MnO2 synthesized via simultaneous-oxidation route

Journal of Alloys and Compounds 532 (2012) 1–9

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Capacitive properties of PANI/MnO2 synthesized via simultaneous-oxidation route Jie Zhang a , Dong Shu a,b,c,d,∗ , Tianren Zhang b , Hongyu Chen a,c,d , Haimin Zhao b , Yongsheng Wang b , Zhenjie Sun a , Shaoqing Tang a , Xueming Fang b , Xiufang Cao a a

School of Chemistry and Environment, South China Normal University, Guangzhou 510006, PR China Tianneng Group, Changxing 313100, Zhejiang Province, PR China c Base of Production, Education & Research on Energy Storage and Power Battery of Guangdong Higher Education Institutes, Guangzhou 510006, PR China d Key Laboratory of Electrochemical Technology on Energy Storage and Power Generation of Guangdong Higher Education Institutes, South China Normal University, Guangzhou 510006, PR China b

a r t i c l e

i n f o

Article history: Received 11 November 2011 Received in revised form 30 March 2012 Accepted 4 April 2012 Available online 10 April 2012 Keywords: Simultaneous-oxidation route PANI/MnO2 composite Inter-molecule contact Supercapacitor Specific capacitance

a b s t r a c t Polyaniline (PANI) and manganese dioxide (MnO2 ) composite (PANI/MnO2 ) was synthesized via a simultaneous-oxidation route. In this route, all reactants were dispersed homogenously in precursor solution and existed as ions and molecules, and involved reactions of ions and molecules generating PANI and MnO2 simultaneously. In this way, PANI molecule and MnO2 molecule contact each other and arrange alternately in the composite. The inter-molecule contact improves the conductivity of the composite. The alternative arrangement of PANI molecules and MnO2 molecules separating each other, and prevents the aggregation of PANI and cluster of MnO2 so as to decrease the particle size of the composite. The morphology, structure, porous and capacitive properties are characterized by scanning electron microscopy, X-ray diffraction spectroscopy, X-ray photoelectron spectroscopy, Branauer–Emmett–Teller test, thermogravimetric analysis, Fourier transform infrared spectroscopy, cyclic voltammetry, charge–discharge test and the electrochemical impedance measurements. The results show that MnO2 is predominant in the PANI/MnO2 composite and the composite exhibits larger specific surface area than pure MnO2 . The maximum specific capacitance of the composite electrode reaches up to 320 F/g by charge–discharge test, 1.56 times higher than that of MnO2 (125 F/g). The specific capacitance retains approximately 84% of the initial value after 10,000 cycles, indicating the good cycle stability. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Electrochemical supercapacitors, also known as supercapacitors or ultracapacitors, are considered as the ideal energy-storage devices. They show greater power density and longer cycle life than batteries, and possess higher energy density compared with conventional capacitors [1]. Recently increasing interest has been focused on the development of electrochemical supercapacitors in communications, transportation, consumer electronics, aviation and so on [2]. Based on charge-storage mechanism, supercapacitors can be categorized as two types [3]: (1) electrical double-layer capacitors (EDLCs), in which energy is stored due to the formation of an electrical double layer at the electrode–electrolyte interface [4]. (2) Pseudocapacitors, which store energy by electrochemical faradic reactions between electrode materials and ions in

∗ Corresponding author at: School of Chemistry and Environment, South China Normal University, Guangzhou 510006, PR China. Tel.: +86 020 39310212. E-mail address: [email protected] (D. Shu). 0925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.04.006

the appropriate conditions. The capacitive performance of supercapacitors strongly depends on the properties of the electrode materials such as their surface area, conductivity, and electrochemical stability [5,6]. Carbon materials with high surface-area have been most widely used in EDLCs. However, their low average specific-capacitance value is a drawback [7]. Materials depending on pseudocapacitance mechanism have higher capacitive nature than carbon materials with electrical double-layer mechanism [8]. Nowadays, great interests are shown by researches on pseudocapacitance materials with excellent electrochemical characteristics such as metal oxides and conducting polymers [9]. Among the metal oxides electrode materials, hydrous RuO2 has been studied extensively. It exhibits high specific capacitance value ranging from 720 to 760 F/g [10]. However, the high cost and toxicity of RuO2 have limited its applications and motivated the research of other transition metal oxides [11]. With the advantages of low cost, natural abundance, environmental safety, and high theoretical specific capacitance (1370 F/g) [12] MnO2 has been applied extensively in supercapacitors [13–16]. However, the poor electrical conductivity of MnO2 limits its

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J. Zhang et al. / Journal of Alloys and Compounds 532 (2012) 1–9

capacitive response [17]. Conducting polymers including polyaniline (PANI), polypyrrole (PPy), and polythiophene (PTh) can offer a cost-effective and flexible alternative to conventional electrode materials due to their fast doping/dedoping abilities [18]. The advantages of easy synthesis and good conductivity make PANI become a unique and promising polymeric material with great potential applications in supercapacitors [19]. But it suffers from the poor electrochemical cyclability [20]. Currently many approaches have been taken to overcome these disadvantages through synthesizing composite material of PANI/MnO2 to combine the good conductivity of PANI and excellent cyclability of MnO2 . [21,22] Such a hybrid material possessing both the advantages of the two moieties have been frequently proposed in past years as the key solution to various challenges in the energy supply, storage and production. Various methods have been reported for the synthesis of PANI/MnO2 composite including traditional template chemical methods, stepped chemical methods [23] and electrochemical routes [24,25]. Many researchers tended to prepare the PANI/MnO2 composite without using any template since the template removal is tedious particularly when hard templates were used. Kim and Park [23] obtained PANI/MnO2 -MWNTs hybrid material by stepped chemical method in which MnO2 -MWNTs was prepared firstly by in situ direct coating method, and then PANI was coated onto the MnO2 -MWNTs to synthesize PANI/MnO2 -MWNTs, in this work the composite was prepared in two separate steps. It was similar to mechanical mixture, and would result in much higher contact resistance between MnO2 and PANI. In comparison with chemical methods discussed above, electrochemical deposition offers a better control of the thickness of the composite film and ease in constructing modified electrodes. Zhang et al. [25] synthesized PANI/MnO2 composite on SnO2 /Ti substrate via potentiodynamic electrodeposition. The composite showed high specific capacitance of 601.48 F/g, which is 1.69 times high as that of PANI electrodes. However, the complicated process and high cost of the electrochemical method limited their mass production. Herein, in this paper, we present a novel and simple simultaneous-oxidation strategy for preparing PANI/MnO2 composites and investigate their electrochemical properties as supercapacitor electrodes. In this strategy, potassium permanganate (KMnO4 ) is used as the oxidizing agent to oxidize aniline and MnCl2 to prepare PANI and MnO2 , and meanwhile KMnO4 is reduced to give product of MnO2 . Compared with those conventional methods, this method can synthesize PANI and MnO2 simultaneously. All the reactants were converted into products totally, avoiding the generation of impurities. An excellent contact in the inter-molecule level between PANI and MnO2 will be formed, and it is beneficial for electron transfer [24]. Furthermore, PANI and MnO2 could be separated from each other. The separation between PANI and MnO2 prevents the aggregation of PANI and cluster of MnO2 . Therefore, the composite maintain nanostructure, providing large specific surface area so as to enlarge the interaction area between the composite and electrolyte. The as-synthesized PANI/MnO2 composite electrode shows a high specific capacitance of 320 F/g and excellent cycle stability. The present synthetic strategy will be a promising fabrication technique for the highly efficient electrode materials of supercapacitors. 2. Experimental 2.1. Materials All chemicals used were of analytical grade. All solutions were prepared from double distilled water. Aniline, MnCl2 ·4H2 O (Aldrich), hydrochloric acid (HCl) (Aldrich) and potassium permanganate (KMnO4 ) (Aldrich) were used as received.

2.2. Preparation of PANI/MnO2 composites Firstly, certain amount of aniline monomer and MnCl2 ·4H2 O were dissolved in 80 mL 0.5 mol/L HCl aqueous solution and sonicated for 15 min. Then aniline was added into the MnCl2 solution and sonicated for 30 min. Finally, KMnO4 dissolved in 80 mL deionized (DI) water was added dropwise into the above mixed solution in ice bath for 30 min under stirring. The reaction was initiated immediately after the addition of KMnO4 . The mixture was stirred for 4 h at constant temperature to generate PANI/MnO2 composite. Then the reaction mixture was suction-filtered. The black precipitate was washed repeatedly with double distilled water and alcohol until the washing solution was colorless. The PANI/MnO2 composite was obtained through the following equations [13]: KMnO4 + aniline → MnO2 ·nH2 O + PANI

2KMnO4 + 3MnCl2 + 2H2 O → 5MnO2 + 2KCl + 4HCl The MnO2 received by the reaction of KMnO4 with MnCl2 ·4H2 O in the same experimental condition was also investigated for comparison experiment. 2.3. Preparation of the working electrode The working electrode was prepared by firstly mixing 75 wt.% active material, 15 wt.% acetylene black and 10 wt.% poly(vinylidene fluoride) (PVDF) in N-methylpyrrolidinone (NMP), and then the slurry was spread onto stainless steel net with 1 cm2 geometry area. The electrodes were dried under vacuum at 60 ◦ C for 24 h to evaporate solvent. 2.4. Characterization of the PANI/MnO2 composite electrode 2.4.1. Physical characterization The morphologies of the composite and MnO2 were observed by scanning electron microscope (SEM) (JSM-6380) with an accelerating voltage of 15 kV. X-ray ˚ radiation diffraction (XRD) analysis was carried out by using CuK␣1 (1.5406 A) on a Y-2000 X-ray generator. The X-ray intensity was measured over a diffraction 2 angle ranging from 5◦ to 80◦ with a velocity of 0.03◦ step−1 and 2◦ min−1 . The measurements of thermogravimetric analysis (TG) were carried out using a STA409PC thermogravimetric analyzer (0–800 ◦ C) at a heating rate of 10 ◦ C min−1 in air atmosphere. The specific surface area and pore size distribution of the composite material and MnO2 were analyzed using the Brunauer–Emmet–Teller (BET, ASAP-2020, Micromeritics, America) method by the adsorption and desorption of N2 . The Fourier transform infrared (FTIR) spectrum (Spectrum One, PerkinElmer instruments, America) was used to determine the functional groups of PANI and PANI/MnO2 . The oxidation states of Mn and N elemental analysis on the surface of the PANI/MnO2 were analyzed using X-ray photoelectron spectroscopy (XPS, ESCALAB 250, Thermo Fisher Scientific Co., America). 2.4.2. Electrochemical characterization All electrochemical experiments, including cyclic voltammetry (CV), charge–discharge (CD) test and electrochemical impedance spectrometry (EIS) were conducted by using Autolab PGSTAT30 (Eco Echemie B.V. company) and CHI 660A (CH Instrument, Inc.) electrochemical working stations in a three-electrode mode (working electrode, a graphite stick as counter electrode, and a saturated calomel electrode (SCE) as reference electrode).

3. Results and discussion 3.1. SEM The morphologies of the PANI/MnO2 composite and MnO2 were investigated by SEM. The results were displayed in Fig. 1. Both MnO2 and PANI/MnO2 composite display compact granular morphology. PANI/MnO2 composite exhibits spherical particles with a radius of about 200 nm, demonstrating a three-dimensional nanostructure. Such a structure allows a shorter ion diffusion path which can improve the rate capability and specific capacitance [15]. Meanwhile it is observed that the granular size of the PANI/MnO2 composite (200 nm) is smaller than that of MnO2 (500 nm). This is because that PANI and MnO2 were synthesized simultaneously and separated from each other. Such a separation between PANI and MnO2 prevents the aggregation of PANI or cluster of MnO2 . Therefore, the composite existed in nanostructure, providing large specific surface area so as to enlarge the interaction area between the composite and electrolyte. Further explanation, as for pure

J. Zhang et al. / Journal of Alloys and Compounds 532 (2012) 1–9

3

Fig. 1. SEM images of the as-prepared (a and c) PANI/MnO2 powder and (b and d) MnO2 .

3.2. TG The thermal stability of the PANI/MnO2 composite was investigated by TG. Fig. 2 depicts the thermograms of MnO2 and PANI/MnO2 composite. According to the TG curve of MnO2 (Fig. 2(a)), 7% weight loss under 170 ◦ C is due to the removal of physically adsorbed water. 10% weight loss from 170 to 250 ◦ C is attributed to the loss of acid dopant anions and water in the interlayer molecule. A small weight loss (3%) between 500 and 800 ◦ C is ascribed to the reduction of manganese from tetravalent to trivalent form accompanied by the evolution of oxygen [26]. In the TG curve of PANI/MnO2 (Fig. 2(b)), 10% weight loss under 180 ◦ C is due to the dissipation of the physically adsorbed water. A further weight loss of 5% observed from 180 to 260 ◦ C is attributed to the loss of acid dopant anions compensating the positive charge of PANI chain. While a small weight loss about 10% between 260 and 800 ◦ C is ascribed to the degradation of the polymer chain. Finally, when the temperature is higher than 800 ◦ C, the percentages of the remaining MnO2 and PANI/MnO2 composite are 82% and 74% respectively. The content difference between the composite and MnO2 is considered as the content of PANI in the composite. If we exclude the small quantity of water and other impurities, the weight ratio of MnO2

and PANI is about 9:1, indicating that MnO2 is predominant in the composite. Besides, the decomposition of MnO2 in the temperature range of 700–800 ◦ C does not appear in the PANI/MnO2 curve, indicating the enhancement of the thermal stability of PANI/MnO2 . It is because good contact and intermolecular force formed between the PANI and MnO2 during the simultaneous-oxidation route. 3.3. XRD The crystal structures of the composite and MnO2 were studied using XRD. Fig. 3 is the XRD patterns of PANI/MnO2 (Fig. 3(b)) and MnO2 (Fig. 3(a)). In Fig. 3(a), the obviously observed peaks at 2 = 37◦ , 42.5◦ , 56◦ and 65.6◦ are corresponding to the (1 3 1),

100

MnO2 Weight/ %

MnO2 prepared in the same experimental condition, direct contact between MnO2 molecules results in the formation of big MnO2 particles. However, in the PANI/MnO2 composite obtained from our work, some MnO2 molecules contact with PANI molecules. The PANI molecules could block the epitaxial growth of MnO2 . In other words, the PANI separated from MnO2 particles. Similarly, the MnO2 could also separate with PANI and prevent PANI from aggregation. So the granular size of the PANI/MnO2 composite is small.

(a)

80

(b) PANI/MnO2 60

40 0

100

200

300

400

500

600

700

o

Temperature/ C Fig. 2. TG patterns of (a) MnO2 and (b) PANI/MnO2 .

800

900

4

J. Zhang et al. / Journal of Alloys and Compounds 532 (2012) 1–9

240

200

220

180

200

160

MnO2

160

(131)

140

140

(160)

(300)

(421)

120 100

(a)

PANI/MnO2

80

(131)

60

Intensity/a.u.

Intensity/a.u.

180

(a)

MnO2

120 100 80

PANI/MnO2

60

40 20

(b)

40

(421)

(b)

20

0

0

0

20

40

(2θ)/

60

1000

80

2000

Wavelength/cm

o

3000 -1

Fig. 3. XRD patterns of (a) MnO2 and (b) PANI/MnO2 .

Fig. 4. FTIR spectrum of (a) MnO2 and (b) PANI/MnO2 .

(3 0 0), (1 6 0) and (4 2 1) planes of ␥-type MnO2 (JCPDS no. 14-0644) respectively, indicating the crystalline state of MnO2 is ␥-MnO2 . The XRD patterns of PANI/MnO2 in Fig. 3(b) are similar to that of MnO2 , indicating the similar crystalline state of ␥-MnO2 . But the broad and weak peak of the composite may be related to the low crystallization degree and the small size of the crystalline grain, verifying a distortion of the crystalline structure of ␥-MnO2 in PANI/MnO2 during the polymerization reaction. [17] As is wellknown that MnO2 can form several kinds of polymorphs such as ␣-, ␤-, ␥- and ␦- types according to the different linkage patterns of basic unit [MnO6 ] octahedral. Among all the structures, ␥-MnO2 which is considered to be an intergrowth of ramsdellite (1 × 2 tunnels) and pyrolusite (1 × 1 tunnels) have wide applications in electrochemical supercapacitors [27], lithium ion battery [28,29] and dry-cell batteries [30]. In previous studies, PANI in its emeraldine salt form shows characteristic peak at 20.8◦ . While in our PANI/MnO2 composite, other two low peaks at 2 = 20◦ , 25◦ are found, due to the periodicity parallel and perpendicular to the polymer chain, respectively [21]. Here in our work, PANI shows only an amorphous halo without any diffraction feature due to the low content and weak crystalline of received PANI. Therefore the XRD peaks of MnO2 are predominant [11].

the characteristics of Mn4+ [31]. In XPS spectra, the full width at half maximum (FWHM) of an element reflects the multiplicity of molecular adsorption states. The N 1s spectrum in Fig. 5 shows a wide FWHM of 3.5 eV (wider than 3 eV), which indicates more than a single type of nitrogen exist (Fig. 5(c)). The spectra of N 1s are reasonably deconvoluted into four Gaussian peaks with the binding energy of 398.4, 399.5, 401.0 and 402.5 eV respectively, which can be ascribed to imine-like structure [ N ], amine-like nitrogen atom [ NH ], [ ph NH ph ], and protonated imine [ N+ ], respectively [22]. The ratio of different elements or statement can be calculated according to the peak area. And the ratio of N and N H components could illustrate the intrinsic doping state of PANI [33–35]. Therefore, the peak areas of the four peaks calculated are 19.7%, 15.8%, 40.2%, and 21.3%, respectively. The N /N H ratio is 41:59, indicating PANI in the composite is half oxidation-half reduction status. In this status, PANI displays the highest electrical conductivity, which is beneficial to improve the electrochemical property of the PANI/MnO2 composites.

3.4. FTIR To obtain more specific information about the PANI/MnO2 , FTIR spectra of the composite and MnO2 were obtained. As seen from Fig. 4, the peaks near 1308, 1490 and 1580 cm−1 are attributed to Ph N, benzenoid structure and quinoid structure, respectively [21]. Bands at 1623 and 1506 cm−1 correspond to the C C stretching of quinoid rings and C C stretching of the benzenoid rings. The bands in the range of 1390–1400 cm−1 are the C N stretching band of the aromatic amine. The band at 3320 cm−1 is attributed to N H stretching. Furthermore, the sharp peak at 560 cm−1 should be ascribed to the contribution of the Mn O vibrations [31]. FTIR spectrum of the PANI/MnO2 composite exhibits both the characteristic band of PANI and MnO2 , which confirms the presence of both components in the composite [32]. 3.5. XPS The surface chemical composition of PANI/MnO2 was investigated by XPS. Mn 2p and N 1s signals can be observed in the survey scan (Fig. 5(a)) of PANI/MnO2 . The Mn 2p region (Fig. 5(b)) consists of a spin-orbit doublet: Mn 2p1/2 with binding energy of 654.2 eV and Mn 2p3/2 with binding energy of 643.7 and 642.3 eV. They are

3.6. BET The specific surface area is an important parameter of nanomaterial, which greatly affects their characteristics such as specific capacitance. The N2 adsorption–desorption isotherm curve and pore size distribution of MnO2 and PANI/MnO2 are shown in Fig. 6. Their isotherms are similar to the shape of type IV [36]. It can be seen that most pores are distributed in a narrow range from 14 to 17 nm, suggesting that both of the MnO2 and PANI/MnO2 exhibit porous structures mainly composed of mesopores (according to IUPAC classification). The measured BET surface area of the PANI/MnO2 is 65.8 m2 /g, which is higher than that of MnO2 (59.4 m2 /g). The specific surface area is closely related to the particle size. Smaller particle size would lead to larger specific surface area [14]. Therefore, the result indicates that PANI/MnO2 composite has smaller particles, which is consistent with the SEM results. 3.7. Electrochemical characterization The super capacitive (SC) behavior of a material can be evaluated from CV and CD. The specific capacitance based formulas of CV or CD cycling can be calculated by the following formulas: SC(CV)(F/g) =

Q (C) E(V)m(g)

(1)

SC(CD)(F/g) =

I(A) × t(s) E(V)m(g)

(2)

J. Zhang et al. / Journal of Alloys and Compounds 532 (2012) 1–9

(a)

(a)

5

0.10

Mn2p3/2 Mn2p1/2

0.08

0.06

Intensity/a.u.

-1

dV/dD/cc g nm

-1

O1s

C1s

N1s

0.04

PANI/MnO2 0.02

MnO2 0.00

1200

1000

800

600

400

200

0

0

-200

20

40

Binding Energy/eV

(b) Volume Absorbed/cm g

Intensity/a.u.

3 -1

(b)

Mn2p 3/2

Mn2p 1/2

Mn2p 3/2

660

655

650

645

640

280 260 240 220 200 180 160 140 120 100 PANI/MnO2 80 60 40 MnO2 20 0 -20 0.0 0.2

635

80

100

120

140

160

0.4

0.6

0.8

1.0

Relative Pressure/(p/p0)

Binding Energy/eV

Fig. 6. (a) Pore size distributions and (b) N2 adsorption–desorption isotherms of MnO2 and PANI/MnO2 .

(c) [-ph-NH-ph-]

Intensity/a.u.

60

Pore size/nm

+ [=N –]

404

402

molar ratio of PANI to MnO2 . Low HCl concentration is beneficial to the deposition of MnO2 , but not beneficial for PANI to obtain good electrical conductivity and electrochemical capacitance. By adjusting the acid concentration, we can control the MnO2 content in the composites. Different concentrations of HCl (0.1, 0.5, 1.0 and 2.0 mol/L) were used to synthesize the composites PM-0.1, PM-0.5 PM-1.0 and PM-2.0, respectively. The CV test was carried out in 1.0 mol/L Na2 SO4 solution. Their specific capacitance calculated from the CV curves in Fig. 7(a) and the content of PANI obtained from TG measurement was presented in Table 1 and Fig. 7(b). It can be found that:

[-NH-]

[=N–]

400

398

396

394

392

Binding Energy/eV Fig. 5. XPS spectrum of (a) survey scan (b) Mn 2p and (c) N 1s for PANI/MnO2 composite.

Here, ‘Q’ is the cathodic charge in coulomb, ‘I’ the discharge current in ampere, ‘t’ the discharge time in second, ‘E’ the potential difference in volt and ‘m’ is the active material mass in gram [35]. 3.7.1. Characterization of PANI/MnO2 materials synthesized with different concentrations of HCl The electrochemical performance of PANI/MnO2 composite is affected greatly by the molar ratio of PANI to MnO2 . The concentration of HCl during the preparation is a key factor to affect the

(i) The concentration of HCl shows great effect on the specific capacitances of PM-0.1, PM-0.5, PM-1.0 and PM-2.0. PM-0.5 exhibits the largest specific capacitance. (ii) The content of MnO2 in the PANI/MnO2 composites increases largely with the decrease of the concentration of HCl. Table 1 The effect of acid concentration on specific capacitance and MnO2 content of composites measured by CV in Na2 SO4 . Sample

Concentration of HCl (mol/L)

PM-2.0 PM-1.0 PM-0.5 PM-0.1

2 1 0.5 0.1

Mass percent of MnO2 in the composite 38% 68% 74% 91%

Specific capacitance of the composite (F/g) 141 172 301 213

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J. Zhang et al. / Journal of Alloys and Compounds 532 (2012) 1–9

(a)

3

0.1M

Current density/mA

2

0.5M

1

1.0M 0

Current/mA

(a)

2.0M

-1

-2 -0.2

0.0

0.2

0.4

0.6

0.8

4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 -2.5 -3.0

1.0

20mV/s 10mV/s 1mV/s

-0.2

0.0

0.2

Potential/V

(b) 350

0.8

1.0

300

Specific capacitance/F g-1

Special capacitance/F g-1

0.6

(b)

300 250 200 150 100 50

250 200 150 100 50

0 0.0

0.4

Potential/V

0.5

1.0

1.5

0

2.0

0

5

10

15

20

-1

Acid concentration/mol L

Therefore, in this work, 0.5 mol/L HCl is selected for the preparation of the composite.

3.7.2. CV and electrochemical stability test Fig. 8(a) shows the CV curves of PANI/MnO2 at different potential scan rates and Fig. 8(b) displays the calculated specific capacitance. The rectangular and mirror images are observed in the CV curve, indicating high electrochemical reversibility. This capacitive nature may be caused by a combination of the double-layer capacitance of the active material and the pseudocapacitance of PANI/MnO2 . It can be observed that the specific capacitance decreases from 301 to 144 F/g with the increase of potential scan rate from 1 to 20 mV/s. It is because the electrolyte ions can only reach the outer surface of the electrode at high scan rates, and the active material that is accessible only through the deep pores does not actively contribute to the pseudocapacitance [10]. Fig. 9 shows the CV curves of the PANI/MnO2 and MnO2 obtained at a scan rate of 1 mV/s in 1.0 mol/L Na2 SO4 solution in the potential range of −0.1 and 0.9 V (vs. SCE). The specific capacitances of MnO2 and PANI/MnO2 composite were calculated to be 125 and 301 F/g, respectively. The larger specific capacitance of PANI/MnO2 composite may be caused by the following reasons: (i) The good contact in the inter-molecule level between PANI and MnO2 formed during the simultaneous-oxidation process.

Scan rate/mVs

Fig. 8. (a) CV curves and (b) specific capacitance of PANI/MnO2 electrode in Na2 SO4 solution at different scan rate.

(ii) Large specific surface area and high electrochemical utilization of the composite material is realized by preventing the composite particles from agglomerating. (iii) The contribution of PANI in the composite may be partially involved in Faradaic pseudocapacitance reaction [17].

3.0 2.5 2.0 1.5

Current/mA

Fig. 7. (a) CV curves and (b) specific capacitance of the composites synthesized with 0.1 M, 0.5 M, 1.0 M and 2.0 M HCl at different scan rates.

-1

1.0 0.5 0.0

MnO2

-0.5 -1.0 -1.5

PANI/MnO2

-2.0 -2.5 -0.2

0.0

0.2

0.4

0.6

0.8

1.0

Potential/V Fig. 9. CV curves of PANI/MnO2 and MnO2 at 1 mV/s in Na2 SO4 solution.

140

1.0

120

0.8

100

0.6

Potential/V

Special capacitence/F g -1

J. Zhang et al. / Journal of Alloys and Compounds 532 (2012) 1–9

80 60 40

0.4 0.2 0.0 -0.2

0 0

2000

4000

6000

8000

10000

Fig. 10. Cyclic performance of PANI/MnO2 electrode in the voltage range of −0.1 to 0.9 V (vs. SCE) in Na2 SO4 solution.

The stability of PANI/MnO2 electrode was investigated by CV scanning for many cycles. Fig. 10 shows the cyclic voltammograms recorded for 10,000 cycles with a scan rate of 20 mV/s in 1.0 mol/L Na2 SO4 solution. The specific capacitance of the PANI/MnO2 electrode material increases with the increase of the cycle number at the first 750 cycles. Afterwards, the capacitance slightly decays and finally retains 84% of the initial value after 10,000 cycles, indicating the excellent cycle stability of the electrode. The excellent cycle stability may be attributed to the cohesive inter-molecular contact between PANI and MnO2 particles which prevents the dissolution of MnO2 . And the capacitance attenuation can be ascribed to two reasons: (i) low dissolution of MnO2 is inevitable. (ii) The redox sites in the PANI are insufficiently stable for many repeated redox processes, which is probably due to the swelling and shrinking of PANI under aqueous environment leading to the degradation of PANI during the long term cycling [22]. In conclusion, both the specific capacitance (320 F/g) and cycle stability (SC retention of 84% over 10,000 continuous cycles) of our result composite displays many advantages comparing to the reported literatures [11,17,21,32,36]. It is particularly worth mentioning that in the existing literature, few PANI/MnO2 composites can have such good cycle stability.

1.0 0.8

Potential/V

0.6 0.4 0.2 0.0 -1

-1

-1

0.70A g

0.35A g

-0.4 0

500

1000

1500

0

500

1000

1500

2000

Time/s

Cycle number

1.30A g

PANI/MnO2

MnO2

20

-0.2

7

2000

Time/s Fig. 11. Charge–discharge curves of MnO2 electrode at different current densities in Na2 SO4 solution.

Fig. 12. Charge–discharge curves of MnO2 and PANI/MnO2 electrode in Na2 SO4 solution.

3.7.3. Galvanostatic CD analysis Fig. 11 shows the galvanostatic CD curves of PANI/MnO2 electrodes at different current densities. Fig. 12 shows the galvanostatic CD curves of the MnO2 and PANI/MnO2 electrodes at a current density of 0.35 A/g. The linear variation of the voltage with time is observed. The responses during anodic charging and cathodic discharging processes are nearly symmetric, demonstrating that both the electrodes display ideal capacitive properties in the potential range. The area under the PANI/MnO2 CD curve is larger than that under the MnO2 CD curve. The specific capacitances of the MnO2 and PANI/MnO2 calculated from the CD curves are 94 and 320 F/g respectively, which are similar to the result of CV. Besides, IR drop is the electrical potential difference between the two ends of a conducting phase during a current flow. It is the product of the current (I) and the resistance (R) of the conductor [21]. In the CD curves IR drop can be observed. The CD curve of PANI/MnO2 composite shows a less IR drop compared to MnO2 . This further confirms the lower contact resistance in the composite.

3.7.4. EIS analysis EIS is a powerful tool for the mechanism analysis of interfacial process and the evaluation of rate constant, ionic and electronic conductivity and double-layer capacitance, etc. [24] The typical Nyquist plot of EIS measurement is shown in Fig. 13. Both PANI/MnO2 and MnO2 exhibit a distorted semicircle in the highfrequency region. It is attributed to the charge-transfer resistance (Rct ) from the interface structure between the electrode surface and electrolyte [37,38]. From the point intersecting with the x-axis in the range of high frequency, a very small internal resistance (Rs ) can be evaluated. The internal resistance includes the ionic resistance of electrolyte, the intrinsic resistance of the active material, and the contact resistance at the interface of active material and current collector. A line almost vertical to the y-axis in low-frequency region is also observed, indicating ideal capacitive behavior [39,40]. Fig. 13(a) and (b) demonstrates the Nyquist plots of MnO2 and PANI/MnO2 at different potential range, and the charge-transfer resistance (Rct ) of PANI and PANI/MnO2 at different potentials is shown in Fig. 13(c). It can be seen that the Rct of MnO2 is much larger than that of PANI/MnO2 . It further proved that the PANI improves the conductivity of the composite, facilitating the charge-transfer of the composite [33].

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concentration plays an important role in the preparation of the composite, and the optimum HCl concentration is 0.5 mol/L. (ii) MnO2 is predominant in PANI/MnO2 composite and its crystalline state is ␥-MnO2 . (iii) PANI/MnO2 composite exhibits larger specific surface area than that of pure MnO2 . (iv) The maximum specific capacitance of the composite electrode reaches up to 320 F/g in CD test, which is 156% higher than that of MnO2 (125 F/g). (v) The specific capacitance of the composite electrode retains approximately 84% of the initial value after 10,000 cycles, indicating good cycle stability of the composite. (vi) The excellent capacitive behavior of the composite is due to the good contact in the inter-molecule level between PANI and MnO2 formed during the preparation, and the high electrochemical utilization of the composite material by preventing the composite particles from agglomerating. The present synthetic strategy can be extended to other conductive polymers and inorganic metal oxides such as polypyrrole, polythiophene and RuO2 . It would also be a promising preparation technique for highly efficient electrode materials of supercapacitors.

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We are grateful for the financial support from the Natural Science Foundation of Guangdong Province, China (grant no. S2011010003416); China Guangdong Province Science and Technology Bureau (grant no. 2010B090400552); China High-Tech Development 863 Program (grant no. 2009AA03Z340) and National Natural Science Foundation of China (grant no. 20877025).

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References

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[1] P. Simon, Y. Gogotsi, Nature Materials 7 (2008) 845–854. [2] H.H. Zhang, Y.Q. Wang, C.W. Liu, H.T. Jiang, Journal of Alloys and Compounds 517 (2012) 1–8. [3] B.E. Conway, Journal of the Electrochemical Society 138 (1991) 1539–1548. [4] A.G. Pandolfo, A.F. Hollenkamp, Journal of Power Sources 157 (2006) 11–27. [5] Y.J. Yang, E.H. Liu, L.M. Li, Z.Z. Huang, H.J. Shen, X.X. Xiang, Journal of Alloys and Compounds 487 (2009) 564–567. [6] Y.Z. Wu, R. Balakrishna, M.V. Reddy, A.S. Naira, B.V.R. Chowdarib, S. Ramakrishna, Journal of Alloys and Compounds 517 (2012) 69–74. [7] Y. Zhang, Y.H. Gui, X.B. Wu, H. Feng, A.Q. Zhang, L.Z. Wang, et al., International Journal of Hydrogen Energy 34 (2009) 2467–2470. [8] Y. Chen, X. Zhang, D.C. Zhang, Y.W. Ma, Journal of Alloys and Compounds 511 (2012) 251–256. [9] W.M. Wu, S.J. Liang, L.J. Shen, Z.X. Ding, H.R. Zheng, W.Y. Su, et al., Journal of Alloys and Compounds 520 (2012) 213–219. [10] J.H. Jang, K. Machida, Y. Kim, K. Naoi, Electrochimica Acta 52 (2006) 1733–1741. [11] F.J. Liu, T.F. Hsua, C.H. Yang, Journal of Power Sources 191 (2009) 678–683. [12] M. Toupin, T. Brousse, D. Bélanger, Chemistry of Materials 16 (2004) 3184–3190. [13] Q. Hao, L.Q. Xu, G.D. Li, Z.C. Ju, C.H. Sun, H.Y. Ma, et al., Journal of Alloys and Compounds 509 (2011) 6217–6221. [14] J.H. Cheng, G. Shao, H.J. Yu, J.J. Xu, Journal of Alloys and Compounds 505 (2010) 163–167. [15] F. Teng, S. Santhanagopalan, Y. Wang, D.D. Meng, Journal of Alloys and Compounds 499 (2010) 259–264. [16] T. Tomko, R. Rajagopalan, M. Lanagan, H. Foley, Journal of Power Sources 196 (2011) 2380–2386. [17] Z.H. Zhou, N.C. Cai, Y.H. Zhou, Materials Chemistry and Physics 94 (2005) 371–375. [18] X.F. Lu, W.J. Zhang, C. Wang, T.C. Wen, Y. Wei, Progress in Polymer Science 36 (2011) 671–712. [19] D.W. Wang, F. Li, M. Liu, G.Q. Lu, H.M. Cheng, Angewandte Chemie International Edition 47 (2008) 373–376. [20] Q. Li, J.H. Liu, J.H. Zou, A. Chunderc, Y.Q. Chen, L. Zhai, Journal of Power Sources 196 (2011) 565–572. [21] C.Z. Yuan, L.H. Su, B. Gao, X.G. Zhang, Electrochimica Acta 53 (2008) 7039–7047. [22] L.J. Sun, X.X. Liu, K.K-T. Lau, L. Chen, W.M. Gu, Electrochimica Acta 53 (2008) 3036–3042. [23] K.S. Kim, S.J. Park, Journal of Solid State Electrochemistry (2012), http://dx.doi.org/10.1007/s10008-012-1694-7. [24] D. Shu, J.H. Zhang, C. He, Y.Z. Meng, H.Y. Chen, Y.S. Zhang, et al., Journal of Applied Electrochemistry 36 (2006) 1427–1431. [25] Y.K Zhang, J.L. Li, F. Gao, X.D. Wang, Advances in Materials Research 239–242 (2011) 1372–1375. [26] F.K. Cheng, C. He, D. Shu, H.Y. Chen, J. Zhang, S.Q. Tang, et al., Materials Chemistry and Physics 131 (2011) 268–273.

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Potential/V Fig. 13. Nyquist plots of (a) MnO2 (b) PANI/MnO2 electrode and (c) charge-transfer resistance (Rct ) of PANI and PANI/MnO2 at different potentials in Na2 SO4 solution.

4. Conclusions PANI/MnO2 composite was prepared by a novel and simple simultaneous-oxidation route. In this method, aniline and Mn2+ are oxidized by KMnO4 simultaneously to generate PANI and MnO2 , and a good contact in the inter-molecule level between them is obtained. The experimental results demonstrate: (i) HCl

J. Zhang et al. / Journal of Alloys and Compounds 532 (2012) 1–9 [27] D.L. Fang, B.C. Wu, A.Q. Mao, Y. Yong, C.H. Zheng, Journal of Alloys and Compounds 507 (2010) 526–530. [28] M. Minakshi, M. Blackford, M. Ionescu, Journal of Alloys and Compounds 509 (2011) 5974–5980. [29] Q. Hao, L.Q. Xua, G.D. Li, Z.C. Ju, C.H. Sun, H.Y. Ma, et al., Journal of Alloys and Compounds 509 (2011) 6217–6221. [30] J.F. Li, B.J. Xi, Y.C. Zhu, Q.W. Li, Y. Yan, Y.T. Qian, Journal of Alloys and Compounds 509 (2011) 9542–9548. [31] A.B. Yuan, Q.L. Zhang, Electrochemistry Communications 8 (2006) 1173–1178. [32] X. Zhang, L.Y. Ji, S.C. Zhang, W.S. Yang, Journal of Power Sources 173 (2007) 1017–1023. [33] J. Zhang, L.B. Kong, B. Wang, Y.C. Luo, L. Kang, Synthetic Metals 159 (2009) 260–266.

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[34] J. Li, L. Cui, X.G. Zhang, Applied Surface Science 256 (2010) 4339–4434. [35] X.P. Zhou, H.Y. Chen, D. Shu, C. He, J.M. Nan, Journal of Physics and Chemistry of Solids 70 (2009) 495–500. [36] L. Chen, L.J. Sun, F. Luan, Y. Liang, Y. Li, X.X Liu, Journal of Power Sources 195 (2010) 3742–3747. [37] Y.L. Xu, J. Wang, W. Sun, S.H. Wang, Journal of Power Sources 159 (2006) 370–373. [38] W.B. Zhong, J.Y. Deng, Y.S. Yang, W.T. Yang, Macromolecular Rapid Communications 26 (2005) 395–400. [39] H.G. Pan, R. Li, Y.F. Liu, M.X. Gao, H. Miao, Y.Q. Lei, et al., Journal of Alloys and Compounds 463 (2008) 189–195. [40] K.K. Liu, Z.L. Hu, R. Xue, J.R. Zhang, Z.J. Zhu, Journal of Power Sources 179 (2008) 858–862.