Chinese Journal of Chemical Physics  2017, Vol. 30 Issue (1): 112-116

#### The article information

Jun-hao Chen, Xia-yu Feng, Wu-feng Chen, Yu-qing Song, Li-feng Yan

Electrochemical Preparation of Polypyrrole/Graphene Films on Titanium Mesh as Active Materials for Supercapacitors

Chinese Journal of Chemical Physics, 2017, 30(1): 112-116

http://dx.doi.org/10.1063/1674-0068/30/cjcp1604092

### Article history

Accepted on: May 9, 2016
Electrochemical Preparation of Polypyrrole/Graphene Films on Titanium Mesh as Active Materials for Supercapacitors
Jun-hao Chen, Xia-yu Feng, Wu-feng Chen, Yu-qing Song, Li-feng Yan
Dated: Received on April 29, 2016; Accepted on May 9, 2016
Department of Chemical Physics, iCHEM, University of Science and Technology of China, Hefei 230026, China
*Author to whom correspondence should be addressed. E-mail:lfyan@ustc.edu.cn
Abstract: Films of polypyrrole/graphene on titanium mesh were prepared by electrochemical reduction of the fresh dried foam films of graphene oxide followed by an electrochemical polymerization of pyrrole. The as-obtained composite had highly surface area, conductivity, and could be used as the electrode for supercapacitors, especially directly used as the active materials in free of binders while the Ti mesh worked as the collector. Plenty of polypyrrole nanoparticles formed on the surface of reduced graphene film, and some fiber-like aggregates could be formed during the polymerization, which worked as the material for pseudo-capacitance. The specific capacitance of the supercapacitor reached 400 F/g and showed high stability with retaining capacitance of 82% after 5000 cycles, indicating that the nanocomposite is a suitable active material for supercapacitors.
Key words: Graphene    Polypyrrol    Thin film    Supercapacitors    Electrochemical
I. INTRODUCTION

Supercapacitors have attracted great interest owing to their high power density, excellent long cycling life and fast charging/discharging rate [1]. The most widely used materials for electrodes of supercapacitors are carbon based materials with high surface area, such as carbon fibers, carbon nanotube, etc. [2]. Since the first preparation of graphene in 2004 [3], it has shown great potential application in supercapacitors due to its high specific area and conductivity [4]. However, electrodes based on graphene powder are usually required using a binder or additives to stick the active materials onto the surface of collector, but the presence of them would decrease the conductivity of the electrode and result in poor performance of the supercapacitors [5, 6]. Recently, free-standing thin film supercapacitors have attracted much attention for it can be directly used as working electrodes in free of binders or additives [7-11]. Electrochemical reduction of graphene oxide (GO) is an effective methods to prepare reduction graphene (rGO) with low defect and high capacitance [12].

Our group developed a method to obtain a fantasy free-standing dried GO foam film, which can be reduced to an ultrathin graphene film [13]. The frame of the titanium meshes was used as both the supports of free-standing micro-films and collector electrode, and the latticed foam films (LFF) of GO were obtained at first by this method, and then it was reduced to latticed rGO foam films, which work directly as the active materials for the electrodes of supercapacitor. In addition, it remedied the defect of a simple rGO foam film that may easily break. Otherwise, the easiness of changing the shape of titanium mesh made it convenient for customers to make a shape that they need, which showed a great application potential in wearable devices.

Compared to traditional carbon based double-layer capacitors, pseudo-capactive electrode materials can increase the electrochemical capacitances and energy densities, and the most selective materials are metal oxides. In addition, some conductive polymers have been paid much attention for their excellent performance for supercapacitors, such as polympyrrole, polyaniline, etc. [8, 11].

Because of the skeleton supports of the titanium mesh, it is easy to make some modification on the rGO LFF. The possibility to be broken is not needed to worry much about. Thus, it is possible to grow polypyrrole (PPy) on the surface of rGO LFF by electrochemical polymerization of its monomer, and PPy/rGO LFFs were obtained. The faradic pseudo-capacitance and the specific surface may be broadened by the composite, which is a binder-free active material.

II. EXPERIMENTS A. Material

Graphite powder, natural briquetting grade, $\sim$8000 meshes, 99.95% (metals basis), and pyrrole (99.7%) were purchased from Aladdin Chemical Regent Company. Titanium mesh (~60 meshes) was purchased from Shijiazhuang Hangxu Company. Analytical grade NaNO3, KMnO4, 98% H2SO4, 30% H2O2 aqueous solution, NH4Cl, and Na2SO4 were purchased from Shanghai Chemical Reagents Company, and were used directly without further purification. Ultra-pure water (18 MΩ) was produced by a Millipore System (Millipore Q, USA).

B. Preparation of GO, GO LFF, rGO LFF, PPy/rGO LFFs and PPy

GO is prepared from graphite by a modified Hummers method [14, 15]. In brief, 2 g of the natural graphite powder was added into a 400 mL beaker and 1 g of NaNO3 and 46 mL of H2SO4 were added subsequently under stirring in an ice-bath. Then 6 g of KMnO4 was added slowly into the beaker under stirring condition and the temperature of the system was controlled up to 20 ℃, the system was kept for 20 h at room temperature. Next 92 mL water was slowly added into the system and stirred for another 20 min. Then 80 mL of hot water (60 ℃) and 3% H2O2 aqueous solution were added to reduce residual KMnO4 until the bubbling disappeared. Finally, the system was centrifuged at 7200 r/min for 20 min, and the residue was washed by warm water until the pH of the upper layer of the suspension reached pH=3-4. The obtained sediment was re-dispersed into water and treated by mild ultrasound for 30 min to get GO solution directly or obtain dried GO powder after freeze-drying of the sediment.

Aqueous solution of GO (5.0 mg/mL) was prepared by dispersing GO in ultra-pure water under mild ultrasound for 15 min, which was used for the GO LFF preparation. Then a piece of titanium mesh was evenly coated with GO to form latticed liquid foam films of aqueous solution of GO. The liquid films were then dried in warm air of about 60 ℃ to obtain a piece of dried GO LFF.

Electrochemical reduction of GO LFF was performed on a CHI660C potentiostat-galvanostat (CH Instruments Inc.) at a voltage of -1.0 V using 1 mol/L NH4Cl as electrolyte.

Electrochemical polymerization of pyrrole monomer onto the surface of rGO LFFs was carried out by cyclic voltammetry (CV) at a scan rate of 40 mV/s and over a potential range of -0.2 V to 1.2 V (vs. SCE) with pyrrole (Py) monomer in 1 mol/L H2SO4 electrolyte.

C. Supercapacitors and performance

Electrochemical impedance spectroscopy (EIS) measurements and galvanostatic charge-discharge (GCD) experiments in a three-electrode electrochemical cell included a piece Ti mesh containing active material (working electrode), a platinum wire (counter electrode) and an Hg/Hg2Cl2 reference electrode. CV and EIS were carried out with 1 mol/L H2SO4 as electrolyte. GCD and cycle GCD were carried out with 1 mol/L Na2SO4 or 1 mol/L H2SO4 as electrolyte.

For CV measurement, the specific capacitance of the LFF was calculated using the equation

 (1)

where S is the integrated area of the CV curve, $m$ is the mass of LFF material in the electrode, v is the scan rate, and U is the working potential window.

For GCD, the specific capacitance Cwt-sp, the energy density Ewt-sp, and the power density Pwt-sp of the LFF were calculated using the following equation:

 (2)
 (3)
 (4)

where I is the constant discharge current, Δt is the discharging time, m is the mass of LFF material in the electrode, and ΔV is the voltage drop upon discharging.

The volume specific capacitance was calculated using the equation Cvol=Cwt-spρ [10, 11].

D. Characterization

The materials structures were measured using a Sirion 200 FESEM at an accelerating voltage of 10 kV. X-ray photoelectron spectroscopy (XPS) was recorded on an Escalab MK II photoelectron spectrometer (VG Scientific Ltd., United Kingdom).

III. RESULTS AND DISCUSSION

Scheme 1 shows the preparation of the latticed PPy/rGO foam films (LFF), which is typically a three-step process. At the first step (step I), nanosheets of GO self-assembled to a series of latticed GO liquid foam films on the surface of Ti mesh, and then they were dried to form the GO LFF. The size of the GO nanosheets is about 2-5 μm, and the self-assembly of the GO nanosheets in the liquid film driven by the liquid crystalline phase formation of 2D sheets results in the formation of a stable and density stacking film of GO in a frame of titanium mesh [13]. In step II, the GO films were reduced to rGO through an electrochemical process. At the last step III, PPy/rGO films were prepared by an electrochemical polymerization of pyrrole on the surface of rGO LFF.

 Scheme 1 Preparation of the PPy/rGO LLF by a typical three-step process.

Figure 1 shows the photoimage of the as-prepared PPy/rGO materials of GO LFF on Ti mesh, and the SEM images of the surface of the materials. Clearly, the Ti mesh is covered by a layer of black film, and the SEM images reveals that there are plenty of polypyrrole globular nanoparticles growing on the surface of rGO film, and some nanowire can be found, indicating the trend to form fiber-like structures of the polypyrrole nanoparticles when much pyrrole monomer was polymerized.

 FIG. 1 Photoimage (a) and SEM images (b) of the as-prepared PPy/rGO LLF on Ti mesh. (c) SEM images of the surface of the material.

XPS measurements could provide direct evidences of the reduction of the GO and the deposition of polypyrrole on the films. Figure 2 shows the typical C1s XPS spectrum of the as-prepared PPy/rGO dried film. The curve was fitted as below: C=C (sp2, 284.4 eV), C-N/C-O (286 eV), C=O/C=N (287.8 eV) [16]. Clearly, the typical peaks of both rGO and polypyrrole can be assigned, and the elemental contents of C, N, and O are 68.42%, 11.3%, and 20.29%, respectively, indicating the success preparation of the target materials.

 FIG. 2 XPS C1s spectrum of PPy/rGO film.

The CV performance of the material is shown in Fig. 3(a), which shows the existence of faradic pseudocapacitance in polymer/rGO LFF. And GCD performance is shown in Fig. 3(b). Figure 4(a) shows the specific capacitance of the materials obtained by the CV measurement and GCD tests. Apparently, PPy/rGO LFF has a better capacitive performance than that of rGO LFF, which performs a high specific capacitance of about 400 F/g measured by CV, much higher than the 150 F/g of rGO LFF. The specific capacitance retains about 200 F/g even at high charging current (Fig. 4(a)). Besides, after 5000 cycles of charge/discharge at a current density of 10 A/g, 82% of capacitance retention remained in acidic condition, indicating the stability of supercapacitor (Fig. 4(b)). In addition, the active material retains about more than 60% of capacitance even in neutral electrolyte (Fig. 4(b)), indicating the PPy/rGO film has a widely potential application.

 FIG. 3 Electrochemical performances of PPy/rGO on Ti mesh measured in a three-electrode system. (a) Cyclic voltammograms at different scan rates of 5, 10, and 50 mV/s. (b) Galvanostatic charge-discharge curves at different current densities.
 FIG. 4 (a) Relationship of specific capacitance of the PPy/rGO composite on the charged current, (b) cycle performance of the supercapacitor in both acidic and neutral electrolytes.

The specific surface areas of the materials were measured with the adsorption of methylene blue, and the concentration change of the methylene blue after the adsorption was measured by UV-Vis. The specific surface area of the PPy/rGO composite is 1850 cm2/g, with a relatively higher value, indicating that the composite is a potential active material for electrode of supercapacitors.

Figure 5 shows the Nyquist plot of the rGO LFF and PPy/rGO LFF over the frequency range from 0.1 Hz to 10 kHz by EIS. As expected, at a low frequency, their imaginary parts increased sharply and nearly vertical lines are observed, indicating their ideal capacitive behaviors. As a comparison of magnified data in the high-frequency range, for the PPy/rGO LFF, a transition between the RC semicircle and the migration of electrolyte was observed at a frequency of about \mbox{966.8 Hz}, corresponding to a smaller resistance of 0.22 Ω than 0.35 Ω at 312.5 Hz of the rGO LFF.

 FIG. 5 Niquist plot of the rGO and PPy/rGO films on Ti mesh and (b) magnified diagram in the high frequency region.

Figure 6 shows the relationship of power density (P) and energy densities (E) (Ragone plot) of the supercapacitor, and the performance was evaluated. The maximum energy density for the asymmetric supercapacitor at a current density of 1 A/g is 35.2 Wh/kg. Importantly, the supercapacitor can provide a high energy density even at high power density, and the energy density still reaches 17.3 Wh/kg when the power density increases to 6610 W/kg.

 FIG. 6 Ragone plot for the supercapacitor of PPy/rGO films on Ti mesh.
IV. CONCLUSION

Free-standing film of rGO has been prepared by a dried foam film technique following an electrochemical reduction on the surface of Ti mesh, which also works as the collector of the electrode. Then, pyrrole was electrochemically polymerized on the surface of the rGO film, and PPy/rGO latticed foam films were obtained. The materials can directly work as the electrode for supercapacitors without adding of binders or additives. The performance of the supercapacitor is high and stable, and it provides a new method to prepare graphene based active materials for supercapacitors.

V. ACKNOWLEDGMENTS

This work is supported by the National Natural Science Foundation of China (No.51373162), and the Natural Science Foundation of Anhui Province (No.1408085MKL03).

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