this figure can be viewed online.)
Fig. 8 – Scan-rate dependent specific capacitance and highrate capacitance percentage of GPN1–4. (A color version of this figure can be viewed online.)
When the length of the nanocones increases, the capacitance retention of the samples after long-time cycle tests reduces. It is mainly ascribed to the relatively poor conductivity of PANI which leads to residual charges on nanocones. The charges cause electrostatic force which damages the integrity of PANI structure, thus PANI nanocone arrays are separated from 3D graphene network. Another reason is the protonation and deprotonation process during the pseudocapacitance reaction. In previous literature, it has been revealed that the application of EMI-TFSI as electrolyte can
avoid the structure change during reaction . It would be a good enlightening to our further research that the electrochemical stability of the materials can be ameliorated with proton-free electrolyte.
In this paper we reported polyaniline nanocone arrays synthesized on three-dimensional graphene network by electrodeposition method. By adjusting the polymerization condition of aniline during electrodeposition process, the shape and length of PANI nanocones can be controlled precisely. Both mass loading of PANI and length of nanocones increase with deposition time, and the length of PANI nanocones has obvious influence on the properties and supercapacitance performances of the materials. The vertically aligned nanocone morphology increases the specific area of PANI, and thus contributes to the charge transfer between electrolyte and electrode. The space between nanocones
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provides unblocked diffusion path for electrolyte ions, and facilitates the protonation and deprotonation process of PANI. These factors promote the capacitance performance of the material, including high specific capacitance, good high-rate performance, and high cyclic stability. Thus, 3D graphene network–PANI nanocone arrays is of great potential for supercapacitor electrodes. Furthermore, this work provides an effective method to prepare other 3D graphene network based materials.
Acknowledgements This work is supported by the National Natural Science Foundation of China (Project No. 21371019), Aero Science Foundation of China (Project No. 2014ZF51066), and the Fundamental Research Funds for the Central Universities.
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