One-step synthesis of iodine doped polyaniline-reduced graphene oxide composite hydrogel with high capacitive properties

Accepted Manuscript One-step synthesis of iodine doped polyaniline-reduced graphene oxide composite hydrogel with high capacitive properties Jing Wang, Baoyan Li, Tao Ni, Tingyang Dai, Yun Lu PII: DOI: Reference:

S0266-3538(15)00030-5 http://dx.doi.org/10.1016/j.compscitech.2015.01.008 CSTE 6019

To appear in:

Composites Science and Technology

Received Date: Revised Date: Accepted Date:

26 October 2014 8 January 2015 10 January 2015

Please cite this article as: Wang, J., Li, B., Ni, T., Dai, T., Lu, Y., One-step synthesis of iodine doped polyanilinereduced graphene oxide composite hydrogel with high capacitive properties, Composites Science and Technology (2015), doi: http://dx.doi.org/10.1016/j.compscitech.2015.01.008

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One-step synthesis of iodine doped polyaniline-reduced graphene oxide composite hydrogel with high capacitive properties Jing Wang, Baoyan Li, Tao Ni, Tingyang Dai*, Yun Lu* Department of Polymer Science and Engineering, State Key Laboratory of Coordination Chemistry, Key Laboratory of High Performance Polymer Materials and Technology of Ministry of Education, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, PR China

* Prof. Dr. Yun Lu, Corresponding author Department of Polymer Science and Engineering, State Key Lab Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China E-mail: [email protected]

Abstract: Iodine doped polyaniline-reduced graphene oxide composite hydrogel was one-step synthesized using iodine as catalyst and dopant. The structure, composition and the surface morphology of the material were characterized by means of X-ray diffraction (XRD), Fourier transform infrared spectrum (FTIR), scanning electronmicroscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results showed that the reduced graphene oxide was iodine-doped and covered uniformly by the polyaniline nanoparticles doped also with iodine. The electrochemical properties of the hydrogel

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were evaluated by cyclic voltammetry, galvanostatic charge/discharge and the impedance spectroscope using a symmetrical two-electrode configuration. With the enhanced specific capacitance as 712.5 F g-1 at a current density of 1 A g-1 and a good cycling stability about 77% (from 440 to 340 F g-1) after 1000 charge/discharge cycles at a current density of 8 A g-1, the composite hydrogel exhibits great potential in the application of high-performance energy storage devices. Key words： A. Polymers A. Nano composites B. Electrical properties

1. Introduction Supercapacitors are acting an important role in the study of new energy storage device due to such merits as faster and higher power capability, wide thermal operating range, long lifecycle and low maintenance cost [1]. The capacitance and charge storage of supercapacitor intimately depend on the electrode materials used, which mainly falls into carbon based materials, conducting polymers and metal oxides[2]. As suitable materials for supercapacitor, conducting polymers such as polyaniline(PANI), polypyrrole(PPy), polythiophene(PT) and their corresponding derivants possess many advantages for examples high conductivity in a doped state and adjustable redox activity through control of doping degree, high storage capacity/porosity/reversibility and low cost/environmental impact[3]. The conducting polymers offer pseudocapacitance behavior through the redox process that came about throughout the

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entire bulk of the polymers, not just on the surface[4]. Unfortunately, swelling and shrinking of the conducting polymers may occur during the charge-discharge process, leading to mechanical degradation of the electrode, fading electrochemical performance after cycling, and thus compromising the conducting polymers as electrode materials. By contrast, the carbon materials exhibit their own unique virtues as electrode materials for supercapacitor, for instance high tensile strength/elasticity and chemical stability, long lifecycle, wide operating temperature range and non-toxicity[5]. Because carbon materials store charges mainly in an electrochemical double-layer formed at the interface between the electrode and the electrolyte rather than in the bulk of the capacitive material, they usually have a limited specific capacitance[6]. In this context, it is very important for the carbon materials to have a big specific surface area, appropriate pore-size distribution, high electrical conductivity and a certain surface functionality. Recently, considerable efforts have been focused on developing novel carbon/polymer composite electrode materials with optimized capacitive properties. For example, by combining the excellent gelation properties of graphene oxide(GO) with the strong interaction between polyaniline and GO sheets[7], GO/polyaniline composite hydrogels have been obtained using polyaniline as crosslinkers. Also, the graphene/polyaniline hydrogels have been made by in situ polymerization of aniline with graphene nano-sheet hydrogel with capacitance of 334 F g−1 at a current density of

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3 A g−1 [8] or obtained by using graphene hydrogel film as a substrate to polymerize aniline with capacitance of 530 F g−1 at 10 A g−1 [9]. Generally speaking, most of these carbon materials/polyaniline composites are obtained by a two-step method, that is, first preparing the reduced GO and then polymerizing aniline on the rGO sheet. In this work, we develop a one step method to obtain a polyaniline-rGO composite hydrogel, in which the reduction of GO is simultaneous with the polymerization of aniline monomers on the freshly generated rGO, using I2 as a catalyst and dopant in the system containing both GO and aniline. During the reaction process, aniline is polymerized with I2, and the HI produced from the oxidation can reduce GO to rGO. By this ingenious method we have obtained successfully the polyaniline-rGO composite hydrogel with high capacitive properties.

2. Experimental 2.1. Materials synthesis 2.1.1. Preparation of GO GO was prepared according to the modified Hummers method[10]. Specifically, 4 g of graphite powder was added to an 80 ºC solution containing 6 ml concentrated H2SO4, 2 g K2S2O8 and 2 g P2O5. The resulting mixture was kept at 80 ºC for 24 h, and then diluted with 200 ml of distilled water. After placing overnight, the mixture was filtered and then washed with distilled water until the rinse water pH became neutral. The

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product was dried in air at ambient temperature overnight. This preoxidized graphite was put into 100 ml of cold (0 ºC) concentrated H2SO4. 12 g KMnO4 was then controlled to add gradually with stirring and cooling, so that the system temperature of the mixture was not exceeding 20 ºC. The solution was then stirred at 35 ºC for 24 h. After that, 200 ml of distilled water was added into the solution. In 2 h, the reaction was terminated by the addition of 0.5 L distilled water and 10 ml of 30% H2O2 solution. The product was filtered and washed with 1 L 1:10 HCl solution, and then subjected to dialysis for a week to completely remove metal ions and acids. 2.1.2. Preparation of iodine doped PANI-rGO hydrogel 2ml aniline was dissolved into 2 ml concentrated HCl. The mixtures were added into 8 ml 10 mg/ml aqueous GO dispersion with an ice bath and stirred for 30 min. Then, into this mixture, 10 ml ethanol with 2 g iodine was added. The above mixtures were heated to 120 ºC for 6 h by hydrothermal method. The product was soaked in ethanol for 3 days to remove excess iodine, and then in 1 M HCl for 3 days to exclude the unreacted aniline. For comparison purposes, rGO-PANI composite without iodine doping was prepared under the same condition. Pure rGO was also prepared by reduction of GO with hydrazine hydrate to avoid the participation of iodine. 2.2. Characterizations The morphology and microstructure of the as-prepared composite hydrogels were investigated by scanning electron microscopy (SEM, Hitachi S-4800) with an

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energy-dispersive spectrometer (EDS) accessory for elemental measurement (Kevex-Sigma). Fourier transform infrared (FTIR) spectra were recorded with Bruker VECTOR22 spectrometer. The X-ray diffraction (XRD) patterns were carried out by an XRD-6000 instrument (Shimadzu, Japan) using Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) analysis was performed on a PHI 5000 Versaprobe system, using monochromatic Al Kα radiation (1486.6 eV) operating at 25 W. The electrical conductivities were measured via the four-probe method at room temperature by inserting four acicular probes into cuboidal samples with the dimension of 1cmⅹ1cmⅹ2mm (RTS-8 4-point probes resistivity measurement system, Probes Tech., China).

2.3. Electrochemical measurements Electrochemical experiments including cyclic voltammetry(CV), galvanostatic charge–discharge(CD) and electrochemical impedance spectroscopy(EIS) tests were carried out at room temperature on a CHI600 electrochemical workstation. The symmetrical two-electrode cell configuration was used to evaluate the material performance for supercapacitors, and the device was fabricated as shown in Fig. 1. The working electrodes were prepared by coating the hydrogel on two platinum sheets with a same quality. The two electrodes were packing into a device with a diaphragm between them. Measurements of cycle-life stability were performed using the computer controlled cycling equipment (LAND CT2001A). The specific capacitance was

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calculated according to the results from CD measurements using the equation of Csp= 2ⅹI/mⅹΔt/ΔE. Where, I is the current, m is the mass of the active material in a single electrode, Δt is the discharge time discharge and ΔE is the voltage range. The mass of the active material was weighed after the electrochemical test by drying at 80 ºC. There was 0.20 mg active material on each electrode.

3. Results and discussion In the formation of the composite hydrogel, the interactions between GO and PANI play an important role. GO has been obtained by placing graphite in a mixture of strong acids and oxidizing agents and thus possessed many oxygen-containing groups on its basal planes and edges[11]. These groups provide electrostatic interactions and hydrogen bonding with the amino-group of PANI, and together with the π-π interactions to form the strong cross-links of the 3D GO network [12]. Therefore, the distribution of aniline into GO is a crucial process in the course of preparation. In general, aniline directly mixing with the aqueous GO dispersion tends to cause the aggregation of GO. In order to get a well-distributed aniline-GO solution, it is essential to dissolve aniline into HCl to form aniline hydrochloride at first. The oxygen-containing groups of GO with a negative charge have an interaction with the aniline hydrochloride ion, thus helping the aniline monomers to distribute uniformly in GO dispersion. As shown in Fig. 2, the PANI-rGO composite is a self-supporting hydrogel, and can be made in different size and shape just based on the reaction containers.

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SEM characterization is used to investigate the structure and morphology of GO nano-sheets and the composite hydrogel, as shown in Fig. 3A and 3B. It can be observed that GO nano-sheets have the quite smooth surface, while for rGO sheets there are tiny particles with a size of 10 to 50 nm uniformly distributed on the surface due to the coating of PANI nanoparticles. The oxygen-containing functional groups on GO sheets act as active sites and enable the in situ polymerization of PANI during the reaction process. Fig. 3C gives the SEM image of PANI-rGO composite hydrogel and the corresponding energy-dispersive X-ray spectroscopy (EDX) mapping for nigrogen, oxygen and iodine, which evidence that the doped PANI is covering on the surface of rGO nanosheets. The chemical composition of the composite hydrogel is obtained according to the EDS analysis with the following results: C 82.66%, N 3.39%, O 5.50% and I 6.52% (Fig. 3D). The presence of N element is attributed to contribution of PANI, thus the component percentage of the composite hydrogel is calculated with 22.76% of PANI, 6.52% of I2 and 70.73% of rGO. Fig. 4A shows the XRD patterns of GO and PANI/rGO composite hydrogel. From Fig. 4A-a, it can be clearly seen that a diffraction peak is located at 2θ=10.6 degree, belonging to the [001] plane of GO sheets[13]. In Fig. 4A-b, the peak at 10.6 degree vanishes while a weak and broad diffraction peak appears at 2θ=24.6 degree. The former indicates the reduction of GO, and the latter implies the amorphous structural feature of the PANI generated. FTIR spectral analysis is performed to confirm the chemical structure of the

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PANI-rGO composite hydrogel. Fig. 4B presents the FTIR spectra of GO, PANI and PANI-rGO composite hydrogel. For pure GO (Fig. 4B-a), the peaks located at 1732, 1400, 1226 and 1053 cm-1 correspond to -C=O/-COOH, C–OH, C–O–C and C–O functional groups of the GO, respectively[14]. The peak around 1620 cm-1 is assigned to the adsorbed water molecules, and may also be descended from skeletal vibrations of unoxidized domain of GO[15]. The peaks at 1621 and 1510 cm-1 (Fig. 4B-b) are due to stretching vibration of quinoid ring and benzenoid ring of PANI. In Fig. 4B-c, the peaks of the oxygen-containing functional groups of GO disappeare, which indicates that the chemical reduction of GO is relatively complete. The peaks of PANI are shifted from 1621, 1510 cm-1 to 1574, 1470 cm-1, due to the π-π interactions between rGO and PANI[12 ]. XPS is used to investigate the chemical states of the iodine in the PANI-rGO composite hydrogel. The I3d5/2 XPS spectrum (Fig. 4C) of PANI-rGO composite hydrogel can be fitted into two peaks at 618.5 and 620.0 eV, which are attributed to the I3- and I5- anions formed during the reaction[16]. Because I3- and I5- are the stable states of iodine atoms in solution[17], they remain in the hydrogel acting as a dopant to graphene and polyaniline improving efficiently the electrical conductivity of the composite hydrogel. The high electrical conductivity of the as-prepared composite hydrogel (0.48 S cm-1) supports the above discussion. Previous research shows that iodine is more effective than protonic acid for doping PANI to obtain a higher conductivity[18], since a highly

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conjugated π system is formed in the iodine doped PANI molecular chains. On the other hand, the iodine doped rGO exhibits a p-type conductive property which leads to a high electrical conductivity[19]. In addition, the iodine reduced GO shows a higher C/O ratio than the GO reduced by other reductants[17]. In the case of doping with the I3- ions, the doped graphene reveals a high catalytic activity and a long-term stability[20]. Moreover, the formation mechanism has been reported concerning the carbon material with high conductivity by using HI to reduce GO [21]. And a PANI/rGO composite film with a relative high specific capacitance of 448 F g-1 at 0.5 A g-1 was prepared by attaching PANI nanospheres on GO sheets, and then reducing GO with HI [22]. Therefore, it can be believed that the iodine doping for the as-prepared composite hydrogel is of the main contribution to the improvement of the charge storage. To explore the electrochemical performance of the PANI-rGO composite hydrogel, CV, EIS and galvanostatic charge/discharge measurements, as well as the cycling life are tested in a two electrode system. The CV tests at different scan rates ranged 5-100 mV/s are depicted in Fig. 5A. It can be seen that the CV curve of PANI-rGO composite hydrogel shows a similar rectangular shape, indicating a double layer electric capacitive behavior. At a lower scan rate, the hydrogel gives a voltammogram close to a standard rectangular shape, which is the characteristic of an activated carbon supercapacitor. The shape remains unchanged at a higher scan rate suggesting the excellent stability. The redox peaks appearing at 0.28/0.22 V in the CV curves can be attributed to the transformation between

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emeraldine and pernigraniline[23], which enhance the integrated area in the CV curve suggesting the higher specific capacitance of the material[24]. Therefore, the capacitance of PANI-based material mainly originates from the pseudo-capacitance behavior of the conducting polymer[25]. In order to investigate the electrochemical behavior at the electrode/electrolyte interface, EIS measurements are implemented. Fig. 5B shows the Nyquist plots for the PANI/rGO composite hydrogel. They exhibit an ideal electrochemical capacitance behavior, that is, a well-defined semicircle in the high to medium frequency regions[26,27] and nearly vertical line in the low frequency region[28]. The small semicircle over the high frequency range is indicative of low interfacial charge-transfer resistance which can be attributed to the high electrical conductivity of these materials. The equivalent series resistance (ESR) of the electrode obtained from the x intercept of the Nyquist plot is about 0.8 Ω, confirming again the high electric conductivity of this material. The galvanostatic charge/discharge is a reliable method to evaluate the electrochemical capacitance of materials. Fig. 5C shows the galvanostatic charge/discharge curves at different current density of the PANI-rGO composite hydrogel at room temperature. All curves exhibits nearly equilateral triangle shape, featuring that the potential of charge/discharge linearly responded to time and indicating a good reversibility during the charge/discharge processes. According to the computational formula of specific capacitance (Csp), the electrode for the PANI-rGO

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composite hydrogel has Csp to be 712.5, 698.8, 567.5 and 402.5 F g-1 at current density of 1.0, 2.5, 5.0 and 10 A g-1, respectively. The high specific capacitance and good rate performance are ascribed to the excellent pseudo-capacitance of PANI. Moreover, to understand the role and influence of iodine in the composite hydrogel, pure rGO and rGO-PANI composite both without iodine doping are prepared. Interestingly, the as-prepared rGO-PANI composite cannot form a self-supporting hydrogel due to the inexistence of iodine, suggesting that the doped iodine also could play a role similar to cross-linking agent via electrostatic interaction with polyaniline chains. The galvanostatic charge/discharge curves of rGO, rGO-PANI composite and PANI-rGO composite hydrogel at a current density of 1 A g-1 are shown in Fig. 6A for comparisons. The specific capacitance of rGO is only 27.0 F g-1, while the rGO-PANI composite has a higher specific capacitance of 71.0 F g-1 attributing to the contribution of the conductive component PANI. By contrast, the PANI-rGO composite hydrogel exhibits a much higher capacitance and electric conductivity (0.48 S cm-1) than rGO-PANI composite (3.0×10-3 S cm-1). The excellent electrical property of the PANI-rGO composite hydrogel largely depends on the doping of iodine, and combining with the advantage of hydrogel novel morphology, the specific capacitance for the PANI-rGO composite hydrogel can be greatly improved. The cycling life of PANI-rGO composite hydrogel based pseudosupercapacitors is measured between 0 and 0.8 V at the current density of 8 A g-1. The hydrogel is coating on the platinum sheets directly and tests in 1 M HCl, which enables to give the real

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performance of the composite hydrogel without destroying its inner structure. The cyclic properties of the rGO and rGO-PANI composite at a current density of 1 A g-1 are carried out for comparisons. The result is shown in Fig. 6B. It can be seen that the specific capacitance of rGO-PANI composite remains 62% of the initial capacitance(72 F g-1) after 5000 cycles at a current density of 1 A g-1. While the PANI-rGO composite hydrogel exhibits a high specific capacitance of 440 F g-1 at a high current density of 8 A g-1, and the capacitance remains 59% (260 F g-1) after 5000 cycles. The capacitance still remains at 340 F g-1 (～77%) after 1000 cycles, reflecting good electrochemical stability and a high degree of reversibility in the repetitive charge/discharge cycling test, which are superior to other PANI-rGO composites[22,29]. We also examine the content of iodine after charging-discharging 5000 cycles, and find that it drops from 6.5% to 4.1%. That is because the iodine may diffuse into the electrolyte during the doping and dedoping process. Thus the loss of specific capacitance may be related to the increase of electrolyte solution resistance and the contact resistance between electrode and electrolyte, as well as the deterioration of ion diffusion during the long time test [30].

4. CONCLUSIONS The iodine doped polyaniline-rGO composite hydrogel is successfully synthesized using a one step process. SEM images show that the rGO nanosheets are covered by PANI nanoparticles uniformly, offering the composite hydrogel excellent pseudo-capacitive property. The iodine doping to both polyaniline and rGO improves

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efficiently the electrical conductivity and electrochemical performance of the composite hydrogel. High specific capacitances are achieved for the composite hydrogel with 712.5 F g-1 at the current density of 1 A g-1. Even at a high current density of 10 A g-1, the capacitance of the composite hydrogel is still 402.5 F g-1. Our research results indicate the iodine doped PANI-rGO composite hydrogel to be a potential candidate of electrode materials for electrochemical energy storage.

Acknowledges

This work was supported by the National Natural Science Foundation of China (No. 21174059, 21374046), China Postdoctoral Science Foundation(2013M530249), Program for Changjiang Scholars and Innovative Research Team in University, Open Project of State Key Laboratory of Superamolecular Structure and Materials (SKLSSM201416) and the Testing Foundation of Nanjing University.

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Figure 1. The device of the symmetrical two-electrode configuration. Figure 2. The photograph of PANI-rGO hydrogel. Figure 3. (A) SEM image of GO, (B) SEM image with different enlargement factor of PANI-rGO composite hydrogel, (C) SEM image and corresponding nitrogen, oxygen and iodine EDX mapping of PANI-rGO composite hydrogel, (D) The EDS of PANI-rGO composite hydrogel. Figure 4. (A) XRD patterns of (a) GO; (b) PANI/rGO composite hydrogel, (B) FTIR spectra of (a)GO, (b)PANI and (c)PANI-rGO composite hydrogel, (C) I3d5/2 XPS spectrum of PANI-rGO composite hydrogel. Figure 5. The electrochemical properties of the PANI-rGO composite hydrogel (A) CV curves at different scan rate, (B) Nyquist plots, (C) charge/discharge curves at different current density.

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Figure 6. (A) charge/discharge curves of the rGO, rGO-PANI composite and PANI-rGO composite hydrogel at a current density of 1 A g-1, (B) cycle stability of the PANI-rGO hydrogel at a current density of 8 A g-1, rGO and rGO-PANI composite at a current density of 1 A g-1.

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