Yongqiang Qina,
Jianfang Zhanga,
Yan Wang*a,
Xia Shua,
Cuiping Yua,
Jiewu Cuia,
Hongmei Zhenga,
Yong Zhangab and
Yucheng Wu*ab
aSchool of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, P. R. China. E-mail: stone@hfut.edu.cn; ycwu@hfut.edu.cn
bKey Laboratory of Advanced Functional Materials and Devices of Anhui Province, Hefei, 230009, P. R. China
First published on 10th May 2016
Highly ordered TiO2 nanotube arrays (TNAs) with enhanced electronic conductivity treated by introducing oxygen vacancies have been considered to be a promising electrode material for supercapacitors. In this work, we fabricated electrochemically doped TiO2 nanotube arrays (ED-TNAs) through a facile cyclic voltammetry method, and then deposited the uniformly dispersed Cu2O nanoparticles onto ED-TNAs to synthesise a high performance electrode for a supercapacitor. The ED-TNAs electrode exhibited a high specific capacitance of 5.42 mF cm−2 at a scan rate of 10 mV s−1, which was about 59 times higher than for the pristine TNAs electrode. Moreover, the ED-TNAs were demonstrated to be an appropriate support for Cu2O nanoparticles. The highest specific capacitance of the Cu2O/ED-TNAs electrode could reach 198.7 F g−1 at the current density of 0.2 A g−1, and approximately 88.7% of the initial capacitance was retained after 5000 cycles of galvanostatic charge–discharge.
Generally, the capacitance performance can be improved by advancing the high surface area, high thermal and chemical stability and high electronic conductivity of the material. So far, highly-ordered TiO2 nanotube arrays (TNAs) have received great attention owing to their high surface area, stability and direct transport pathways.13 However, the poor electric conductivity of TNAs leads to a relatively low specific capacitance. To overcome this limitation, many efforts have been made in recent years. For instance, Salari et al.14 found that the capacitance of TNAs can be enhanced through annealing treatment under a reducing atmosphere and confirmed that the introduction of oxygen vacancies in TNAs further improves the capacitance due to the faradic reaction. Yat Li's group15 reported a new and general strategy for improving the capacitive properties of TiO2 in a hydrogen atmosphere at high annealing temperatures, and the H–TiO2 prepared at 400 °C yielded the largest specific capacitance of 3.24 mF cm−2 with a remarkable rate capability and outstanding long-term cycling stability. It is worthy of noting that the treatment in an atmosphere at high annealing temperatures is not suitable for practical application. A simple and easy method is highly required for modification of TNAs to reduce the electric resistance and improve the capacitance properties. Recently, Wu et al.,16 showed that TNAs exhibited a high specific capacitance of 20.08 mF cm−2 at the current of 0.05 mA cm−2 through a facile treatment of electrochemical doping approach. More interestingly, the capacitance of TiO2 can be recovered by the same doping process after several cycles.
On the one hand, the highly ordered electrochemically doped TiO2 nanotube arrays (ED-TNAs) offer high surface area and greatly improved electronic conductivity, which has attracted much attention in charge storage devices; on the other hand, Cu2O is a promising material for pseudo-capacitor owing to its large specific capacitance. The ED-TNAs can serve as a good support for Cu2O electrode materials to form composite structures. Motivated by this, we employed ED-TNAs as a support for Cu2O nanoparticles to form Cu2O/ED-TNAs composite electrodes for supercapacitor. Herein, the ED-TNAs were fabricated by electrochemical doping process using a facile cyclic voltammetry method, and then Cu2O nanoparticles were deposited onto ED-TNAs though square wave voltammetry (SWV) deposition method to synthesis the Cu2O/ED-TNAs electrodes. By taking the advantages of both ED-TNAs and Cu2O nanoparticles, the Cu2O/ED-TNAs electrodes achieved high electrochemical performance and excellent long-term cycling stability.
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Fig. 1 Schematic diagram illustrating the synthesis procedures of Cu2O/ED-TNAs electrodes on Ti foil. |
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Fig. 3 FE-SEM images of Cu2O/ED-TNAs on the top surface prepared by the different concentration of CuSO4 solution: (a) 0.005; (b) 0.01; (c) 0.05; (d) 0.1 M. |
Fig. 4a shows the typical TEM images of Cu2O/ED-TNAs composites prepared from 0.05 M CuSO4 solution (0.05-Cu2O/ED-TNAs). The uniformly distributed Cu2O nanoparticles with a small average diameter of 5 nm can be clearly observed on the inner surface of ED-TNAs. As shown in Fig. 4b, the selected area of the HRTEM image depicts the Cu2O nanoparticles embedded inside the TiO2 nanotubes, where the lattice planes with a spacing of 0.246 nm and 0.351 nm corresponds to the (111) plane of Cu2O and the (101) plane of anatase TiO2, respectively, further confirming the crystalline phase of the Cu2O/ED-TNAs heterojunction composites.
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Fig. 4 (a) Typical TEM images of the 0.05-Cu2O/ED-TNAs; (b) HRTEM image of the corresponding area of the 0.05-Cu2O/ED-TNAs. |
Fig. 5 shows the X-ray diffraction (XRD) patterns of the different crystalline structures of the samples. The samples show the diffraction peaks at 2θ = 25.2, 37.8 and 48.0°, which correspond to the (101), (004), and (200) planes of the TiO2 anatase phase (JCPDS card no. 21-1272), respectively. The Cu2O/ED-TNAs fabricated by depositing Cu2O in the CuSO4 solution of 0.01, 0.05, and 0.1 M display clear peak at 2θ = 36.42°, which is ascribed to the (111) reflection of Cu2O (JCPDS card no. 05-0667). Moreover, a distinguishing peak at 2θ = 42.29° can be observed both in 0.05-Cu2O/ED-TNAs and 0.1-Cu2O/ED-TNAs samples, which correspond to the (200) reflection of Cu2O, and the peak of Cu2O (220) plane at 2θ = 61.34° appear in the sample of 0.1-Cu2O/ED-TNAs. However, no Cu2O phase was observed in 0.005-Cu2O/ED-TNAs, which is attributed to the low content of the Cu2O phase that cannot be detected through XRD technique. In addition, it can be seen clearly, that when the concentration of Cu2O increased, the peaks of Cu2O also increased, indicating that the amount of Cu2O is related to the deposition voltage. Furthermore, the characteristic peaks of Cu and CuO were not found, which indicated that the deposited nanoparticles were Cu2O rather than Cu or CuO.
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Fig. 5 XRD patterns of the ED-TNAs and Cu2O/ED-TNAs prepared by the different concentration of CuSO4 solution. |
In order to detect the elemental composition and chemical states of 0.05-Cu2O/ED-TNAs samples, X-ray photoelectron spectroscopy (XPS) test was carried out and the results are shown in Fig. 6. The typical peaks of Ti 2p, O 1s, Cu 2p, and C 1s in the survey spectrum can be seen clearly in Fig. 6a. Fig. 6b presents a typical high-resolution XPS spectrum of O 1s. The peak at 529.9 eV can be assigned to the oxygen atoms of Ti–O–Ti, and the shoulder peak at higher binding energy of 531.8 eV can be attributed to the hydroxyl group of Ti–OH, which was reported to be located at ∼1.5–1.8 eV higher than the oxygen atoms of Ti–O–Ti.15,18 This result demonstrates that the hydroxyl group was formed on the sample surface and the TiO2 surface was activated by electrochemical doping process. The high resolution XPS spectrum of Cu 2p is displayed in Fig. 6c, and it can be observed that the Cu 2p3/2 and Cu 2p1/2 were located at binding energies of 932.5 eV and 952.5 eV, respectively, which are in good agreement with the reported values of Cu2O.19–21 In order to further distinguish between Cu2O and Cu or CuO, the Cu LMM Auger transitions was employed, as displayed in Fig. 6d. We can notice clearly that there is only one peak at 570 eV corresponding to the binding energy of Cu2O.
Fig. 7a shows the dynamic CV performance for electrochemical doping process of TNAs electrode. During the first anodic scan, the oxidation current density is very low starting at −1 V, which means the capacitance is small. When in the return at cathodic scan, the reduction current density increases tremendously with bubbles evenly generated around the counter electrode. To a great extent, both the oxidation current density and reduction current density are much larger in the second scan than in the first scan, indicating that the oxygen vacancies were introduced into TNAs to enhance the capacitance. Furthermore, the CV curves of ED-TNAs electrode obtained at various scan rates in the wide range from 10 to 500 mV s−1 display the unchanged quasi-rectangular shapes, shown in Fig. 7b, indicating the good capacitive behavior and higher-rate capability of ED-TNAs.22 Fig. 7c displays the galvanostatic charge–discharge curves at different current densities. The discharge curves are almost symmetrical to the corresponding charge curves even at a high current density of 4.0 mA cm−2, indicating a good capacitive behavior of the ED-TNAs electrode. Fig. 7d shows the calculated specific capacitance of the ED-TNAs based on the CV curves. The specific capacitance of the ED-TNAs electrode are calculated to be 5.42 mF cm−2 at a scan rate of 10 mV s−1, which is about 59 times more than pristine TNAs electrode (0.092 mF cm−2). Additionally, the specific capacitance of ED-TNAs can be kept at 4.35 mF cm−2 even at high scan rate of 500 mV s−1, indicating a good rate capability and a good capacitance retention capability.
To further analyze the mechanism of electrochemical doping process, EIS was carried out at open circuit potential with an amplitude of 5 mV. Comparing the Nyquist plots of pristine TNAs and ED-TNAs electrode, the ED-TNAs electrode has a smaller semicircle in the high-frequency region as shown in Fig. 8. It is generally accepted that a semicircle reflects the electrochemical reaction impedance of the electrode, and a smaller semicircle means smaller charge transfer resistance.23 As shown in the inset image of Fig. 8, the EIS data of ED-TNAs was simulated by an equivalent circuit. The equivalent circuit consists of the bulk solution resistance (Rs), the charge transfer resistance (Rct), the double-layer capacitance (CPE1), the interface resistance (Rc) and capacitance (CPE2) of the space charge layer within the bulk TNAs and a Warburg diffusion element (W). From the result of equivalent circuit, we note that the bulk solution resistance (Rs) of ED-TNAs is about 32.7 Ω, which is much closer to that of pristine TNAs (45.4 Ω); however, the charge transfer resistance (Rct) between the ED-TNAs and electrolyte interface is only 169 Ω, much lower than that of original TNAs (2.13 KΩ), indicating that the electronic conductivity of TNAs can be increased by electrochemical doping process to facilitate the charge transfer.
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Fig. 8 Nyquist plots of pristine TNAs and ED-TNAs electrodes and corresponding equivalent circuit, the inset showing the Nyquist plot of the experiment and fitting result of the ED-TNAs. |
Fig. 9a–d shows the CV curves of 0.005-Cu2O/ED-TNAs, 0.01-Cu2O/ED-TNAs, 0.05-Cu2O/ED-TNAs and 0.1-Cu2O/ED-TNAs electrodes, respectively. It can be examined that the CV curves of those film electrodes display the same shapes; however, the intensity of the redox peaks of those film electrodes firstly increase and then decreased with the loading contents of Cu2O nanoparticles. The evenly distributed Cu2O nanoparticles can reduce the internal resistance of the electrode but excess Cu2O nanoparticles prevented the diffusion of electrons and ions in the ED-TNAs. This further verifies the loading content dependence of the enhancement of the capacitive performance of Cu2O/ED-TNAs electrodes. All the CV curves show similar shapes even at a high scan rate of 100 mV s−1, which indicates the excellent capacitive behavior and high rate capability. Fig. 9e displays the CV curves of Cu2O/ED-TNAs electrodes obtained from different concentrations of CuSO4 solution at a scan rate of 100 mV s−1. It can be observed that there are remarkable redox peaks in all electrodes at the same scan rate. A pair of redox peaks in the CV curves owing to the combination effects of Cu2O nanoparticles, indicating that faradic reactions were generated during the electrode charging and discharging process and the capacitance of Cu2O/ED-TNAs electrodes were attributed to the redox pairs of Cu+/Cu2+. Moreover, the 0.05-Cu2O/ED-TNAs electrode displayed the largest anodic and reduction current densities than that of other electrodes in the CV curves, indicating that it possess the largest specific capacitance. Besides, it can be observed from Fig. 9f that both the oxidation peak current density and reduction peak current density of those electrodes increased almost linearly with the scan rate as high as 100 mV s−1, which reveals the excellent kinetic performance and further proves that these electrodes have ideal capacitive behaviors.24
The GCD test provides a reliable method for evaluating the specific capacitance of materials. Typical discharge curves of all Cu2O/ED-TNAs electrodes were investigated at various current densities in the range of 0.2 to 5 A g−1, as shown from Fig. 10a–d. It can be seen that all the discharge curves are not ideal straight line, indicating that these electrodes have faradic capacitive performance.25 The shapes of discharge curves for each Cu2O/ED-TNAs electrode have no changes even at a high current density of 5 A g−1, which reveals these Cu2O/ED-TNAs electrodes have high rate capability. As shown in Fig. 10e, the charge–discharge time of the 0.05-Cu2O/ED-TNAs electrodes was significantly enhanced than that of other electrodes at the current density of 1 A g−1, implying that the 0.05-Cu2O/ED-TNAs electrodes possessed the highest specific capacitance. This result is good in accordance with the above results of CV tests. The specific capacitance of these film electrodes can be calculated by the following equation:
Fig. 11 gives the Nyquist impedance spectra of ED-TNAs and Cu2O/ED-TNAs electrodes at open circuit potential. The Cu2O/ED-TNAs electrodes have much smaller semicircles in the high-frequency region than the ED-TNAs film electrode, indicating that the resistance of the Cu2O/ED-TNAs electrodes are much lower than that of the ED-TNAs electrode. For example, the bulk solution resistance (Rs) estimated from the equivalent circuit is about 10.5 Ω, and the charge transfer resistance (Rct) is about 32.4 Ω for the 0.05-Cu2O/ED-TNAs electrode film, which is much lower than that of the ED-TNAs (32.7 Ω for the Rs and 169 Ω for the Rct). These remarkable results further demonstrate that the ED-TNAs film electrode deposited with Cu2O nanoparticles can substantially reduce the charge transfer resistance and accelerate electronic transmission between the electrode and electrolyte.
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Fig. 11 Nyquist plots of ED-TNAs electrode and Cu2O/ED-TNAs electrodes obtained from different concentration of CuSO4 solution. |
The cycling life test for 0.05-Cu2O/ED-TNAs film electrodes were carried out through a cyclic galvanostatic charge–discharge process over 5000 cycles at current density of 1 A g−1. The specific capacitance of 0.05-Cu2O/ED-TNAs electrode as the function of the cycle number is shown in Fig. 12. The specific capacitance maintained 152.8 F g−1 after 5000 cycles, which is about 88.7% of the initial capacitance. Additionally, the charge–discharge curves are more or less the same as that of the first cycle after 5000 cycles of measurement. This result illustrates the 0.05-Cu2O/ED-TNAs electrode has a good capacitance recovery and excellent long-term stability.
This journal is © The Royal Society of Chemistry 2016 |