Abstract
Polyaniline/carbon doped TiO2 composite nanotube arrays (PANI/C-TiO2 NTAs) have been prepared successfully by electrodepositing PANI in C-TiO2 NTAs which were prepared by directly annealing the as-anodized TiO2 NTAs under Ar atmosphere. The organic residual in the TiO2 NTAs during the process of anodization acts as carbon source and is carbonized in Ar atmosphere to manufacture the C-TiO2 NTAs. The specific capacitance of the PANI/C-TiO2 electrode is 120.8 mF cm−2 at a current density of 0.1 mA cm−2 and remains 104.3 mF cm−2 at a current density of 2 mA cm−2 with the calculated rate performance of 86.3%. After 5000 times of charge-discharge cycling at a current density of 0.2 mA cm−2, the specific capacitance retains 88.7% compared to the first cycle. All these outstanding performances of the as-prepared PANI/C-TiO2 NTAs indicate it will be a promising electrode for supercapacitor.
1. Introduction
Energy shortage crisis and environmental pollution are global problems which promote the scientific research to develop clean energy and corresponding energy storage system [1, 2]. Electrochemical capacitor, also called ultracapacitor, is considered as one of the most promising energy storage devices, which is widely used to satisfy the energy demand in mobile equipments, hybrid electric vehicles, and aerospace fields because of its high power density, energy density, and excellent cycle performance [3–5]. Recently, the faradic pseudocapacitors-type supercapacitors (SCs) have attracted much attention derived from the exhibited higher specific capacitance stemming from the reversible redox reactions compared with the electric double-layer capacitors. Various pseudocapacitive materials, such as conducting polymers and transition-metal oxides/hydroxides, have been studied extensively. Among them, polyaniline (PANI) is a promising capacitive material because of its large theoretical faradic pseudocapacitance (as high as 1145 F g−1), high conductivity, low cost, facile synthesis, and fast doping/dedoping rate [6–9]. However, the relatively poor cycle stability of PANI due to the swelling and consequent breakage of the structures during the charge-discharge process hinders its wide application [10]. Thus, maintaining the structure stability in cycling is crucial for high performance PANI-based supercapacitor [11]. PANI supported by other mechanically stable materials such as carbon nanotube, carbon nanofiber [12], graphene [13], Si [14] exhibited improved cycle stability. However, most of these supporting materials are of powder which need further mixing with conductive agent and binder before the supercapacitor assembling process, which will decrease the specific capacitance in some sense.
TiO2 nanotube arrays (NTAs) fabricated directly and adhered strongly to the underlying Ti substrate by simple anodization is a promising supporting material for PANI loading, which provides large active surface areas and robust scaffold. Its hydrophilic surface and controllable dimension (including diameter and tube spacing) in favor of the uniform deposition and efficient utilization of conducting polymer [15, 16]. However, TiO2 with poor conductivity hinders electrodepositing PANI and goes against the rate ability of supercapacitor. In very recently, we improved the conductivity of TiO2 by directly nitriding TiO2 NTAs to form well conductive TiN NTAs acting as support materials for loading PANI and MoOx, which exhibits high specific capacitance and excellent cycle stability [17, 18].
In this work, we demonstrated another conductive support material, carbon doped TiO2 nanotube arrays (C-TiO2 NTAs), to maintain the PANI structure. PANI can be easily deposited on the surface to form uniform PANI and C-TiO2 nanotube arrays (PANI/C-TiO2 NTAs) by the electrochemical deposition. Because of directional channel for electrons transport and the suitable tube space for the electrolyte permeating, the PANI/C-TiO2 NTAs electrode exhibits high specific capacitance of 120.8 mF cm−2 at the current density of 0.1 mA cm−2 and it also remains 86.3% when the current density increases to 2 mA cm−2. It expresses wonderful cycling performance with the specific capacitance retention of 88.7% even after 5000 cycles. The results demonstrate that the synthesized PANI/C-TiO2 NTAs nanocomposite has great potential applications for SCs.
2. Experimental Details
All chemicals used are of analytical grade. Electrochemical anodization was adopted for TiO2 preparation with a typical two-electrode configuration of electrochemical anodization where a piece of Ti foil (1·1·0.5 cm3) was used as anode and a piece of graphite plate was served as cathode with 1 cm separation, powered by a direct current power supply (IT6834, ITECH, China). The electrolyte was NH4F (0.5 wt.%), CH3OH (5 vol.%), and deionized water (5 vol.%) in ethylene glycol. The applied potential was 60 V and lasted for 30 min.
To produce the C-TiO2 NTAs, we developed an easy annealing processing by utilizing the residual organic agency after anodization in ethylene glycol based electrolyte. In detail, the as-anodized TiO2 NTAs was directly calcined in a tube furnace at 500°C for 3 h in Ar atmosphere with the heating rate of 15°C/min [19, 20].
Controlled electropolymerization was used in a three-electrode system with a Pt foil (2·2 cm2) as the counter electrode, saturated calomel electrode (SCE) as the reference electrode, TiO2 NTAs and C-TiO2 NTAs as the working electrode to fabricate the composite NTAs. In general, the C-TiO2 NTAs was used as working electrode which was subjected to repeating potential scanning in 0.5 M H2SO4 solution containing 0.05 M aniline in the range from −0.6 to 1.0 V at the scan rate of 50 mV/s for 15 and 30 cycles, respectively.
The samples were characterized by SEM (FE-SEM, FEI Nova 400 Nano), X-ray diffraction (XRD, Philips X′ Pert Pro) with Cu Kα radiation of wavelength of 1.5416 Å in the range of 20–80° (), FTIR Spectrometer (Perkin Elmer 1600), and X-ray photoelectron spectroscopy (ESCALB MK-II, VG Instruments, U.K.).
Electrochemical measurements were carried out using a CHI 660C instrument. The three-electrode cell consisted of a SCE as the reference electrode, a 2·2 cm2 platinum plate as the counter electrode, and PANI/C-TiO2 sample (1·1 cm2) as the working electrode. 1 M H2SO4 solution served as the electrolyte at room temperature. The cyclic voltammetry (CV) was performed between 0 and 1 V (versus SCE) at different scanning rate. Galvanostatic charge-discharge curves were measured at different current density over the voltage range of 0–0.6 V (versus SCE). Cycling stability performance was conducted with a charge-discharge tester (Land CT2001A, Wuhan LAND Electronics Co., Ltd., China).
3. Results and Discussion
Figures 1(a) and 1(b) show the SEM images of TiO2 NTAs and C-TiO2 NTAs obtained from annealing as-anodized TiO2 NTAs under air and Ar, respectively. It is obvious that the morphologies of TiO2 NTAs and C-TiO2 NTAs are of no significant difference. The as-obtained highly ordered and well-separated C-TiO2 NTAs have a length of about 9 μm, interior diameter of about 110 nm, and tube wall thickness of about 20–25 nm. As the XRD pattern of TiO2 NTAs and C-TiO2 NTAs shown in Figure 1(c), the diffraction peaks can be attributed to anatase TiO2 and titanium substrate; and no distinguished difference is observed between the TiO2 NTAs and C-TiO2 NTAs (2). Figure 1(d) displays the XPS full spectra for TiO2 NTAs and C-TiO2 NTAs (2). C, Ti, and O peaks are observed for C-TiO2 NTAs, while only Ti and O peaks are observed for TiO2 NTAs. The top right corner inset shows the high-resolution C 1s spectra. The peak at 281.9 and weak peak at 284.6 eV are attributed to Ti-C and C-C [21]. The XPS results indicate that carbon was doped into the TiO2 NTAs through the residual ethylene glycol reacting with the as-anodized amorphous TiO2 during annealing in Ar. While during heating in air, the residual ethylene glycol can react with O2 forming CO2 and H2O resulting in formatting pure TiO2 NTAs. Figures 1(e) and 1(f) show the electrochemical impedance spectroscopy of TiO2 NTAs and C-TiO2 NTAs, respectively, to identify their conductivity. Compared to the resistance of the TiO2 NTAs (20 kΩ), the resistance of C-TiO2 NTAs decreased by 3 orders of magnitude to 18 Ω. It indicates that the C-TiO2 NTAs has an excellent conductivity, which will be of benefit for aniline monomer electropolymerizing on tube wall.
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Figures 2(a) and 2(b) show the SEM images of the PANI deposited on TiO2 NTAs and C-TiO2 NTAs by electropolymerization in 0.05 M aniline sulfuric acid solution for 15 cycles, named PANI/TiO2-15 and PANI/C-TiO2-15, respectively. The typical PANI/C-TiO2-15 nanotube has a smaller interior diameter of 80 nm and its tube wall is 35–40 nm which is thicker than that of the C-TiO2 NTAs due to the PANI depositing. However, for the PANI/TiO2-15, the diameter and tube wall are the same as the origin TiO2 nanotube. The inset is corresponding electrochemical CV curves of the PANI deposition process of them. As for C-TiO2 NTAs, obvious redox peaks were observed and the peak current increases with the cycle number increasing which indicates that polymerization reaction has occurred. However, redox peak was not appeared at the CV curves collected for TiO2 NTAs during whole deposition. These results suggest that C-TiO2 NTAs are easier for electrochemical deposition for PANI due to the enhanced conductivity compared to TiO2 NTAs. The SEM image of PANI/C-TiO2-30 is shown in Figure 2(c) indicating that the C-TiO2 nanotubes are packed by the deposited PANI after polymerizing for 30 cycles. It would block the electrolyte from contacting the inner PANI resulting in the fact that it will not be possible to make full use of the as-polymerized PANI. On the other hand, the accumulated PANI because of lacking firmly support may be easy to topple down which is antagonistic to the cycle stability and rate capability.
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The FTIR spectrum of the C-TiO2 NTAs before and after depositing PANI is shown in Figure 3. For C-TiO2 in curve , the peak at 1350 cm−1 is attributed to the formation of carbonate or carboxylate species and the peak at 1600 cm−1 corresponding to the stretching vibration modes of water or hydroxyl groups absorbed on C-TiO2 [22]. Besides, the two peaks at 1349 cm−1 and 1599 cm−1 are attributed to the formation of carbonate or carboxylate species and hydroxyl groups, respectively; other peaks are found for PANI/C-TiO2 (curve ). The peaks at 1561 and 1483 cm−1 belonged to the C=C stretching of the quinoid ring and benzenoid ring, respectively. Those at 1263 and 1168 cm−1 are attributed to C-N stretching of the secondary aromatic amine. The peak at 813 cm−1 is ascribed to the out-of-plane bending of C-H on the 1,4-disubstituted ring. The peak at 1220 cm−1 can be attributed to various stretching and bending vibrations associated with the C-C bond [23]. The peak at 1168 cm−1 is referred to as the “electronic-like band,” and its intensity is considered as a measure of the degree of delocalization of electrons on polyaniline [24]. From the FTIR spectra, it can be further concluded that PANI was deposited onto the C-TiO2 NTAs nanocomposite electrode after the electrochemical deposition process.
CV tests were performed to study the electrochemical properties of the PANI/C-TiO2 NTAs electrode in 1 M H2SO4 solution. As shown in Figure 4, CV curve of PANI/C-TiO2-15 NTAs displays a larger area compared with that of the original C-TiO2 NTAs at the scan rate of 50 mV s−1. It exhibits four pairs of redox peaks of PANI, which implies a high pseudocapacitance. The first pair of redox peaks A/A′ (at 0.23/0.15 V) can be ascribed to the transformation between emeraldine and pernigraniline states [15]; the peaks B/B′ (at 0.34/0.25 V) and C/C′ (at 0.50/0.41 V) derived mainly from the redox reactions of dimers, oligomers, and the degradation products including p-benzoquinone, quinoneimine, p-aminodiphenylamine, hydroquinone, and p-aminophenol [23, 25] and the peaks D/D′ (at 0.58/0.56 V) can be ascribed to the transformation between emeraldine and pernigraniline states [17].
Figure 5(a) shows the charge/discharge curves of PANI/C-TiO2-15 NTAs electrode at the current density of 0.1, 0.2, 0.5, 1, and 2 mA cm−2. As a comparison, the charge/discharge curves of C-TiO2, PANI/C-TiO2-15 and PANI/C-TiO2-30 at 0.1 mA cm−2 are shown in Figure 5(b). Their specific capacitances are calculated according to the formula as follows:where is the specific capacitance, is the charge/discharge current density, is the discharge time, is the potential range, and denotes apparent area of the electrode. The specific capacitance of PANI/C-TiO2-15 NTAs is calculated to be 120.8, 119.3, 116.1, 111.7, and 104.3 mF cm−2 at a current density of 0.1, 0.2, 0.5, 1, and 2 mA cm−2 shown in Figure 5(c). In contrast, the specific capacitance of C-TiO2 and PANI/C-TiO2-30 NTAs is calculated to be 16.1, 14.5, 13.5, 12.5, and 12.3 mF cm−2 and 202.4, 163.8, 109.8, 89.9, and 79.7 mF cm−2 at the corresponding current density, respectively. It can be seen that both PANI/C-TiO2-15 and PANI/C-TiO2-30 NTAs show enhanced performance after the electropolymerization. It is worth mentioning that the specific capacitance of PANI/C-TiO2-30 NTAs is higher than that of the PANI/C-TiO2-15 NTAs at a lower current density because of the large amount of the loading mass of PANI per cm2. However, when the current density increases to 0.5 mA cm−2, the areal capacitance becomes smaller than the PANI/C-TiO2-15 NTAs due to the poor ion diffusion in blocked nanotube of the PANI/C-TiO2-30.
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Cycling performance is also one of the most important properties of supercapacitor. Cycling test of PANI/C-TiO2-15 NTAs electrode was carried out at a current density of 0.2 mA cm−2 in 1 M H2SO4 solution in the potential range of 0–0.6 V (versus SCE). Figure 5(d) shows the curve of capacitance changes with the cycle number increasing. After 5000 cycles, the capacitance retention is as high as 88.7%. This excellent cycle stability of PANI/C-TiO2-15 NTAs electrode resulted from the structure stability of C-TiO2 NTAs during charge-discharge process.
4. Conclusions
In summary, the PANI/C-TiO2 NTAs electrode was prepared by depositing PANI on the conductive C-TiO2 NTAs obtained from directly annealing the as-anodized TiO2 NTAs under Ar atmosphere. The PANI/C-TiO2-15 NTAs electrode performs a good specific capacitance of 120.8 mF cm−2 and good rate ability with capacitance retention of 86.3% when the current density increases 20 times. Due to the structure stability of C-TiO2 NTAs, PANI/C-TiO2 nanocomposite electrode expresses wonderful cycling performance that the specific capacitance remains 88.7% even after 5000 cycles. All these outstanding performances make this composite material a promising electrode in the field of energy storage.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Authors’ Contribution
Chengcheng Zhang and Changjian Peng contributed equally to this work.
Acknowledgments
This work was financially supported by Young Scientific and Technological Backbone Cultivating Project of Wuhan University of Science and Technology (no. 2013xz002), the Outstanding Young and Middle-Aged Scientific Innovation Team of Colleges and Universities of Hubei Province (T201402), the Natural Science Foundation of Hubei Province (2013CFB327), and the Applied Basic Research Program of Wuhan City (2013011801010598).