TiO2 nanowires for supercapacitors

Electrochimica Acta 88 (2013) 526–529

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Preparation and electrochemical properties of the ternary nanocomposite of polyaniline/activated carbon/TiO2 nanowires for supercapacitors Qiangqiang Tan ∗ , Yuxing Xu, Jun Yang, Linlin Qiu, Yun Chen, Xiaoxiao Chen State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China

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Article history: Received 9 October 2012 Received in revised form 26 October 2012 Accepted 26 October 2012 Available online 2 November 2012 Keywords: Nanocomposite Polyaniline TiO2 (B) nanowires Polymerization Supercapacitor

a b s t r a c t We herein report the synthesis of ternary nanocomposites consisting of polyaniline (PANI), activated carbon, and TiO2 (B) components, which involves the preparation of activated carbon/TiO2 (B) nanowires (ACTB) using sonochemical–hydrothermal method, and their subsequent composites with PANI via in situ polymerization. The morphology and structure of ACTB/PANI ternary nanocomposites are characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier transform infrared spectra (FTIR) and X-ray diffraction (XRD). Morphology analysis shows that the porous network layer of PANI homogeneously coated on the outer surface of ACTB support. The electrochemical properties of the ternary nanocomposite as the electrode material for electrochemical capacitors are examined by cyclic voltammetry and galvanostatic charge/discharge test in an organic electrolyte (1.0 M LiClO4 in propylene carbonate). The results show that the ternary nanocomposites have a specific capacitance as large as 286 F g−1 in the potential range from −3 to 3 V (vs. SCE) at a charge–discharge current density of 1.0 A g−1 , which is a significant improvement compared to those of the three separate components, demonstrating that the ACTB/PANI nanocomposites are promising materials for supercapacitor electrode. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction Supercapacitors have attracted considerable attention over the past decades because they can combine the advantages of the high power density of conventional capacitors with the high energy density of rechargeable batteries [1–3]. The performances of supercapacitors are primarily determined by the electrode materials [4]. The active materials of electric double-layer capacitors (EDLCs) and pseudo-capacitors including conducting polymers, carbon materials and transition metal oxides have their own advantages and disadvantages. Carbon materials have high rate capability and good electrical conductivity, long life-cycles, but low specific capacitance [5–9]. On the other hand, conducting polymers are cost-effective, flexible, but poor in cyclability [10,11]. Among the conducting polymers, polyaniline (PANI) is considered as one of the most promising candidates for the application of supercapacitor electrode due to its ease of synthesis, electrochemical reversibility and high capacitance [12–15]. The composite materials of PANI and carbon materials have attracted dramatic attention as they can

∗ Corresponding author. Tel.: +86 10 82545008; fax: +86 10 82545008. E-mail address: [email protected] (Q. Tan). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.10.126

integrate the advantages of their separate components and show a higher capacitance and better rate capability [16,17]. Nanostructured electrode materials have garnered sustained research interest due to their more favourable rates and capacitances than those of traditional materials. The inorganic 1D nanomaterials have been studied widely as the electrode materials for energy-storage devices owing to their size/dimensional dependent properties. Among those inorganic nanomaterials, TiO2 nanowires are seemed to be extremely attractive for hybridsupercapacitor applications because of their excellent physical and chemical properties [18,19]. In addition, the modification of activated carbon by TiO2 nanowires could decrease the ion concentration in the double-layer of activated carbon since the charges on the surface of TiO2 are higher than those of the other regions, resulting in the reduction of the polarization of activated carbon [20]. Herein, we reported a facile process for the design and fabrication of the activated carbon/TiO2 (B) nanowires (ACTB), which was used subsequently as the supporting materials for the preparation of ternary nanocomposites of ACTB/PANI using an in situ polymerization method. The introduction of TiO2 (B) nanowires could greatly improve the long-term cycle stability of ACTB/PANI composite. The morphologies, microstructures and electrochemical performances of the resulting products were investigated and

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the effect of TiO2 (B) nanowires on the electrochemical performance was also studied.

The ACTB nanowires were prepared by sonochemical–hydrothermal method. Firstly, the granular titanium dioxide powders were mixed with activated carbon with an average surface area of 1690 m2 /g in mass ratio of 8:100 in 70 mL of 10 M aqueous NaOH solution, followed by vigorous ultrasonication for 2 h. Then, the hydrothermal reaction was carried out in a sealed teflon-lined autoclave at 160 ◦ C for 24 h. After being cooled down to room temperature, the suspension was neutralized with 0.2 M nitric acid to neutral or weak acidic state and then filtered to collect the products. The products thus obtained were washed with deionized water and anhydrous ethanol several times, respectively, and then dried at 80 ◦ C under vacuum. The products obtained above were heated in air at 400 ◦ C for another 2 h to give rise to ACTB nanowires. Subsequently, 4.2 g of ACTB and 4.5 mL of aniline were added into 100 mL of distilled water, and the mixture was then stirred for sufficient dispersion. After adequately mixing and stirring the solution, 50 mL of 0.25 M aqueous ammonium peroxydisulfate solution (APS) was added dropwise as an oxidant to polymerize the aniline monomer. The polymerization process was carried out at 0–5 ◦ C for 16 h under stirring. Finally, the obtained powders were collected by filtration and washed with deionized water and anhydrous ethanol several times and then dried in a vacuum oven at 60 ◦ C for 24 h. The morphologies and structures of the resulting nanocomposites were characterized by X-ray diffraction (XRD, D/Max-RB, Cu K␣), transmission electron microscopy (TEM, JEOL JEM-2100CX) and scanning electron microscopy (SEM, JEOL JSM-6301F). Fourier transformation infrared (FTIR) spectra of the samples were recorded on a Bruker EQUINOX55 spectrometer in the transmission mode. 2.2. Fabrication of the working electrodes The fabrication of the working electrodes was carried out as follows: Briefly, the electroactive materials, carbon black and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 85:10:5 and dispersed in N-methylpyrrolidone (NMP). The resulting composite was stirred mechanically for 6 h to form a sticky slurry. The resulting slurry was pasted onto a clean Al foil, followed by drying at 80 ◦ C under vacuum for 24 h. The constant area of the electrodes used in this study was 1 cm2 (1 cm × 1 cm) and the electrolyte was organic electrolyte (1.0 M LiClO4 in propylene carbonate). The capacitance properties were studied by cyclic voltammetry (CV) and galvanostatic charge/discharge tests. Cyclic voltammetry measurement was performed between −3 and 3.0 V using CHI-604B model electrochemical working station (Shanghai, China). The galvanostatic charge/discharge performance of the electrodes was evaluated using a Land-2001A battery workstation system (Wuhan Jinnuo Company) within the potential range of −3 to 3.0 V. 3. Results and discussion The XRD patterns of TiO2 (B) nanowires, ACTB and ACTB/PANI nanocomposites are shown in Fig. 1. The phase structure of the product in Fig. 1a could be identified as TiO2 (B) (JCPDS 74-1940) since all the diffraction peaks are consistent with TiO2 (B) reported in the literature [19,21]. In Fig. 1c, the diffraction peaks of the sample could be assigned to ACTB and PANI, confirming the formation of

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2 θ / degree Fig. 1. XRD patterns of TiO2 (B) (a), ACTB (b), and ACTB/PANI (c), respectively.

ACTB/PANI in the ternary nanocomposite. The crystalline peaks at 2Â = 20.5◦ and 25◦ respectively correspond to the (0 2 0) and (2 0 0) planes of PANI in its emeraldine salt form [22]. In addition, the diffraction peaks of TiO2 (B) are observed both in Fig. 1b and c, indicating the presence of TiO2 (B) in ACTB nanowires and ACTB/PANI nanocomposites. Fig. 2a and b shows the FTIR spectra of PANI and ACTB/PANI nanocomposites, respectively. The characteristic peaks of PANI (Fig. 2a) were assigned as follows: the characteristic peaks at 1562 and 1486 cm−1 can be ascribed to C N and C C stretching mode for the quinoid and benzenoid rings, which indicate the oxidized state of the emaraline salt of PANI [23]. The peaks at 1300 and 1245 cm−1 are corresponding to C N stretching mode for the benzenoid ring. While the peak at 1131 cm−1 , which is the characteristic of PANI conductivity and the degree of delocalization of electrons, was attributed to the inplane bending vibration of C H [24,25]. Fig. 2b indicates that the main characteristic peaks of PANI are visible in FTIR spectra of ACTB/PANI composite. The presence of ACTB particles leads to the shift of some peaks and changes of relative intensity in Fig. 2a. In particular, the peak at 1562 cm−1 shifted to higher wavenumber of 1574 cm−1 , could be assigned to the stretching mode of C N. In addition, the peak associating with the doping of PANI also shifts from 1131 to 1139 cm−1 . These obvious changes suggested the existence of an interaction between ACTB nanowires and PANI since the transition metal titanium has a tendency to form coordination compound with the nitrogen atom in PANI [26]. Fig. 3 shows the TEM and SEM images of as-prepared samples. As seen in Fig. 3b, TiO2 (B) nanowires were well distributed

Intensity / a.u.

2.1. Preparation of ACTB nanowires

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Fig. 2. FTIR spectra of PANI (a) and ACTB/PANI nanocomposite (b), respectively.

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Fig. 3. TEM image of TiO2 (B) nanowire (a); SEM images of ACTB (b) and ACTB/PANI nanocomposites (c and d), respectively.

among the activated carbon particles. The length and the diameter of TiO2 (B) nanowires were approximate 1–3 ␮m and 100–200 nm, respectively, which were confirmed by TEM observation in Fig. 3a. In Fig. 3c and d, the morphology of the ternary nanocomposite seemed to be a nanometer-scale, porous network layer homogeneously coating on the outer surface of the ACTB due to the high chemical activity and surface area of ACTB. The activated carbon was used as the main supporting material for the deposition of PANI particles and TiO2 (B) nanowires provided enhanced wires interconnected between activated carbon and PANI particles, which would be beneficial to further improve the mechanical strength of the composite. The CVs (within the potential window from −3 to 3 V vs. a saturated calomel electrode (SCE) at scan rate of 20 mV s−1 ) of ACTB and ACTB/PANI composites are shown in Fig. 4. Fig. 4a shows that the CV of ACTB nanowires is similar to that of activated carbon, which produces a curve close to the ideal rectangular shape. The shape

of CV curves in Fig. 4b indicates that the capacitance characteristic of ACTB/PANI composites is distinct from the electric double-layer capacitance of ACTB. The two pairs of redox and reduction peaks observed in Fig. 4b, which would be caused by the chemical state of PANI, resulted in the redox capacitance and indicated the pseudocapacitance besides the electric double layer capacitance. The variation of specific capacitance with cycle number for activated carbon/PANI(AC/PANI) and ACTB/PANI capacitors is given in Fig. 5. The result indicates that ACTB/PANI composite electrode (Fig. 5a) has the specific capacitance at approximate 286 F g−1 at the first cycle, which could be remained at approximate 230 F g−1 after 2000 cycles (above 80% of the original value). As for the ACTB electrode (Fig. 5b), the initial specific capacitance is 292 F g−1 and the capacitance declined dramatically to 124 F g−1 after 2000 cycles. This result indicated that the ACTB/PANI nanocomposite electrode possesses long-term cycle stability over 2000 charge–discharge 300

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Voltage / V Fig. 4. Cyclic voltammorgrams within the potential window −3 to 3 V vs. SCE for ACTB (a) and ACTB/PANI (b), respectively, scan rate: 20 mV s−1 .

Fig. 5. Charge/discharge cycle at a current density of 1 A g−1 within the potential window −3 to 3 V vs. SCE for AC/PANI (a) and ACTB/PANI nanocomposites (b), respectively.

Q. Tan et al. / Electrochimica Acta 88 (2013) 526–529

tests. This is mainly because of the presence of TiO2 (B) nanowires with good mechanical intensity, which are very advantageous to reduce the electrochemical degradation of AC/PANI composite and improve its cycle stability as electrode material [27]. Furthermore, the network structure layer of PANI with nano-sized pores creates electrochemical accessibility for electrolyte ions during the charging or discharging process, which is a significant effect in enhancement of capacitance. In addition, the activated carbon with high surface area has been widely used as a support for supercapacitor and battery electrode materials [28–33]. 4. Conclusion In this paper, the ternary ACTB/PANI nanocomposites were synthesized via in situ polymerization. The initial specific capacitance of ACTB/PANI composite electrode was 286 F g−1 , which could be remained at approximate 230 F g−1 after 2000 cycles (above 80% of the original value compared to 42% for AC/PANI electrode), demonstrating that ACTB/PANI electrode had excellent cycle stability. The ternary ACTB/PANI nanocomposites, which integrated the mechanical strength of TiO2 (B) nanowires, highly porous network structure of PANI, and high surface area of activated carbon, displayed remarkably improved electrochemical performances. Therefore, we believe that the ACTB/PANI composites can integrate the advantages of its three separate components and will be a potential low-cost candidate for supercapacitor. Acknowledgment This work supported by the Knowledge Innovation Program of the Chinese Academy of Sciences (Grant No. KGCX2-YW-341) References [1] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nature Materials 7 (2008) 845. [2] B.E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Plenum Publishers, New York, 1999. [3] J.R. Miller, P. Simon, Electrochemical capacitors for energy management, Science 321 (2008) 651. [4] J.P. Zheng, P.J. Cygan, T.R. Jow, Hydrous ruthenium oxide as an electrode material for electrochemical capacitors, Journal of the Electrochemical Society 142 (1995) 2699. [5] D.H. Jurcakova, X. Li, Z.H. Zhu, R.D. Marco, G.Q. Lu, Graphitic carbon nanofibers synthesized by the chemical vapor deposition (CVD) method and their lectrochemical performances in supercapacitors, Energy & Fuels 22 (2008) 4139. [6] K. Hung, C. Masarapu, T. Ko, B.Q. Wei, Wide-temperature range operation supercapacitors from nanostructured activated carbon fabric, Journal of Power Sources 193 (2009) 944. [7] C. Portet, J. Chmiola, Y. Gogotsi, S. Park, K. Lian, Electrochemical characterizations of carbon nanomaterials by the cavity microelectrode technique, Electrochimica Acta 53 (2008) 7675. [8] C.M. Yang, Y.J. Kim, M. Endo, H. Kanoh, M. Yudasaka, S. Iijima, K. Kaneko, Nanowindow-regulated specific capacitance of supercapacitor electrodes of single-wall carbon nanohorns, Journal of the American Chemical Society 129 (2007) 20.

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