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Preparation and electrochemistry of nanostructured PPy/graphite nanosheets/rare earth ions composites for supercapacitor.

INTRODUCTION

Supercapacitors have been developed to close the gap between conventional capacitors and batteries, such as in digital communication and electric vehicles that require electrical energy at high power levels in relatively short pulses [1-3]. Pseudocapacitors have received increasing attention as devices have greater energy storage capacity, and rapid Faradic electron-transfer processes utilizing electroactive conducting polymers and metal oxides (or carbon nanomaterial) dominate their charge/discharge cycles [4]. Composites of conducting polymers and carbon have been reported in the literature as promising prototype materials for supercapacitor applications.

Polypyrrole (PPy), one of the most important conducting polymers, has been studied extensively since PPy having the unique 7t-conjugated system and provided an effective route for the flow of electronic charges [5], Conducting polymer composites are formed by dispersing different functional fillers into a polymer matrix and have attracted many researchers to explore their potential applications as high-performance material [6, 7], The properties of the conducting polymer composites could be further tailored by the filler materials, filler loadings, surface functionalities of the fillers, and the nature of the polymer matrix. The different fillers such as carbon nanotubes [8, 9], metal or metal oxide [10-12], and graphene [13-15] have been studied for many years. Some PPy-inorganic composites have already been reported in the literatures, such as PPy/Mn[O.sub.2] [6], PPy/Ag [7], PPy/Au [10], PPy/Fe2[O.sub.3] [11], PPy/NiO [12], PPy/ W[O.sub.3] [16], PPy/ZnS [17], and PPy/graphene [13-15],

Graphite has been used as the conducting filler for making conducting polymer composites due to its excellent performance such as flexibility, easy processing, and high conductivity [18], Moreover, there are no reactive ion groups on the graphite layers. It is rather difficult to intercalate PPy directly into the interlayer of graphite through ion exchange reaction to prepare the PPy/graphite composites. Therefore, graphite nanosheets (nanoG) is considered as a promising conducting filler because nanoG maintain the layered structure similar to natural flake graphite and possess of good electrical conductivity (with an electrical conductivity of about [10.sup.4] S/cm at ambient temperature) and lower cost, which are responsible for the low percolation threshold and high electrical conductivity [19, 20]. These excellent properties may be relevant at the nanoscale if expanded graphite (it was obtained by heating the expandable graphite in a muffle furnace at high temperature) can be exfoliated into graphite nanosheets [21]. Therefore, nanoG as filler has not only the low percolation threshold but can significantly improve the property of PPy/nanoG composites.

Literature survey reveals that there is dearth of reports on three-phase polypyrrole/nanoG/rare earth ions (PPy/nanoG/ [RE.sup.3+]) composites. Because the combination of different components, at the molecular level, can provide a method to design new nanocomposite materials as well as the ability to improve the conductivity and electrochemical properties of both components. In this work, the PPy/nanoG/[RE.sup.3+] composites are produced in order to understand how the nanoG and [RE.sup.3+] could be exploited for electrode materials application. The PPy/ nanoG/[RE.sup.3+] nanocomposites are prepared using different molar ratios of nanoG and [RE.sup.3+] via in-situ polymerization method with p-toluenesulfonic acid as a dopant and Fe[Cl.sup.3] as an oxidant. This study shows the characterization of the supercapacitor application of PPy/nanoG/[RE.sup.3+] composites characterized by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) techniques, Cyclic Voltammetry, respectively.

EXPERIMENTAL

Materials

The graphite used for preparing nanoG was expandable graphite, provided by Shandong Qingdao Graphite Company (China). The pyrrole monomer (Py), purchased from Shanghai Chemical Company (China), was purified by vapor distillation before use. The rare earth chloride (La[Cl.sub.3], Sm[Cl.sub.3], EU[Cl.sub.3], Gd[Cl.sub.3], and Tb[Cl.sub.3]) was prepared by gadolinium oxide ([La.sub.2][O.sub.3] [Sm.sub.2][O.sub.3], [EU.sub.2][O.sub.3], Gd203i and Tb704). The hydrogen peroxide, ferric chloride crystal (Fe[Cl.sub.3]-6[H.sub.2]O), polyethylene glycol 400 (PEG-400), p-toluenesulfonic acid (p-TSA) and orthodichlorobenzene at analytical reagent grade were obtained from Shanghai Chemical Company (China) and used as received.

Preparation of nanoG

The expanded graphite was prepared according to previous literature [22]. The synthetic process was as follows: the expanded graphite was immersed in ethyl alcohol and subjected to powdering in an ultrasonic bath at 80 W for 8 h, and then filtered and washed with enough ethyl alcohol and deionized water. The resulting suspension was dried in the vacuum oven at 60[degrees]C for 24 h to obtain nanoG.

Preparation of PPy/nanoG/[RE.sup.3+] Composites

As the preparation of PPy/nanoG/[RE.sup.3+] composites, 1 mL PEG-400, 7.22 mmol pyrrole, and a certain amount of nanoG and [RE.sup.3+] were mixed into 10 mL [C.sub.2][H.sub.5]OH and the mixture was sonicated for about 2-3 h at room temperature. Then a dark gray colloidal system was obtained. After stirring for 15 min at 0-5[degrees]C, 2 mmol p-TSA was injected into the above mixture and stirred for 20 min. Finally, 50 mL 0.34 mol/L Fe[Cl.sub.3] x 6[H.sub.2]O aqueous solution (Fe[Cl.sub.3]pyrrole = 2.35:1 molar ratio) was dropped into the reactor and a rapid oxidation occurred. The resulted suspension was stirred at 0-5[degrees]C for 2 h. The polymerization was allowed to proceed for 22 h at room temperature with a continuous stirring. The obtained product was filtered and washed with deionized water and ethanol, and dried at 50[degrees]C in vacuum for 24 h.

Characterization

Scanning electron microscope (SEM; Hitachi, Japan, JEOL, JSM-6330F) was used to observe the morphologies of the composites. Prior to the characterization, the specimen was coated with a very thin layer of gold atoms. Transmission electron microscope (TEM; JEOLIOOCX-II model) was run at 100 kV accelerated voltage to observe the morphologies. The observations were carried out after retrieving the slices onto Cu grids. The electrical conductivity measurements were made by the conventional four-point probes resistivity measurement system (china model RTS-B) on pressed pellets of composite prepared at room temperature (25[degrees]C).

Electrochemical Tests

The working electrodes were prepared according to the following steps. The typical mass load of the electrode materials is 10 mg. 75 wt% of active materials was mixed with 7.5 wt% of acetylene black and 7.5 wt% of conducting graphite in an agate mortar until a homogeneous black powder was obtained. To this mixture, 10 wt% polytetrafluoroethylene was added with a few drops of ethanol. The resulting paste was pressed at 10 MPa to a stain less-steel (SS) grid that served as a current collector. Each electrode contained about 4 mg of electroactive material and had a geometric surface area of about 1 [cm.sup.2]. A typical three-electrode experimental cell equipped with a working electrode, a platinum foil counter electrode, and a saturated calomel reference electrode (SCE) was used for measuring the electrochemical properties of the working electrode and its performance as a Faradaic electrochemical capacitances (ECs). All electrochemical measurements were carried out on CHI660B electrochemical working station in 1 M [H.sub.2]S[O.sub.4] aqueous solution as electrolyte at 25[degrees]C.

RESULTS AND DISCUSSION

Synthesis Design

It is rather difficult to intercalate organic molecules or polymers directly into the interlayer of graphite through ion exchange reaction to prepare the polymer/nanoG composites. However, we can deal with expanded graphite (EG) and prepare graphite nanosheets. The EG contains abundant multipores ranging from 2 to 10 nm (Fig. 2b) [18, 19]. The properties of multi-pores and the little OH groups could make EG have good affinity to polymers, therefore, polymers may able to absorbed into the pores and galleries of EG by proper method to produce conductive polymer/nanoG composites [18-21]. The schematic of synthesis process is shown in Fig. 1.

Morphology

The expandable graphite was heat treated at high temperature to obtain EG. Figure 2a and b shows the SEM microstructures of EG. Figure 2c (SEM) and d (TEM) shows the micrograph of the prepared nanoG using EG by ultrasonic irradiation in alcohol solution for 8 h. It can be seen clearly that the EG was exfoliated into nanosheets with a thickness ranging 5-10 nm. This result is similar to that reported by other researchers [23]. Such nanoscale dispersion facilitates the formation of the electrical conductive network in the polymer matrix at very low filler loading [22].

Figure 3 shows the SEM microstructures of PPy (a), PPy/ nanoG/[La.sup.3+] (b), PPy/nanoG/[Sm.sup.3+] (c), PPy/nanoG/[Eu.sup.3] (d), PPy/nanoG/[Gd.sup.3+] (e), and PPy/nanoG/[Tb.sup.3+] (f), respectively. The composites of PPy/nanoG/[La.sup.3+], PPy/nanoG/[Sm.sup.3+], PPy/ nanoG/[Eu.sup.3+], PPy/nanoG/[Gd.sup.3+], and PPy/nanoG/[Tb.sup.3+] are observed to exhibit PPy with a fairly uniform distribution in the nanoG. layers. However, spheroidal PPy are observed to be slightly packed around the edge of the nanoG compared with the nanoG layers. The large flake and fairly uniform distribution of nanoG can improve the effective contact between nanoG and nanoG layer or nanoG layer and PPy, which are beneficial to form a good electrically conductive channel.

Figure 4 shows TEM images of PPy/nanoG/[Gd.sup.3+] and PPy/ nanoG/[Eu.sup.3+] composites dispersed in ethyl alcohol. The gray Hakes represented the nanoG. It can be seen that the nanoscale patterns of the nanoG uniform dispersed in PPy matrix. This kind of nanoG as observed had thickness of 2-10 nm, which was a little thinner than that of the result in Fig. 2c and d due to the PPy presence prevented the overlay of nanoG. The TEM morphologies of PPy/nanoG/[Gd.sup.3+] and PPy/nanoG/[Eu.sup.3+] were very consistent with the result of Fig. 3d and e. This structure of composites effectively confirmed that there was strong interaction among nanoG, [RE.sup.3+], and PPy.

Conductivity Characteristics

From Figure 5, it is easily observed that the conductivity of PPy/nanoG composites in PPy matrix can be rapidly increased compared with PPy with the increase of nanoG amount. The PPy/nanoG composites containing a nanoG loading of 1, 2, 3, 5, 6, and 10 wt% of PPy show a room temperature conductivity of about 3.69, 5.18, 15.62, 17.24, 18.52, and 20.00 S/cm, respectively. The conductivity 15.62 S/cm found with 3 wt% nanoG at room temperature, which was seven times greater than the PPy. The interaction between the unique [pi]-stacked coplanar structure of PPy and the aromatic structure of the nanoG may be contributed to the formation of electrically conductive network [18, 24].

Figure 6 shows the conductivity of PPy/nanoG/[RE.sup.3+] composites with [RE.sup.3+] loadings of 0, 1, 3, 5, 7, and 9 wt%, respectively. Keeping nanoG content of 3 wt% in the PPy/nanoG/ [RE.sup.3+] composites, its conductivities increase with the increase of [RE.sup.3+] content. Maximum conductivity of PPy/nanoG/[RE.sup.3+] composites about 16.36 S/cm found with 5 wt% of [Gd.sup.3+] at room temperature. So, [RE.sup.3+] presence at a low loading may be helped conducting network formation in PPy/nanoG/[RE.sup.3+] composites. The conductivity decreases slowly with the [RE.sup.3+] increases, which may be ascribed to the percolation threshold of the composites. At the same time, the [RE.sup.3+] introduced into the PPy/nanoG may serve as an efficient bridge effect to improve conductivity of the PPy/nanoG [16].

Electrochemical Characterization

To further evaluate the applicability of PPy, PPy/nanoG, and PPy/nanoG/[Gd.sup.3+] (the conductivity of PPy/nanoG/[Gd.sup.3+] is the best one compared with others, so we only researched its electrochemical properties) composites in supercapacitors, the mass specific capacitance and the cyclic voltammetry behavior of composites have been investigated in 1 M [H.sub.2] S[O.sub.4] aqueous solution.

Figure 7 shows the charge-discharge curves of PPy, PPy/ nanoG, and PPy/nanoG/[Gd.sup.3+] composites electrodes in 1 M [H.sub.2] S[O.sub.4] aqueous solution. The SC of PPy increases with doping nanoG and [Gd.sup.3+], and PPy, PPy/nanoG, and PPy/nanoG/[Gd.sup.3+] composites reaches 103, 132, and 175 F/g, respectively. This may also be attributed to the doping effect associated with the formation of a charge-transfer complex between PPy and nanoG rather than the weak vander waals interactions between them, which may due to that nanoG with some oxygen-containing groups is a good electron acceptor, PPy is a fairly good electron donor, and then nanoG and PPy may form a charge-transfer complex in their ground state [14, 25], Therefore, PPy/nanoG/ [Gd.sup.3+] composites with strong interactions leads to charge stabilization and thus effectively promote the protonation of pure PPy, which may then serve as condensation nuclei to increase the propensity for PPy to coat on the surface of nanoG to form the PPy/nanoG/[Gd.sup.3+] composite.

Cyclic voltammetry of PPy, PPy/nanoG, and PPy/nanoG/ [Gd.sup.3+] composite in 1 M [H.sub.2] S[O.sub.4] aqueous electrolyte over the potential range of -0.2 to +0.8 V (vs. SCE) at the scan rate of 20-100 mV/s are Fig. 8. As observed, the pure PPy showed substantially lower charge capacity as compared to PPy/nanoG and PPy/nanoG/[Gd.sup.3+] composite. From the CV curves, it can be observed that the PPy/nanoG/[Gd.sup.3+] has larger voltammetric current response compared with PPy, revealing higher charge storage capability of Py/GR/[Gd.sup.3+]. All the CVs were observed to be almost rectangular in shape under various scan rates, which indicated the electrode is charged and discharged at a pseudo-constant rate over the complete voltammetric cycle. However, with the scan rate increasing, the effective interaction between the ions and the electrode gradually reduced, the deviation from rectangularity of the CV becomes obvious. Figure 9 shows a comparative study of the cyclic voltammograms for PPy, PPy/ nanoG, and PPy/nanoG/[Gd.sup.3+] electrodes. The capacitance of PPy was improved in the electrochemical performance of the composites after introduced into nanoG and [Gd.sup.3+] from Fig. 9.

The instability of the capacitors based on conducting polymer during long-term charge/discharge cycling is one of their most lethal deficiencies. As shown in Fig. 10, the capacitance of PPy lost 19.4% (from 103 to 83 F/g) after 800 charging/discharging cycles at a current density of 1 A/g. However, under the same conditions, the capacitance of the PPy/nanoG/[Gd.sup.3+] composite decreased only 5.1% (from 175 to 166 F/g). The improved electrochemical stability of the PPy/nanoG/[Gd.sup.3+] composite can be explained as follows. In the composite, the nanoG and [Gd.sup.3+] can partly prevent PPy component from severely swelling and shrinking during cycling. Thus, the morphological and electrochemical property changes of PPy induced by charge/discharge cycling were greatly reduced [13].

CONCLUSIONS

A series of PPy/nanoG/[RE.sup.3+] composites have been successfully prepared by in-situ polymerization and showed an obvious increase of electrical conductivity and electrochemical performance compared with pure PPy. Systematic studies on the electrical conductivity and electrochemical performance of PPy/ nanoG/[RE.sup.3+] composites revealed that the nanoG and [RE.sup.3+] may be simultaneously contributed to increase of the conductivity of PPy/nanoG/[RE.sup.3+] composites. The PPy/nanoG/[RE.sup.3+] composites show high electrical conductivities and electrochemical performance which are essential for their applications as electrode materials. High specific capacitances are achieved for PPy/nanoG/ [Gd.sup.3+] with the highest specific capacitance of 175 F/g at a current density of 1 A/g. Our results prove that high specific capacitance can be obtained either by doping PPy with a small amount of nanoG and [RE.sup.3+] due to the significant change of the composite structure.

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Wanhong Sun, (1) Zunli Mo, (2) Hailing Li, (1) Yu Sun, (1) Yanqing Zhou (3)

(1) Experiment Center of Northwest University for Nationalities, Lanzhou 730030, China

(2) College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China

(3) Department of Petrochemical Engineering, Lanzhou Petrolchemical College of Vocational Technology, Lanzhou, Gansu 730060, People's Republic of China

Correspondence to: Zunli Mo; e-mail: mozl@163.com

Contract grant sponsor: Central Universities Fundamental Research; contract grand numbers: 31920130025, 31920130027; Contract grant sponsor: Education Department of Gansu Province; contract grand number: 2013A-141.

DOI 10.1002/pen.23821

Published online in Wiley Online Library (wileyonlinelibrary.com).
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Author:Sun, Wanhong; Mo, Zunli; Li, Hailing; Sun, Yu; Zhou, Yanqing
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:1USA
Date:Dec 1, 2014
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