polyaniline films for supercapacitor electrodes with a high specific capacitance

Accepted Manuscript Title: Electrochemical Reduction of Ultrathin Graphene Oxide/Polyaniline Films for Supercapacitor Electrodes with a High Specific Capacitance Author: Ashis K. Sarker Jong-Dal Hong PII: DOI: Reference:

S0927-7757(13)00654-7 http://dx.doi.org/doi:10.1016/j.colsurfa.2013.08.043 COLSUA 18614

To appear in:

Colloids and Surfaces A: Physicochem. Eng. Aspects

Received date: Revised date: Accepted date:

1-7-2013 14-8-2013 20-8-2013

Please cite this article as: A.K. Sarker, J.-D. Hong, Electrochemical Reduction of Ultrathin Graphene Oxide/Polyaniline Films for Supercapacitor Electrodes with a High Specific Capacitance, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2013), http://dx.doi.org/10.1016/j.colsurfa.2013.08.043 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Electrochemical Reduction of Ultrathin Graphene Oxide/Polyaniline Films for

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Ashis K. Sarker and Jong-Dal Hong*

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Supercapacitor Electrodes with a High Specific Capacitance

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Department of Chemistry, University of Incheon, 119 Academy-ro Yeonsu-gu, Incheon

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406-772, Republic of Korea

To whom all correspondence should be addressed. *Jong-Dal Hong TEL. 82-32-835-8234, FAX. 82-32-835-8238, e-mail: [email protected]

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Electrochemical Reduction of Ultrathin Graphene Oxide/Polyaniline Films for

Ashis K. Sarker and Jong-Dal Hong*

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Supercapacitor Electrodes with a High Specific Capacitance

Department of Chemistry, University of Incheon, 119 Academy-ro Yeonsu-ku, Incheon

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406-772, Republic of Korea

Abstract

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The article describes ultrathin film electrode composed of polyaniline (PANi) and electrochemically reduced graphene oxide (ERGO) bilayers, which was achieved using layer-by-

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layer (LBL) assembly method. The performance of the electrode composed of 30 PANi/ERGO bilayers denoted to PANi-ERGO30 was analyzed in a three-electrode cell using aqueous 1 M

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H2SO4 electrolytes. The electrode exhibited a specific capacitance of 1563 F/cm3 (at a current

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density of 3.0 A/cm3), and achieved one of new record values among carbon-based devices including conducting polymers, to the best of our knowledge. This breakthrough was made

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possible by the development of a unique process that minimized the morphological damage to the thin film electrodes, and prepared optimal doping and oxidation state of PANi in the multilayer films for achieving excellent electronic conductivities and ionic transport. The LBLassembly method provides a tool for preparing well-organized homogeneous PANi/ERGO composites.

Keywords: Supercapacitor, Graphene-Polyaniline Composite, Electrostatic SelfAssembly, Electrochemical Reduction, Electrical Conductivity, Ultrathin Film Fabrication 2

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1. Introduction

The development of sufficiently compact and efficient energy-storage devices has attracted great interest in recent years in response to the trend toward miniaturization and weight reduction

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in modern portable and wireless electronic equipment. Supercapacitors are very attractive power sources, because their recharge times are generally short, they yield long life cycle, and they

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yield high power; however, the energy densities of supercapacitors still fall short of the values

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achieved in batteries or fuel cells. The need for highly efficient supercapacitors supplying energy for long periods of time motivated the world community including the ‘U.S. Department of

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Energy’ to improve the energy density of supercapacitors and to approach the energy densities of batteries [1]. Supercapacitors function via two energy storage mechanisms: electrical double-

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layer (EDL) capacitance or pseudocapacitance [2]. EDL capacitance arises from the accumulation of charge at the electrode/electrolyte interface. By contrast, pseudocapacitance is

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generated by Faradic, redox reactions involving electrode materials, such as electrically conducting polymers or metal oxides.

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Carbon-based materials could provide excellent candidates for lightweight and compact

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devices owing to their unique combination of properties, including a high surface area, low weight, good electrical conductivity, corrosion resistance in aqueous electrolytes, and highly modifiable nanostructures [3–5]. Carbon materials, such as activated carbons (ACs), carbon nanotubes (CNTs), and graphenes, usually exhibit good stability, but limited specific capacitance [4–7]. Carbon-based supercapacitors are mainly dominated by EDL capacitance. One approach to enhancing the poor specific capacitance of carbon materials has involved modifying the EDL materials using a pseudocapacitance pair such as conducting polymers that undergo reversible Faradaic redox reaction. Among the conducting polymers, polyaniline (PANi) has been

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considered as one of the most promising electrode material due to its high flexibility and relatively high conductivity [8]. Graphene/PANi composite [9], graphene/PANi nanofiber composite [10], electrochemically polymerized PANi/reduced graphene oxide (RGO) films [11],

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PANi nanorods array on graphene oxide (GO) nanosheets [12], graphene/PANi porous silica MCM-41 [13], and PANi-coated curved graphene active materials [14] have been developed. A

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critical barrier to the preparation of the PANi/graphene composites is presented by the tendency

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of graphene to precipitate in solution via aggregate formation driven by strong π-π interaction among 2D graphene sheets [15,16]. The relatively low specific capacitance of the PANi-

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containing electrodes may be improved by developing new methods for preparing homogeneous composites. For instance, a hybrid film composed of PANi polymerized in situ on RGO

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exhibited a specific capacitance of 160 F/cm3 at 0.3 A/g [17] and 238 F/cm3 at 0.07 A/cm3 [18]. Electrostatic self-assembly (ESA) method that involves the layer-by-layer (LBL)

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deposition of uniform nanoscale films [19–21] has been suggested for nanostructured electrode materials composed of PANi and carbon, (such as CNT or RGO) for use in energy storage

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applications, such as battery or supercapacitor [18,22]. LBL self-assembly is particularly

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important for the fabrication of electrochemical capacitance (EC) electrodes, which store energy via nanoscopic charge separation at the interface between an electrolyte and an electrode. The assemblies provide excellent control over the film thickness and the nanoscale roughness via adjustments to the charge reversal mechanism and interpenetration. The energy storage capacity is inversely proportional to the thickness of the double layer. These capacitors, therefore, have extremely high energy densities compared to conventional dielectric capacitors. Electrodes were fabricated using ultrathin multilayer films that had been LBL-assembled via multiple interactions (electrostatic, hydrogen bonding and π–π) between the layer components including the positively

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charged PANi and the negatively charged GO, as shown in Fig. 1. The PANi-GO electrode was electrochemically reduced to give the PANi-electrochemically reduced graphene oxide (ERGO) electrode assembled in a supercapacitor.

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Herein reported are the results of a study of LBL-assembled PANi-ERGO electrode that exhibited an exceptionally high specific capacitance. The supercapacitor performance of an

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electrode composed of 30 PANi-ERGO bilayers (denoted PANi-ERGO30) was assessed using

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cyclic voltammetry (CV) and galvanostatic charge/discharge measurements. The outstanding specific capacitance of the PANi-ERGO30 electrode (1563 F/cm3) can push the supercapacitor

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devices to a high level of specific energy density. The morphologies of the multilayer films composed of PANi-GO30 or PANi-ERGO30 bilayers were investigated using UV/visible, Raman,

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x-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) in an effort to identify the cause of the significant improvement in the electrode performance by the

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electrochemical treatment of PANi-GO30 electrodes.

X

H N

X

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N

O

O

O C OH O C O O

X

n

N

H

O O C

H N

H

H O

O C OH

C

PANi

O

O

GO C

OH

O

O H

H N

X

N

N

H

H

H N

n

PANi

Fig. 1. Schematic view of multilayer film LBL-assembled based on multiple interactions (electrostatic, hydrogen bonding, and π-π) between the positively charged PANi and the negatively charged GO.

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2.

Experimental 2.1. Materials Natural graphite powder (particle size <20 μm), PANi in emeraldine base form (MW 50

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000), dimethyacetamide (DMAc, 99%), and hydroiodic acid (HI, 55%), were purchased from Sigma-Aldrich, and used as received. Besides, concentrated sulfuric acid (H2SO4) was obtained

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from Daejung Chemical & Metal Co. Ltd. (Korea). Silicon wafers (d = 100 mm) and tin-doped

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indium oxide (ITO) (8 Ω cm−1, d = 100 mm) were purchased from MEMC Electronic Materials Inc. (Malaysia) and Libby Owens Ford (USA), respectively. Ultrapure water (Millipore Milli-Q

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Plus, 18.2MΩ cm) was used for all experiments including the preparation of sample solutions,

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films, and cleaning steps.

2.2. The Preparation of PANi-ERGO Electrodes

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Synthetic method of GO and preparation of PANi solution were in detail described in the supporting information (SI), though. Here is a brief description about the

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synthesis of PANi solution. The emeraldine base form of PANi (MW 50 000; 200 mg) was

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dissolved in 10 ml dimethylacetamide overnight with stirring, resulting in a concentration of 20 mg/mL. This solution was 10-fold diluted with H2O (pH 3.0). After that, the pH of the solution was then quickly adjusted to 2.6 using 1 N HCl for optimally doped PANi. Prior to use, the PANi solution was filtered through a membrane filter with a 0.2 μm pore size and diluted with H2O to different concentration for the purpose of experiments. The solution was stable for at least 2 weeks. ITO glass substrate (cleaned by a successive rinsing in acetone, ethanol, and H2O) was then dried under a gentle stream of nitrogen, and immersed into a freshly prepared piranha

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solution at RT for 10 s. For a typical dip-coating-based LBL-assembly [19–21], the freshly cleaned ITO substrates were first immersed into a solution of positively charged PANi (0.5 mM, pH 2.6) for 15 min, and rinsed in H2O (pH 2.6) for 1 min per each of the three washing steps.

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Subsequently, they were dried under a gentle stream of nitrogen gas. Again, the substrates coated with the PANi layer were immersed into a solution of negatively charged GO (0.125 mg/mL, pH

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3.5) for 15 min to allow the deposition of an additional GO layer. The washing and drying steps

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were followed for the substrates coated with the PANi-GO bilayer. The LBL-assembly of the first three PANi-GO bilayers was manually controlled in a successive manner. Then, the

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substrate coated with 3 PANi-GO bilayers was vertically connected to a homemade holder in an automatic dipping robot (HMS 70, MICROM GmbH/Germany) for the additional deposition of

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PANi and GO layers (15 min adsorption time, 3× washing steps). It is noteworthy that the pH of washing or GO solution for the LBL assembly was set to be 2.6 and 3.5, respectively, in order to

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minimize the transformation of emeraldine salt (partially doped state) into the emeraldine base (undoped state). The molarity of the PANi solution was calculated based on a repeat unit of the

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emeraldine base (Mo = 181) containing two aniline structural units; a benzene ring, an amine

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unit, a quinoid ring, and an imine unit. The ITO electrodes fabricated with 30 PANi-GO bilayers was connected to a potentiostat (IviumStat) for scanning the potential from 0 to -1.3 V (vs. Ag/AgCl) in a rate of 50 mV/s, and transformed to a PANi-ERGO30 electrode.

2.3. Methods

UV/visible absorption spectra were recorded on a spectrometer (Perkin-Elmer, Lambda 40) in order to monitor the LBL-assembly of PANi and GO films onto a fused silica substrate and also to investigate the optical properties of the resulted multilayer films composed of PANi-GO

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bilayers. The thickness of the multilayer film deposited on silicon wafers was measured using a real-time spectroscopic ellipsometer (EllipsoTechnology, Elli-SE-F) with a Xe arc lamp(350−820 nm) equipped with a rotating polarizer, a liquid cell with optical access at an incidence

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angle of 60°, an analyzer, and a multichannel detection system. Employing a self-made computer program, the elliptical azimuth and phase angle were calculated for both the cleaned reference

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substrate and the multilayer films. Note that at least 3–5 sampling points were measured to

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obtain the average thickness. Field emission scanning electron microscopy (FE-SEM) was carried out on a JSM-7001F SEM microscope at an accelerating voltage of 20 kV.

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Raman spectroscopy was performed on the multilayer film composed of either PANiGO30, or PANi-ERGO30 bilayers LBL-assembled on ITO substrates using a HRLAB HR-800

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UV apparatus (Horiba Jobin-Yvon) equipped with an excitation laser (λ = 633 nm). XPS analysis was performed on a PHI 5000 VersaProbe II X-ray photoelectron spectrometer. The AFM

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measurements were performed on a ITO substrate in air at RT using a Nanoscope IV multimode microscope (Veeco). Using a 125 μm long Si cantilever, with tip radius of less than 10 nm, and a

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resonance frequency 320 kHz (Nanoworld) with a force constant of 42 N/m, topographic images

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were recorded in tapping mode (1 μm × 1 μm size) at a scan rate of 0.854 Hz.

2.4. Capacitance Measurements CV measurements were performed in an aqueous electrolyte solution (1 M H2SO4) at different scan rate of (mV/s) using IviumStat Technologies system connected to a three-electrode cell, which includes ITO with a PANi-ERGO30 multilayer films as the working electrode. The electrolyte solutions were purged with nitrogen for 20 min before performing the electrochemical experiments. The volumetric

capacitance of the electrodes was

determined using

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charge/discharge method based on 0.125 cm2 active area and the thickness of the multilayer film composed of 30 PANi-ERGO bilayer, which was determined by optical ellipsometry. No binder or conductive additives were used. The specific capacitance of the single electrode was

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calculated from the galvanostatic discharge process, according to the following equation: C = I! t/V! E, where I is the constant current and V is the volume of PANi-ERGO30 electrode, ! t is the

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discharge time, and ! E is the voltage change during the discharge process.

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3. Results and discussion

3.1. UV/visible Absorption Spectra and Thickness Measurement

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The LBL-assembly of GO and PANi layers on solid substrates, such as fused silica, silicon, and ITO was carried out, as described in detail in our previous paper [22] and in SI. Here, the pH

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of the PANi solution was adjusted to 2.6, which provides optimal doping condition for forming green protonated emeraldine, which displays a semiconductor-like conductivity on an order of

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100 S cm–1 [23]. The regular deposition of PANi/GO bilayers was monitored at the characteristic

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absorption peaks (λ = 310 and 880 nm) of the PANi (Fig. 2a). The regular deposition of PANi/GO bilayers on a silicon substrate was confirmed based on thickness measurements using optical ellipsometry, yielding a PANi/GO bilayer thickness of 2.39±0.08 nm (Fig. 2b). The result agreed well with the thickness (2.34 nm) estimated based on the cross-sectional scanning electron microscopy image of a multilayer film composed of 50 PANi/GO bilayers (Fig. 2c). The thicknesses of the PANi and GO monolayers were estimated to be 1.06 and 1.32 nm, respectively. The thickness of each GO monolayer agreed well with the literature value (1.3 nm) [24].

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0.6

15 0.4 0.2

0.3 0.2 0.1 0.0 0 2 4 6 8 10 12 14 16

Number of bilayers

10 5 1

(c)

(b)

120 100 80 60 40 20

0.0

0 300

450

600

750

900

1050

0

10

Wavelength (nm)

20

30

40

50

60

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Number of bilayers

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0.8

140

0.4

Thickness (nm)

(a)

Absorbance (a.u)

Absorbance (a.u.)

1.0

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Fig. 2. (a) UV/visible absorption spectra of the multilayer films composed of different number of PANi/GO bilayers deposited onto a fused silica. Inset: Plot of the absorbance at 310 (■) and 880 nm (□) vs. the number of the bilayers. (b) Plot of the film thickness measured using ellipsometry vs. the number of the bilayers. (c) The cross-sectional view of the multilayer film composed of 50 PANi/GO bilayers on silicon substrate, obtained by scanning electron microscopy.

3.2. The Preparation of PANi-ERGO30 Electrodes

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The electrochemical reduction (e-reduction) of GO in the PANi-GO30 multilayer films selfassembled on ITO electrode was performed in 1 M H2SO4 while scanning the potential from 0 to

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-1.3 V (vs. Ag/AgCl) at a rate of 50 mV/s. The e-reduction was monitored by CV, as shown in Fig. S1 (SI). The e-reduction of GO took place at the electrochemical triple-phase boundary of

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working electrode, electrolyte, and Pt counter electrode, which permitted access to both electrons

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from the electrode and H+ ions from the electrolyte at very low potentials [25]. The e-reduction of GO in the multilayer films was apparent from the electrode color change from green (before reduction) to black (after reduction) (Fig. S1, inset), indicating the restoration of extended π-orbital conjugation on RGO sheets. The e-reduction of the PANi-GO30 multilayer film using the acidic electrolyte supported the preparation of an optimal doping state in the PANi layers, as verified by Raman and XPS analysis, described in the following sections. It should be noted that the e-reduction of GO was confirmed an irreversible process based on the featureless reverse scan (from -1.3 to 0.0 V) detected after the second forward scan (Fig. S1). 10

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3.3. Supercapacitor Performances The PANi-ERGO30 electrode was prepared via the electrochemical treatment of a cell assembled on an ITO charge collector from ultrathin films composed of 30 PANi/GO bilayers.

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The electrochemical performance of the PANi-ERGO30 electrode was investigated in a threeelectrode cell using aqueous 1 M H2SO4 electrolytes. The CVs (Fig. 3a) revealed that the PANi-

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ERGO30 electrode yielded better rectangular-shaped curve over the potential range of -0.2 to 0.8

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V at scan rate of 10, 20 and 30 mV/s compared to the HI-reduced electrode denoted as PANiRGO, which exhibited an excellent capacitive behavior [22]. This indicated that charge

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propagation was enhanced in the PANi-ERGO30 electrode compared to the PANi-RGO, supporting favorable capacitive behavior. The potential utility of the ultrathin film electrodes as

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supercapacitors was assessed using galvanostatic charge/discharge methods. The average volumetric capacitance values, C (F/cm3) of the electrodes were estimated based on the discharge

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process [26]. Representative charge/discharge curves for the PANi-ERGO30 electrode at current density of 10, 20 and 30 A/cm3 (Fig. 3b) showed the near-triangular shape of the

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charge/discharge plots. The PANi-ERGO30 electrode displayed a specific capacitance of 1563

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F/cm3 at a current density of 3.0 A/cm3, which is more than twice the value of the PANi-RGO electrodes (584 F/cm3), which was obtained using a HI reduction method [22]. This is remarkable improvement in the capacitance, compared to supercapacitors based on pure RGO [27], or pure PANi, which also revealed poor stability during the charge/discharge cycling [28]. The discharge curve of the PANi-ERGO30 electrode showed a distinct “IR” drop over the range 0.80 – 0.60 V and a gently sloping descent over the ranges 0.60 – (-0.20) V. The former stage, with relatively short discharge duration, is ascribed to pure EDL capacitance. The latter stage, with a much longer discharge duration, was associated with a combination of EDL and faradaic

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capacitance of the PANi component [17,29]. The “IR” drop of the discharge curve for the PANiERGO30 electrode was much lower than that observed for PANi-RGO electrode [22], indicating much lower internal device resistance [17,30]. A low internal resistance is important for energy

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storing devices; less energy is wasted by producing unwanted heat during the charging/discharging processes. The specific capacitance of the PANi-ERGO30 electrode (1563

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F/cm3) at a current density of 3 A/cm3 sets one of best records for capacitance based on carbon

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material including conducting polymers (see Table 1). Importantly, the e-reduced electrode yielded more than twice the capacitance values of the HI-treated electrode at all current

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densities.[22]

Electrode Materials

Specific Capacitance

Composite RGO-PANi nanofiber film LBL-PANi/multiwall carbon nanotube film LBL-PANi/RGO film

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Table 1. Comparison of literature values in electrochemical capacitance of various hybrid electrodes composed of PANi and RGO. Current Density

References

0.3 A/g

ACS Nano (2010)17

238 F/cm3

0.07 A/cm3

ACS Nano (2011)18

584 F/cm3

3 A/cm3

Langmuir (2012)22

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160 F/cm3

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The electrode rate performance was evaluated based on the volumetric capacitances, which were obtained from the charging/discharging curves over the range 3–100 A/cm3, as shown in Fig. 3c. The maximum specific capacitance of PANi-ERGO30 electrode decreased exponentially upon increasing the current density, and approached to a value of 512 F/cm3 at 100 A/cm3. The durability of the electrode materials is extremely important for the long-term maintenance of supercapacitors. The maximum specific capacitance of the PANi-ERGO30 electrode decreased only by 18% even after 1000 charge/discharge cycles at a current density of 3 A/cm3, indicating a high durability as shown in Fig. 3d. The electrochemical stability of the PANi-ERGO30 12

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electrode was mainly ascribed to the durability of the LBL-deposited multilayer films. These films displayed improved ionic and electronic conductivity, as a result of both the optimized doping state of PANi layers and the lower oxygen content of ERGO sheet frameworks, which

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prevent the films from severely swelling or shrinking during the charging/discharging cycles. The electrochemical properties of the PANi-ERGO30 electrodes relied heavily on the

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morphology and the PANi state, which were significantly affected by the reduction method used

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to convert GO to RGO in the multilayer films. The PANi-ERGO30 exhibited an excellent volumetric capacitance, good cycling stability, and rapid charge/discharge rates, which are

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required for supercapacitors. The outstanding capacitance of the PANi-ERGO30 electrode was attributed to the high electron and ionic conductivities of the multilayer films (Fig. S2 and S3 in

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the SI). The conductivities were assisted by their morphology, structure and composition of the

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sections.

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materials, which were analyzed using Raman spectrometry, XPS, and AFM in the following

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-10 -20

(a)

-30 -0.2

0.0 0.2 0.4 0.6 E (V vs. Ag/AgCl)

3

Capacitance (F/cm )

1800

1200 900 600 300 0

20 40 60 80 100 3 Current density (A/cm )

0.0

(b)

0

50

100 150 Time (s)

200

250

140

(d)

120 100 80 60 40 20 0

0

200

400 600 800 Number of cycle

1000

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0

0.2

-0.2

0.8

(c)

1500

0.4

cr

0

0.6

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10

10 A/cm 3 20 A/cm 3 30 A/cm

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20

E (V vs. Ag/AgCl)

Current (A)

30

3

0.8

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10 mV/s 20 mV/s 30 mV/s

Capacitance retention (%)

40

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Fig. 3. (a) Cyclic voltammograms, (b) Galvanostatic charge/discharge curves of the electrode based on PANi-ERGO30. (c) Volumetric capacitance at various current densities, (d) Cycle stability tests of the electrode based on PANi-ERGO30.

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3.4. Raman Spectrometry of the Multilayer Films

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The electrochemical transformation of the layer components in the multilayer PANi-GO30 films was analyzed using Raman spectroscopy, as shown in Fig. 4. In general, the Raman spectrum of GO includes a prominent G band at 1600 cm-1 corresponding to the first-order scattering of the E2g mode, and a D band at 1343 cm-1, attributed to the reduced in-plane sp2 domain sizes in the pristine graphite, possibly due to extensive oxidation [31]. First, the reduction of GO to ERGO in the multilayer assemblies was clearly identified by the increased D/G intensity ratio (1.32) compared to the ratio of the virgin films (1.08), where the D and G bands appeared at 1332 and 1583 cm-1, respectively. The enhanced ratio of the D/G intensities

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resulted from the formation of more numerous new graphitic domains, along with a decrease in the average size of the sp2 domains upon reduction of the exfoliated GO [22]. A Comparison of the HI-reduction and e-reduction approaches revealed the clear

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introduction of prominent features corresponding to the in-plane aromatic C-H group bending modes (Q, 1156 cm-1), C-N+ (SQR, 1215 cm-1), CH=CH (Q, 1405 cm-1) and C=N (Q, 1455 cm-1)

cr

of the quinoid in PANi. The results indicate a high level of oxidation and an optimal doping state

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in PANi layers. These goals were achieved more effectively through e-reduction of the multilayer films. The conductivity of PANi was directly proportional to the doping state and

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oxidation state of PANi. By contrast, the C-N+ at 1215 cm-1 and C=N at 1455 cm-1 were not more prominent after HI-reduction [22]. This difference could be ascribed to the less doping

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state of HI-reduced PANi, which favored realignment of the PANi backbones and increased conjugation, which were directly affected by the weakened electrostatic interactions among the

PANi-GO30 PANi-ERGO30

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Intensity (a.u.)

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positively charged PANi and the negatively charged GO reduced to a neutral ERGO.

1000

1200

1400 1600 1800 -1 Raman shift (cm )

2000

Fig. 4. Raman spectra of PANi-GO30 and PANi-ERGO30 films.

3.5. XPS Analysis of the Chemical Composition of the Multilayer Films

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The chemical compositions of the multilayer films composed of PANi-GO30 films after the ereduction were analyzed using XPS. These results were compared with those of the virgin state, as shown in Fig. 5a. The composition of the multilayer films were calculated based on the areas

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of the XPS peaks, as listed in Table 2. High-resolution scan analysis of C, O, and N spectra in Fig. 5a led to the determination of the composition of the virgin PANi-GO30 multilayer film:

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72.5% C1s (284 eV), 23.2% O1s (532 eV), 4.4% N1s (399 eV). These values were 81.5%,

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11.4%, and 3.1% after e-reduction, respectively. It should be noted that trace amounts of Sn, In, and S were detected from the PANi-ERGO30 films, indicating the contamination from the ITO

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substrates. A distinctive feature of the composition analysis is the decrease in the oxygen content in the exfoliated GO after e-reduction, that is, a 11.4% oxygen content was present in graphene,

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versus 23.2% in GO. This indicated considerable deoxygenation during the e-reduction process. The e-reduction method can be regarded as a slightly more effective than the HI reduction

electrodes [see Fig. S4].

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method at reducing GO to RGO, based on the film composition of the HI-reduced multilayer

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The XPS spectrum of the PANi-GO30 film shown in Fig. 5b shows the C1s peak, which

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consists of a weakly resolved peak at 287.8 eV due to C=O (carbonyl) groups, and two intense peaks centered at 286.0 and 283.5 eV from C-O (hydroxyl and epoxy) and C=C/C-C [32–37]. The contribution from the O-C=O (carboxyl, ~289.5 eV) group was nearly negligible. After the e-reduction process, the peaks at 287.8 and 289.5 eV are not clearly observed from the XPS spectrum of the PANi-ERGO30 film (Fig. 5c). This observation indicates that sp2 carbon atoms were restored by e-reduction which is better than HI-reduction method (Table S1). Note that the GO compounds tended to contain batch-to-batch variability in the functional group compositions, although the C:O ratios tended to be comparable [37].

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The nitrogen content did not vary, indicating that the e-reduction process did not significantly affect the PANi layers. The N1s core–level spectra shown in Fig. 5d and 5e, reveal three peaks in the deconvolution of the spectra, corresponding to different electronic states: The

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benzenoid amine with a binding energy (BE) centered at 398.9 eV, the quinoid amine with a BE at 397.5 eV, and the nitrogen cationic radical (N+) with a BE at 401.0 eV [11]. The benzenoid

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amine (-NH-) content decreased from 62.3% to 29.14%, in contrast with the nitrogen cationic

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radical (N+), which increased significantly from 30.5% to 43.65%. The strong composition changes were ascribed to the effective electrochemical oxidation of the benzenoid amine to the

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nitrogen cationic radical of PANi, which contribute to improve the conductivity of the films, as evidenced by electron transfer rate constant measurement (Fig. S3). The quinoid amine content

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also increased significantly from 7.1% to 27.20% (see Table 2). These observations indicate that the doping level of PANi in the multilayer films was strongly enhanced during e-reduction of the

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PANi-GO30 films in the H2SO4 electrolyte solution.

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C 1s

S 2p

Counts (s)

PANi-ERGO30

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Sn 3d5 In 3d5

O 1s

N 1s

(a)

PANi-GO30

600

500

400

300

200

cr

700

Binding energy (eV)

(b)

292 290 288 286 284 282 280 Binding energy (eV)

M

pt

(c)

PANi-ERGO30

-NH-

=N-

(e)

N+

Counts (s)

=N-

402 400 398 396 Binding energy (eV)

394 404

402 400 398 396 Binding energy (eV)

394

Ac ce

404

C-O

ed

Counts (s)

-NH-

N+

C/O = 7.1

292 290 288 286 284 282 280 Binding energy (eV)

(d)

PANi-GO30

C-C/C=C

us

Counts (s)

C=O

Counts (s)

C-O

C/O = 3.1

O-C=O

PANi-ERGO30

C-C/C=C

an

PANi-GO30

Fig. 5. XPS spectra of the PANi-GO30 and PANi-ERGO30 films. (a) Survey scan, (b,c) C 1s, (d,e) N 1s spectra.

Table 2. The composition (%) of high resolution C 1s and N 1s peaks in XPS of the LBLassembled PANi-GO30 and PANi-ERGO30 films.

Samples

C-C 283.5

C-O 286

C=O 287.8

O-C=O 289.5

=N– 397.5

–NH– 398.9

N+ 401.0

PANi/GO30

56.29

36.15

3.74

3.80

7.1

62.3

30.5

PANi/ERGO30

64.51

35.48

-

-

27.20

29.17

43.65

18

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3.6. AFM Topographical Images of the Multilayer Films The morphologies of the PANi-GO30 before and after reduction were characterized by AFM. Fig. 6a shows the surface morphologies of the PANi-GO30. Fig. 6b highlight several differences

ip t

in the morphologies of PANi-ERGO30. The sheets appeared to be crimped after reduction treatment due to the reduction of GO (one homogeneous domain resulted from the reduction

cr

process). The domain size was higher as a result of e-reduction comapred to the domain size

us

produced by HI-reduction [22], however, the rms roughness values of the multilayer PANi-GO30 films (9.74 nm) decreased to 4.37 nm for the HI-reduction and 5.22 nm for the electrochemically

an

reduced films. (a)

M 1

8 0 -8 0

0.5

1

Ac ce

pt

0.5

nm

ed

1 µm

1 µm nm

10 0 -20 0

(b)

Fig. 6. Topographical 2D and 3D AFM images of the multilayer PANi-GO30 virgin film (a) PANi-ERGO30 (b) on ITO substrate. The RMS roughness values of the multilayer PANi-GO30 and PANi-ERGO30 films were 9.74, and 5.22 nm, respectively.

4. Conclusions and Perspectives The PANi-ERGO30 electrode yielded a specific capacitance of 1563 F/cm3 at a current density of 3 A/cm3. The specific capacitance of the electrode fabricated using PANi/ERGO 19

Page 19 of 27

ultrathin films is one of the best values yet achieved among carbon-based materials, including conducting polymers and metal oxides [38,39]. This breakthrough electrode is made possible by the realization of well-organized homogeneous composite PANi-GO30 electrode using LBL-

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assembly, and also the proper selection of an electrode reduction method, which minimized the matrix disturbances and established optimized doping and oxidation state in the PANi

cr

component.

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The LBL approach to the preparation of PANi-RGO composites provides a unique method for controlling the deposition of single-layer graphene films. The LBL-approach also provides

an

uniform film compositions with nano-sized pores that are useful for the preparation of electrochemical cells. It should be noted that the PANi-GO complex precipitated immediately

M

after mixing acidic PANi and GO solutions (Fig. S5). The precipitation ultimately gave rise to non-uniform films (Fig. S6). The uniformity of the PANi-RGO composites provided optimal

ed

conditions for forming a charge transfer complex between PANi and ERGO [40]. This structure maximized the synergy between ERGO and PANi and significantly enhanced the specific

pt

capacitance of the resulting device. Supercapacitors based on the e-reduced PANi-ERGO would

Ac ce

play a significant role in a wide range of energy storage applications due to the excellent cell performance.

Acknowledgements

This research was supported by Basic Science Research Pro-gram through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (20110015807), and partially by the Research Grant of Incheon National University in 2011.

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“For Table of Contents Use Only”

Electrochemical Reduction of Ultrathin Graphene Oxide/Polyaniline Films for

cr

Ashis K. Sarker and Jong-Dal Hong*

ip t

Supercapacitors with an Ultrahigh Specific Capacitance

us

Department of Chemistry, University of Incheon, 119 Academy-ro Yeonsu-gu,

-2 -4 -6 -1.5

-1.2

-0.9

-0.6

-0.3

0.3

40 30 20

10 mV/s 20 mV/s 30 mV/s

10 0

Ac ce

pt

E (V vs. Ag/AgCl)

0.0

(B)

M

1st cycle 2nd cycle 3rd cycle 4th cycle 5th cycle

Current (A)

(A)

ed

Current (mA)

0

an

Incheon 406-772, Republic of Korea

Text for the Table of Contents

This article describes a unique process for preparing supercapacitor electrodes fabricated with ultrathin film composed of polyaniline (PANi) and electrochemically reduced graphene oxide (ERGO) bilayers, which yielded a specific capacitance of 1563 F/cm3 (at 3.0 A/cm3). The ultrahigh specific capacitance could be achieved through fine-tuning of film structure using layer-by-layer assembly method as well as electrochemical reduction of PANi-GO electrodes.

26

Page 26 of 27

Highlights Electrochemical reduction effect on electric capacitance of LBL-PANi/GO films. Supercapacitor with PANi/ERGO electrodes yielded capacitance of a record value. Electrochemical reduction maintained optimal doping and oxidation state of PANi.

Ac ce

pt

ed

M

an

us

cr

ip t

LBL-method optimized morphology of multilayer film electrodes for high capacitance.

27

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