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A general strategy to construct N-doped carbon-confined MoO2 and MnO for high-performance hybrid supercapacitors
Vacuum 165 (2019) 179–185
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A general strategy to construct N-doped carbon-confined MoO2 and MnO for high-performance hybrid supercapacitors
T
Yaotian Yana, Jinghuang Lina, Jing Jiangb, Haohan Wanga, Junlei Qia,∗, Zhengxiang Zhongb, Jian Caoa, Weidong Feia, Jicai Fenga a
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin, 150001, China MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, State Key Laboratory of Urban Water Resource and Environment, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, China
b
ARTICLE INFO
ABSTRACT
Keywords: Polypyrrole Hybrid supercapacitor MoO2 MnO N-doped carbon
Herein, a general strategy is provided to construct N-doped carbon-confined MoO2 and MnO by the hydrothermal process, polymerization process and carbonation process, which serves as negative electrode and positive electrode for hybrid supercapacitors. The N-doped carbon as shell can provide fast electron pathways and short ion diffusion paths, resulting in the improved performances at the high rates. Consequently, the obtained N-doped carbon-confined MoO2 and MnO possess the good electrochemical performances, including high specific capacitance, excellent cycling stability and good rate capability. Furthermore, the as-fabricated hybrid supercapacitor using N-doped carbon-confined MoO2 and MnO shows a high energy density up to 44.82 Wh kg−1 at a power density of 900 W kg−1, as well as good cycling performance. This work may provide a general strategy for constructing high-performance energy storage devices.
1. Introduction As the new emerging energy devices, supercapacitors are the promising candidates due to the high power density, fast charge-discharge rate and environmental friendliness [1–3]. However, the relatively low energy density of supercapacitors greatly limits the practical applications [4,5]. It is well known that the voltage window (V) and specific capacitance (C) are the two key parameters for supercapacitors, according to equation E = 1/2 CV2 [6]. Thus, it is a general strategy to construct hybrid supercapacitors by coupling two kinds of electrode (positive electrode and negative electrode), which could greatly enlarge the voltage window and then improve the energy density [7,8]. Consequently, it is urgently to design and fabrication of advanced materials for both positive electrode and negative electrode. As for the positive electrodes, the limited potential window (usually less than 0.6 V) for most transition metal oxides (TMOs, like CoOx, NiO, NiCo2O4) could not fully enlarge the voltage windows for ASCs [9–12]. As one kind of TMOs, manganese oxides (MnO, MnO2 et al.) can provide a wider potential window more than 0.8 V, which shows great potentials as positive electrode for ASCs [13,14]. Among these manganese oxides, MnO is the promising candidate because of the redox reaction [14,15]. However, MnO still suffers from the limited cyclic
∗
stability and poor conductivity [16,17]. As for the negative electrodes, the limited specific capacitances greatly constraints the electrochemical performances of hybrid supercapacitors, according to equation 1/ C = 1/Cp + 1/Cn [18]. Commonly used carbon materials often show the low specific capacity due to the charge stored on the electric-double layer capacitance (EDLC) mechanism [19,20]. In this regard, many molybdenum oxides as anode materials, such as MoO3, MoO2 and MoO3-x could be the promising alternatives, because they show better electrochemical performances than that of carbon materials [21,22]. However, the low electronic conductivity of molybdenum oxides profoundly affects their electrochemical performance [23,24]. Even worse, these molybdenum oxides often suffer from the molybdenum oxides and large volume changes during cycling tests, and thus resulting in inferior rate capability and poor cycling stability [25,26]. To resolve this challenge for MnO and molybdenum oxides, one effective strategy is to combine these transition metal materials with carbonaceous materials [21,27–30]. According to previously reported researches, it has been well demonstrated that metal oxides are subtly combined with carbon materials can not only improve the conductivity, but also maintain the structural integrity, thus resulting in improved rate capability and cycling stability [27–30]. Among various carbon materials, nitrogen-doped carbon has attracted particular research interest. The
Corresponding author. E-mail address: [email protected] (J. Qi).
https://doi.org/10.1016/j.vacuum.2019.04.033 Received 28 March 2019; Received in revised form 14 April 2019; Accepted 15 April 2019 Available online 16 April 2019 0042-207X/ © 2019 Elsevier Ltd. All rights reserved.
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nitrogen doping in carbon materials has been proven that can create defect sites and enhance the electrical conductivity and wettability in electrolyte [31–33]. Stimulated by these considerations, we design and construct of polypyrrole-derived N-doped carbon-encapsulated MoO2 and MnO with yolk-shelled nanostructures (MoO2@NCT and MnO@NCT) for hybrid supercapacitors. The whole synthesis step consists of the by the hydrothermal process, polymerization process and carbonation process. The N-doped carbon as shell could provide fast electron pathways and short ion diffusion paths, resulting in the improved performances at the high rates. Further, the yolk-shelled nanostructures in MoO2 and MnO could effectively relieve volume variation during cycling tests, leading to good cycling stability. As a result of the structural, morphological and compositional advantages, the as-synthesized yolk-shelled MoO2@ NCT and MnO@NCT display excellent electrochemical performances. In addition, the as-fabricated hybrid supercapacitor using MoO2@NCT as negative electrode and MnO@NCT as positive electrode can achieve a high energy density of 44.82 Wh kg−1 at a high power density of 900 W kg−1 and desirable electrochemical stability.
SEM, Helios Nanolab 600i), transmission electron microscopy (TEM, Tecnai G2 F30) equipped with an energy dispersive X-ray spectrometer (EDS), X-ray diffraction (XRD, D8 Advance) and X-ray photoelectron spectroscopy (XPS, Thermo Fisher). Thermogravimetric analysis was investigated by a thermal analyzer (TGA) under Air flow with a heating rate of 10 °C min−1. 2.3. Electrochemical measurements To prepare the working electrode, a slurry was prepared by mixing active materials, acetylene black and PVDF (mass ratio of 8:1:1). Then, the mixed slurry was coated onto a carbon cloth with a mass loading of 2 mg cm−2, and dried at 80 °C overnight. All electrochemical performances of obtained samples were investigated by electrochemical workstations (CHI 760E). The electrochemical performance was test in the three-electrode configuration using 1 M Na2SO4 as electrolyte, where samples on carbon cloth, Pt and saturated calomel electrode (SCE) were used as working, counter and reference electrodes. In addition, a hybrid supercapacitor was constructed by MnO@NCT as positive electrode and MoO2@NCT as negative electrode. The specific capacity (C, F g−1), energy density (E, Wh kg−1) and power density (P, W kg−1) were calculated [34–37], as following:
2. Experimental 2.1. Materials synthesis
C = IΔt/mV
2.1.1. Synthesis of the MoO3 nanobelts Firstly, a mixed solution of 60 mL deionized water, 1.21 g Na2MoO4·2H2O and 0.6 g NaCl was adjusted to pH 1.0. Then, the mixed solution was transferred into 100 mL Teflon-lined stainless-steel autoclave and maintained at 180 °C/24 h. The synthesized products were collected and washed for several times, and dried under vacuum at 60 °C/24 h.
E = 1/2CV2 P = EΔt where I, V, m and Δt mean the specific capacitance, current density, potential window, mass loading and discharging time. 3. Results and discussion
2.1.2. Synthesis of the MnO2 nanowires In a typical procedure, a mixed solution of 75 mL deionized water, 0.7 KMnO4 and 3.6 mL concentrated HCl was transferred into 100 mL Teflon-lined stainless-steel autoclave and maintained at 140 °C/13 h. The synthesized products were collected and washed for several times, and dried under vacuum at 60 °C/24 h.
To better understanding the pyrolysis of PPy, TGA analysis was conducted to investigate the thermal properties of pure MoO3 and PPy, as shown in Fig. 1a. It can be found that the main weight loss for pure MoO3 occurs in 700–800 °C, which is mainly attributed to the decomposition of MoO3. As for pure PPy, the weight loss below 150 °C is due to the vaporization of water/moisture molecules, and the main weigh loss occurs in 300–800 °C, owing to the complete decomposition of PPy and the formation of the nitrogen-doped carbon tubes. Consequently, different pyrolysis temperature (600, 700 and 800 °C) was chosen to investigate the mutual reaction of MoO3 and PPy. Fig. 1b shows the XRD patterns of MoO3 and MoO3@PPy. Obviously, all the diffraction patterns in both samples could be indexed to the α-MoO3 phase of the hexagonal crystals (JCPDS no. 05–0508). And it also demonstrated that there are no crystalline changes for the MoO3 upon the in situ polymerization of PPy. Fig. 1c and d shows the XRD patterns of MoO2@ NCT-600–800 and MoO3-700. It can be found that the XRD pattern of MoO3-700 is indexed to MoO3 and MoO2, suggesting the partially reduction of MoO3 after annealing at 700 °C. However, as for MoO2@ NCT-600 and MoO2@NCT-700, all the peaks could be indexed to MoO2 (JCPDS no. 32–0671), suggesting the completely reduction of MoO3. As for MoO2@NCT-800, it mainly consists of MoO2 and Mo0.42C0.58 (JCPDS no. 36–0863) phases. There are no obvious peaks for carbon materials in MoO2@NCT-600–800, which may be due to the relatively low amounts of the nitrogen-doped carbon tubes. Meanwhile, XPS measurements are employed to further investigate the composition of MoO2@NCT-700, as shown in Fig. 1e–h. As for Mo 3d spectrum in Fig. 1e, it could be deconvoluted into four peaks, where the peaks at 229.7 and 232.9 eV were corresponded to Mo 3d5/2 and Mo 3d3/2 of Mo4+, and the peaks at 232.3 and 235.8 eV were indexed to Mo 3d5/2 and Mo 3d3/2 of Mo6+ due to the MoO2 surface oxidation under ambient conditions [38,39]. From the O 1s spectrum (Fig. 1f), the characteristic peaks at 530.7 eV, 532 eV, and 533.3 eV belong to the Mo-O, C-O, and C-OH bonds, respectively [21,40]. As for C 1s spectrum
2.1.3. Synthesis of the MoO2@NCT Firstly, MoO3@PPy was synthesized by in situ oxidative polymerization process. 0.42 g MoO3 was dispersed in 160 mL deionized water, and 1.6 mL pyrrole was added by vigorous stirring at 0 ± 2 °C for 30 min. Then a mixed solution of 3.2 g ammonium persulfate and 80 mL deionized water was added dropwise to the above suspension. The reaction mixture was stirred for 10, 20, 30 and 60 min to obtain different content of PPy (denoted as MoO3@PPy-10–60 for convenience). The reaction products were filtered, washed for several times, and dried overnight. Secondly, to obtain MoO2@NCT, as-synthesized MoO3@PPy was annealed at Ar atmosphere at 600, 700 and 800 °C for 2 h (denoted as MoO2@NCT-600–800 for convenience). For comparison, pure MoO3 was also annealed at 700 for 2 h (denoted as MoO3-700 for convenience). 2.1.4. Synthesis of the MnO@NCT Firstly, the synthesis process of MnO2@PPy was similar to that of MoO3@PPy, and the reaction mixture was stirred for 30 min. Secondly, to obtain MnO@NCT, as-synthesized MnO2@PPy was annealed at Ar atmosphere at 700 °C for 2 h (denoted as MnO@NCT for convenience). Similarly, the pure MnO2 sample was annealed at Ar atmosphere at 700 °C for 2 h to obtain MnO sample. 2.2. Materials characterization The morphology, structure, phase and chemical valence states of samples were investigated by field-emission electron microscopy (FE180
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Fig. 1. (a) TGA curves of pure MoO3 and PPy. (b) XRD patterns of MoO3 and MoO3@PPy, (c) XRD patterns of MoO2@NCT-600, MoO2@NCT-700 and MoO3-700, (d) XRD pattern of MoO2@NCT-800. XPS spectra of (e) Mo 3d, (f) O 1s, (g) C 1s and (h) N 1s in MoO2@NCT-700.
(Fig. 2g), it can be deconvoluted into C-C, C-N, C-O, and O=C-N bonds at 284.6, 285.7, 286.4 and 287.7 eV, respectively [41].According to previously reported researches [42,43], the functional groups in NCT may improve the electrochemical reaction kinetics. As for N 1s, three peaks could be indexed to pyridinic N (398.5 eV), pyrrolic N (400.3 eV), and graphitic N (401.3 eV), respectively [44]. It indicates the successful incorporation of nitrogen into graphitic carbon. According to previously reported researches [45,46], N doped in carbon shells could improve the storage capacitance and rate capability. Fig. 2 shows the morphological characterizations of MoO3, MoO3@ PPy, MoO3-700 and MoO2@NCT. As shown in Fig. 2a, as-synthesized MoO3 possess a 1D nanobelt structure. After coating PPy as shell, the surface of MoO3@PPy is rougher than that of pure MoO3 (Fig. 2b). For comparison, we also synthesized different content of PPy on MoO3 nanobelts by controlling the reaction time, as shown in Fig. S1. Obviously, it can be found that with the increase of reaction time, the surface of MoO3@PPy becomes much rougher, indicating that more PPy is coated on the surface of MoO3 nanobelts. After annealing in 700 °C at the Ar atmosphere, it can be found that the MoO3-700 becomes more bulky, as shown in Fig. 2c. However, after coating PPy as shell, MoO2@NCT maintains the nanobelt morphologies after high-
temperature pyrolysis (Fig. 2d). Further, it is worth noting that the solid MoO3 nanobelt core converts into hierarchically assembled MoO2 nanosheets. Fig. S2 shows the SEM images of MoO2@NCT-10–60, where all obtained samples maintain the nanobelt morphologies after hightemperature pyrolysis. In addition, we also investigated the effect of pyrolysis temperature on the MoO2@NCT, as shown in Fig. S3. It can be found that MoO2@NCT-800 are aggregated with each other, which may be due to the low melting points of MoO3 [47]. The TEM and high-resolution TEM (HRTEM) images of MoO3 and MoO2@NCT-700 are shown in Fig. 3. As shown in Fig. 3a, MoO3 shows the nanobelt structure with a smooth surface. The lattice spacing of ∼0.381 nm could be assigned to the (110) plane of α-MoO3 (Fig. 3b). After the pyrolysis process, the overall nanobelt morphology of MoO2@ NCT-700 (Fig. 3c) is well maintained with a width of 200–300 nm. During the pyrolysis process, the PPy shell changes into NCT hollow shell and the shell thickness of the NCT was about 10 nm (Fig. 3d). At the same time, the solid MoO3 nanobelt core has been converted into assembled MoO2 nanosheets. The elemental mappings in Fig. 3e show that the element C and N are mainly distributed in hollow carbon walls, while Mo and O show a uniform distribution in the MoO2 core. It clearly indicates that N is successfully doped into the carbon shells.
Fig. 2. SEM images of (a) MoO3, (b) MoO3@PPy, (c) MoO3-700 and (d) MoO2@NCT-700. 181
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Fig. 3. (a) TEM and (b) HRTEM images of MoO3 nanobelts, (c) TEM, (d) HRTEM images and corresponding EDS element mapping images of Mo, O, C, and N of MoO2@NCT-700.
Fig. 4. SEM images of (a) MnO2 nanowire and (b) MnO@NCT. (c) TEM and (d) HRTEM images of hollow MnO2 nanowire, (e) TEM, (f) HRTEM images and corresponding EDS element mapping images of Mn, O, C, and N of MnO@NCT.
and are shown in Fig. S7. From the GCD curves at 2 A g−1, the optimum parameters for MoO2@NCT are that the oxidative polymerization process lasts 30 min and pyrolysis temperature is 700 °C. According to the above results, the following discussion is mainly focused on the MoO2@ NCT obtained at 700 °C with 30 min polymerization process. Fig. 5a shows the CV curves of MoO3, MoO3@PPy, MoO3-700 and MoO2@NCT with voltage window from −1.0 to 0.0 V (vs. SCE) at 100 mV s−1. Notably, MoO2@NCT shows the largest enclosed CV curve area among four samples, demonstrating the highest chare-storage capacitance. Fig. 5b shows the CV curves of MoO2@NCT at various scan rates, and all of the CV curves exhibit an approximately rectangular shape without any obvious redox peaks, indicating the fast, reversible successive surface redox reactions [48–50]. In addition, Fig. 5c shows GCD curves for the as-fabricated MoO2@NCT at different current densities. From calculations of the discharge time from the GCD curves, the specific capacitances for all the samples were calculated and are shown in Fig. 5d. A desirable specific capacitance of 254.8 F g−1 is obtained for the MoO2@NCT electrode at a current density of 2 A g−1, compared to 109.5, 98.6 and 148.7 F g−1 for the pure MoO3, MoO3@PPy and MoO3700 electrodes, respectively. Even at 20 A g−1, the specific capacitance of the MoO2@NCT still remains at 124.9 F g−1, demonstrating the good
At the same time, we also synthesized MnO@NCT by the similar process, as shown in Fig. 4. It can be found that pure MnO2 shows a regular nanowire structure with an average dimeter of about 30 nm (Fig. 4a), while MnO@NCT becomes much thicker after the growth of NCT as shell (Fig. 4b). The XRD patterns in Fig. S5 show that both samples match well MnO2 (JCPDS no. 44–0141) and MnO (JCPDS no. 75–0626). Further, the TEM image in Fig. 4c shows the hollow morphology of MnO2 nanowire, and the lattice fringe of ∼0.692 nm in Fig. 4d corresponds to the (110) plane of MnO2. Fig. 4e shows the TEM image of MnO@NCT. During the high-temperature pyrolysis, the solid MnO2 nanowire core converts into hierarchically assembled MnO nanosheets, which is encapsulated by NCT shells. And the shell thickness of the NCT was estimated to be ∼10 nm (Fig. 4f). In addition, the elemental mappings of MnO@NCT in Fig. 4g confirm the coexistence and the dispersion of Mn, O, C and N elements. We evaluate the electrochemical performances by testing cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) measurements in a three-electrode cell using 1 M Na2SO4 as electrolyte. In order to investigate the effect of PPy content and pyrolysis temperature, electrochemical performances were evaluated with different PPy content and pyrolysis temperature, and the corresponding GCD analysis 182
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Fig. 5. (a) CV curves of MoO3, MoO3@PPy, MoO3-700 and MoO2@NCT at 100 mV s−1. (b) CV and (c) GCD curves of MoO2@NCT. (d) The corresponding specific capacitance, (e) Nyquist plots and (f) cycling performances of MoO3 and MoO2@NCT. (g) CV curves of MnO2, MnO, MnO2@PPy and MnO@NCT at 100 mV s−1. (h) CV curves of MnO@NCT. (i) The corresponding specific capacitance of MnO2, MnO, MnO2@PPy and MnO@NCT.
delivers a high capacitance up to 306.4 F g−1 at 2 A g−1, and still holds a capacitance up to 191.2 F g−1 at 20 A g−1, confirming the good rate capability. Additionally, the synthesized MnO@NCT electrode exhibits a capacitance retention of about 94.9% after 10000 cycles (Fig. S10). Therefore, we believe that the as-synthesized MnO@NCT electrode is an excellent positive electrode for the design of hybrid supercapacitors. In addition, we fabricated hybrid supercapacitors by using MnO@ NCT as positive electrode and MoO2@NCT as negative electrode. Fig. 6a shows the CV curves of MnO@NCT//MoO2@NCT in different potential windows at 100 mV s−1. It can be found that the potential window of 0–1.8 V is the most suitable potential window. Fig. 6b and c shows the CV and GCD curves of MnO@NCT//MoO2@NCT at different scan rates and different current densities. As shown in Fig. 6b, the CV curves maintain the quasi-rectangular shapes as the scan rate increases from 10 to 100 mV s−1, implying the ideal capacitive behaviors and high rate capabilities. The almost symmetric sharp of GCD curves in Fig. 6c demonstrates the good coulombic efficiency and excellent reversibility of MnO@NCT//MoO2@NCT. Based on the GCD curves, the calculated specific capacitance of MnO@NCT//MoO2@NCT is shown in Fig. 6d. The MnO@NCT//MoO2@NCT device exhibited maximum capacitance of 99.6 F g−1 at 1 A g−1. As expected, a remarkable rate capability has also been achieved by the MnO@NCT//MoO2@NCT, which retained about 56.2% of the initial capacitance as the current density increased from 1 to 10 A g−1. Fig. 6e shows the cycling performance of MnO@NCT//MoO2@NCT device tested by GCD at 5 A g−1. After 10000 cycling tests, the capacitance retains about 92.5%, suggesting the good cycling stability. Fig. 6f shows the Ragone plot of
rate capability. Consequently, it is apparent that incorporation of NCT as shell is beneficial and effective for improving the capacitive performance. Further, the enhanced electron transfer from both highly conductive NCT and MoO2 is beneficial for improved rate capability. As for the long term cycle stability, as-synthesized MoO2@NCT achieved a good cycling stability with 96.3% capacitance retention after 10000 cycles (Fig. 5e). Electrochemical impedance spectrum (EIS) tests were carried out to investigate the electrochemical performances of obtained samples, as shown in Fig. 5f. In the low-frequency region, the slope of the MoO2@NCT samples is much closer to 90° than that of pure MoO3 sample, suggesting the better capacitive behavior and faster ion transportation [51,52]. The charge transfer resistance (Rct) could be estimated by the semicircle in the high-frequency region [53]. It can be found that MoO2@NCT shows a Rct value of about 0.4 Ω, which is much lower than that of pure MoO3 (about 0.9 Ω). It indicates that MoO2@ NCT faster electron transfer reaction kinetics [53,54], which may be due to the combination with conductive NCT layers. The electrochemical performances of MnO@NCT electrode were measured in a three-electrode cell, as shown in Figs. 5g–6i. As shown in Fig. 5g, the CV integral area of MnO@NCT electrode is the largest among four samples at 100 mV s−1, suggesting the superior capacitance. As for MnO@NCT, all CV curves in Fig. 5h maintained a quasirectangular and symmetric shape, suggesting the fast reversible faradaic reactions. Further, as the scan rate increases from 2 to 50 mV s−1, the CV sharp shows no obvious changes, indicates the excellent rate capability. Based on the discharge times, the specific capacitances of obtained samples are shown in Fig. 5i. The as-fabricated MnO@NCT 183
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Fig. 6. (a) CV curves of the obtained hybrid supercapacitor at different voltage windows. (b) CV and (c) GCD curves and (d) the corresponding specific capacitance of the obtained hybrid supercapacitor. (e) The cycling performance of the obtained hybrid supercapacitor tested using GCD at 5 A g−1. (f) Ragone plot of the obtained hybrid supercapacitor.
MnO@NCT//MoO2@NCT device. The MnO@NCT//MoO2@NCT device could achieve a high energy density of 44.82 Wh kg−1 at a power density of 900 W kg−1, which is better than previously reported device, such as MoO3@CNT//MnO2@CNT (27.8 Wh kg−1 at the power density of 524 W kg−1) [21], β-MnO2//activated graphene oxide (40.4 Wh kg−1 at the power density of 300 W kg−1) [36], CNTs-MnO2//MoO3PPy (21.03 Wh kg−1 at the power density of 220 W kg−1) [54], and RGO/MnO2//RGO/MoO3 (42.6 Wh kg−1 at the power density of 276 W kg−1) [55]. The good electrochemical properties of MnO@NCT and MoO2@NCT can be contributed to following advantages. (1) One-dimensional nanostructures in MnO@NCT and MoO2@NCT with a high aspect ratio could provide efficient electron and ion transport. (2) The well-defined inner voids in yolk-shelled nanostructures could not only serve as “electrolyte container” to create sufficient electron paths, but also relax the electrode strains during cycles. (3) The N-doped carbon as shell could not only improve the electrical conductivity, but also buffer the volume changes during cycling tests.
(Grant Nos 51575135, 51622503, U1537206 and 51621091) is highly appreciated. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.vacuum.2019.04.033. References [1] X. Wei, H. Peng, Y. Li, Y. Yang, S. Xiao, L. Peng, Y. Zhang, P. Xiao, ChemSusChem 11 (2018) 3167–3174. [2] X. Liu, G. Sheng, M. Zhong, X. Zhou, Nanoscale 10 (2018) 4209–4217. [3] H. Wang, D. Wang, T. Deng, X. Zhang, C. Zhang, T. Qin, D. Cheng, Q. Zhao, Y. Xie, L. Shao, H. Zhang, W. Zhang, W. Zheng, J. Power Sources 399 (2018) 238–245. [4] S. Dong, X. He, H. Zhang, X. Xie, M. Yu, C. Yu, N. Xiao, J. Qiu, J. Mater. Chem. 6 (2018) 15954–15960. [5] S. Liu, V.S. Kalimuthu, K. Aniruddha, M. Ming, K. Jang-Yeon, S.C. Jun, ACS Appl. Mater. Interfaces 9 (2017) 21829–21838. [6] T. Deng, Y. Lu, W. Zhang, M. Sui, X. Shi, D. Wang, W. Zheng, Adv. Energy Mater. 8 (2018) 1702294. [7] S. Liu, S.H. Kwan, N.H. Kwun, H. Li, W.N. Kar, J. Xu, Z. Tang, C.J. Seong, J. Mater. Chem. 5 (2017) 19046–19053. [8] C. Wan, Y. Jiao, D. Liang, Y. Wu, J. Li, Adv. Energy Mater. 8 (2018) 1802388. [9] M. Li, W. Yang, J. Li, M. Feng, W. Li, H. Li, Y. Yu, Nanoscale 10 (2018) 2218–2225. [10] B.R. Wiston, M. Ashok, Vacuum 160 (2019) 12–17. [11] X. Wei, Y. Li, H. Peng, M. Zhou, Y. Ou, Y. Yang, Y. Zhang, P. Xiao, Chem. Eng. J. 341 (2018) 618–627. [12] J. Lin, H. Wang, X. Zheng, Y. Du, C. Zhao, J. Qi, J. Cao, W. Fei, J. Feng, J. Power Sources 401 (2018) 329–335. [13] M. Li, W. Lei, Y. Yu, W. Yang, J. Li, D. Chen, S. Xu, M. Feng, H. Li, Nanoscale 10 (2018) 15926–15931. [14] J.G. Wang, F. Kang, B. Wei, Prog. Mater. Sci. 74 (2015) 51–124. [15] M. Ramadan, A.M. Abdellah, S.G. Mohamed, N.K. Allam, Sci. Rep. 8 (2018) 7988. [16] J. Shin, D. Shin, H. Hwang, T. Yeo, S. Park, W. Choi, J. Mater. Chem. 5 (2017) 13488–13498. [17] S. Ortaboy, J.P. Alper, F. Rossi, G. Bertoni, G. Salviati, C. Carraro, R. Maboudian, Energy Environ. Sci. 10 (2017) 1505–1516. [18] Y. Li, J. Xu, T. Feng, Q. Yao, J. Xie, H. Xie, Adv. Funct. Mater. 27 (2017) 1606728. [19] J. Jia, X. Liu, R. Mi, N. Liu, Z. Xiong, L. Yuan, C. Wang, G. Sheng, L. Cao, X. Zhou, X. Liu, J. Mater. Chem. 6 (2018) 15330–15339. [20] S. Liu, Y. Yin, K.S. Hui, K.N. Hui, S.C. Lee, S.C. Jun, Adv. Sci. (2018) 1800733. [21] T.H. Lee, D.T. Pham, R. Sahoo, J. Seok, T.H.T. Luu, Y.H. Lee, Energy Storage Mater 12 (2018) 223–231. [22] J. Lin, H. Liang, H. Jia, S. Chen, J. Guo, J. Qi, C. Qu, J. Cao, W. Fei, J. Feng, J. Mater. Chem. 5 (2017) 24594–24601.
4. Conclusion In summary, we have successfully developed a simple strategy to fabricate the N-doped carbon-confined MoO2 and MnO with yolkshelled nanostructures for hybrid supercapacitors. Based on SEM, TEM and Raman results, N-doped carbon coating has been successfully prepared on MoO2 and MnO. The N-doped carbon as shell could not only effectively improve the electron transfer, but also maintain the structural integrity. Consequently, electrochemical tests demonstrated the yolk-shelled MoO2 and MnO showed the enhanced electrochemical performances, when compared to pristine MoO3 and MnO2. Further, the as-fabricated hybrid supercapacitor of MnO@NCT//MoO2@NCT showed a high energy density up to 44.82 Wh kg−1 at a power density of 900 W kg−1, as well as good cycling performance. Acknowledgements The support from the National Natural Science Foundation of China 184
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A general strategy to construct N-doped carbon-confined MoO2 and MnO for high-performance hybrid supercapacitors
T
Yaotian Yana, Jinghuang Lina, Jing Jiangb, Haohan Wanga, Junlei Qia,∗, Zhengxiang Zhongb, Jian Caoa, Weidong Feia, Jicai Fenga a
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin, 150001, China MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, State Key Laboratory of Urban Water Resource and Environment, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, China
b
ARTICLE INFO
ABSTRACT
Keywords: Polypyrrole Hybrid supercapacitor MoO2 MnO N-doped carbon
Herein, a general strategy is provided to construct N-doped carbon-confined MoO2 and MnO by the hydrothermal process, polymerization process and carbonation process, which serves as negative electrode and positive electrode for hybrid supercapacitors. The N-doped carbon as shell can provide fast electron pathways and short ion diffusion paths, resulting in the improved performances at the high rates. Consequently, the obtained N-doped carbon-confined MoO2 and MnO possess the good electrochemical performances, including high specific capacitance, excellent cycling stability and good rate capability. Furthermore, the as-fabricated hybrid supercapacitor using N-doped carbon-confined MoO2 and MnO shows a high energy density up to 44.82 Wh kg−1 at a power density of 900 W kg−1, as well as good cycling performance. This work may provide a general strategy for constructing high-performance energy storage devices.
1. Introduction As the new emerging energy devices, supercapacitors are the promising candidates due to the high power density, fast charge-discharge rate and environmental friendliness [1–3]. However, the relatively low energy density of supercapacitors greatly limits the practical applications [4,5]. It is well known that the voltage window (V) and specific capacitance (C) are the two key parameters for supercapacitors, according to equation E = 1/2 CV2 [6]. Thus, it is a general strategy to construct hybrid supercapacitors by coupling two kinds of electrode (positive electrode and negative electrode), which could greatly enlarge the voltage window and then improve the energy density [7,8]. Consequently, it is urgently to design and fabrication of advanced materials for both positive electrode and negative electrode. As for the positive electrodes, the limited potential window (usually less than 0.6 V) for most transition metal oxides (TMOs, like CoOx, NiO, NiCo2O4) could not fully enlarge the voltage windows for ASCs [9–12]. As one kind of TMOs, manganese oxides (MnO, MnO2 et al.) can provide a wider potential window more than 0.8 V, which shows great potentials as positive electrode for ASCs [13,14]. Among these manganese oxides, MnO is the promising candidate because of the redox reaction [14,15]. However, MnO still suffers from the limited cyclic
∗
stability and poor conductivity [16,17]. As for the negative electrodes, the limited specific capacitances greatly constraints the electrochemical performances of hybrid supercapacitors, according to equation 1/ C = 1/Cp + 1/Cn [18]. Commonly used carbon materials often show the low specific capacity due to the charge stored on the electric-double layer capacitance (EDLC) mechanism [19,20]. In this regard, many molybdenum oxides as anode materials, such as MoO3, MoO2 and MoO3-x could be the promising alternatives, because they show better electrochemical performances than that of carbon materials [21,22]. However, the low electronic conductivity of molybdenum oxides profoundly affects their electrochemical performance [23,24]. Even worse, these molybdenum oxides often suffer from the molybdenum oxides and large volume changes during cycling tests, and thus resulting in inferior rate capability and poor cycling stability [25,26]. To resolve this challenge for MnO and molybdenum oxides, one effective strategy is to combine these transition metal materials with carbonaceous materials [21,27–30]. According to previously reported researches, it has been well demonstrated that metal oxides are subtly combined with carbon materials can not only improve the conductivity, but also maintain the structural integrity, thus resulting in improved rate capability and cycling stability [27–30]. Among various carbon materials, nitrogen-doped carbon has attracted particular research interest. The
Corresponding author. E-mail address: [email protected] (J. Qi).
https://doi.org/10.1016/j.vacuum.2019.04.033 Received 28 March 2019; Received in revised form 14 April 2019; Accepted 15 April 2019 Available online 16 April 2019 0042-207X/ © 2019 Elsevier Ltd. All rights reserved.
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nitrogen doping in carbon materials has been proven that can create defect sites and enhance the electrical conductivity and wettability in electrolyte [31–33]. Stimulated by these considerations, we design and construct of polypyrrole-derived N-doped carbon-encapsulated MoO2 and MnO with yolk-shelled nanostructures (MoO2@NCT and MnO@NCT) for hybrid supercapacitors. The whole synthesis step consists of the by the hydrothermal process, polymerization process and carbonation process. The N-doped carbon as shell could provide fast electron pathways and short ion diffusion paths, resulting in the improved performances at the high rates. Further, the yolk-shelled nanostructures in MoO2 and MnO could effectively relieve volume variation during cycling tests, leading to good cycling stability. As a result of the structural, morphological and compositional advantages, the as-synthesized yolk-shelled MoO2@ NCT and MnO@NCT display excellent electrochemical performances. In addition, the as-fabricated hybrid supercapacitor using MoO2@NCT as negative electrode and MnO@NCT as positive electrode can achieve a high energy density of 44.82 Wh kg−1 at a high power density of 900 W kg−1 and desirable electrochemical stability.
SEM, Helios Nanolab 600i), transmission electron microscopy (TEM, Tecnai G2 F30) equipped with an energy dispersive X-ray spectrometer (EDS), X-ray diffraction (XRD, D8 Advance) and X-ray photoelectron spectroscopy (XPS, Thermo Fisher). Thermogravimetric analysis was investigated by a thermal analyzer (TGA) under Air flow with a heating rate of 10 °C min−1. 2.3. Electrochemical measurements To prepare the working electrode, a slurry was prepared by mixing active materials, acetylene black and PVDF (mass ratio of 8:1:1). Then, the mixed slurry was coated onto a carbon cloth with a mass loading of 2 mg cm−2, and dried at 80 °C overnight. All electrochemical performances of obtained samples were investigated by electrochemical workstations (CHI 760E). The electrochemical performance was test in the three-electrode configuration using 1 M Na2SO4 as electrolyte, where samples on carbon cloth, Pt and saturated calomel electrode (SCE) were used as working, counter and reference electrodes. In addition, a hybrid supercapacitor was constructed by MnO@NCT as positive electrode and MoO2@NCT as negative electrode. The specific capacity (C, F g−1), energy density (E, Wh kg−1) and power density (P, W kg−1) were calculated [34–37], as following:
2. Experimental 2.1. Materials synthesis
C = IΔt/mV
2.1.1. Synthesis of the MoO3 nanobelts Firstly, a mixed solution of 60 mL deionized water, 1.21 g Na2MoO4·2H2O and 0.6 g NaCl was adjusted to pH 1.0. Then, the mixed solution was transferred into 100 mL Teflon-lined stainless-steel autoclave and maintained at 180 °C/24 h. The synthesized products were collected and washed for several times, and dried under vacuum at 60 °C/24 h.
E = 1/2CV2 P = EΔt where I, V, m and Δt mean the specific capacitance, current density, potential window, mass loading and discharging time. 3. Results and discussion
2.1.2. Synthesis of the MnO2 nanowires In a typical procedure, a mixed solution of 75 mL deionized water, 0.7 KMnO4 and 3.6 mL concentrated HCl was transferred into 100 mL Teflon-lined stainless-steel autoclave and maintained at 140 °C/13 h. The synthesized products were collected and washed for several times, and dried under vacuum at 60 °C/24 h.
To better understanding the pyrolysis of PPy, TGA analysis was conducted to investigate the thermal properties of pure MoO3 and PPy, as shown in Fig. 1a. It can be found that the main weight loss for pure MoO3 occurs in 700–800 °C, which is mainly attributed to the decomposition of MoO3. As for pure PPy, the weight loss below 150 °C is due to the vaporization of water/moisture molecules, and the main weigh loss occurs in 300–800 °C, owing to the complete decomposition of PPy and the formation of the nitrogen-doped carbon tubes. Consequently, different pyrolysis temperature (600, 700 and 800 °C) was chosen to investigate the mutual reaction of MoO3 and PPy. Fig. 1b shows the XRD patterns of MoO3 and MoO3@PPy. Obviously, all the diffraction patterns in both samples could be indexed to the α-MoO3 phase of the hexagonal crystals (JCPDS no. 05–0508). And it also demonstrated that there are no crystalline changes for the MoO3 upon the in situ polymerization of PPy. Fig. 1c and d shows the XRD patterns of MoO2@ NCT-600–800 and MoO3-700. It can be found that the XRD pattern of MoO3-700 is indexed to MoO3 and MoO2, suggesting the partially reduction of MoO3 after annealing at 700 °C. However, as for MoO2@ NCT-600 and MoO2@NCT-700, all the peaks could be indexed to MoO2 (JCPDS no. 32–0671), suggesting the completely reduction of MoO3. As for MoO2@NCT-800, it mainly consists of MoO2 and Mo0.42C0.58 (JCPDS no. 36–0863) phases. There are no obvious peaks for carbon materials in MoO2@NCT-600–800, which may be due to the relatively low amounts of the nitrogen-doped carbon tubes. Meanwhile, XPS measurements are employed to further investigate the composition of MoO2@NCT-700, as shown in Fig. 1e–h. As for Mo 3d spectrum in Fig. 1e, it could be deconvoluted into four peaks, where the peaks at 229.7 and 232.9 eV were corresponded to Mo 3d5/2 and Mo 3d3/2 of Mo4+, and the peaks at 232.3 and 235.8 eV were indexed to Mo 3d5/2 and Mo 3d3/2 of Mo6+ due to the MoO2 surface oxidation under ambient conditions [38,39]. From the O 1s spectrum (Fig. 1f), the characteristic peaks at 530.7 eV, 532 eV, and 533.3 eV belong to the Mo-O, C-O, and C-OH bonds, respectively [21,40]. As for C 1s spectrum
2.1.3. Synthesis of the MoO2@NCT Firstly, MoO3@PPy was synthesized by in situ oxidative polymerization process. 0.42 g MoO3 was dispersed in 160 mL deionized water, and 1.6 mL pyrrole was added by vigorous stirring at 0 ± 2 °C for 30 min. Then a mixed solution of 3.2 g ammonium persulfate and 80 mL deionized water was added dropwise to the above suspension. The reaction mixture was stirred for 10, 20, 30 and 60 min to obtain different content of PPy (denoted as MoO3@PPy-10–60 for convenience). The reaction products were filtered, washed for several times, and dried overnight. Secondly, to obtain MoO2@NCT, as-synthesized MoO3@PPy was annealed at Ar atmosphere at 600, 700 and 800 °C for 2 h (denoted as MoO2@NCT-600–800 for convenience). For comparison, pure MoO3 was also annealed at 700 for 2 h (denoted as MoO3-700 for convenience). 2.1.4. Synthesis of the MnO@NCT Firstly, the synthesis process of MnO2@PPy was similar to that of MoO3@PPy, and the reaction mixture was stirred for 30 min. Secondly, to obtain MnO@NCT, as-synthesized MnO2@PPy was annealed at Ar atmosphere at 700 °C for 2 h (denoted as MnO@NCT for convenience). Similarly, the pure MnO2 sample was annealed at Ar atmosphere at 700 °C for 2 h to obtain MnO sample. 2.2. Materials characterization The morphology, structure, phase and chemical valence states of samples were investigated by field-emission electron microscopy (FE180
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Fig. 1. (a) TGA curves of pure MoO3 and PPy. (b) XRD patterns of MoO3 and MoO3@PPy, (c) XRD patterns of MoO2@NCT-600, MoO2@NCT-700 and MoO3-700, (d) XRD pattern of MoO2@NCT-800. XPS spectra of (e) Mo 3d, (f) O 1s, (g) C 1s and (h) N 1s in MoO2@NCT-700.
(Fig. 2g), it can be deconvoluted into C-C, C-N, C-O, and O=C-N bonds at 284.6, 285.7, 286.4 and 287.7 eV, respectively [41].According to previously reported researches [42,43], the functional groups in NCT may improve the electrochemical reaction kinetics. As for N 1s, three peaks could be indexed to pyridinic N (398.5 eV), pyrrolic N (400.3 eV), and graphitic N (401.3 eV), respectively [44]. It indicates the successful incorporation of nitrogen into graphitic carbon. According to previously reported researches [45,46], N doped in carbon shells could improve the storage capacitance and rate capability. Fig. 2 shows the morphological characterizations of MoO3, MoO3@ PPy, MoO3-700 and MoO2@NCT. As shown in Fig. 2a, as-synthesized MoO3 possess a 1D nanobelt structure. After coating PPy as shell, the surface of MoO3@PPy is rougher than that of pure MoO3 (Fig. 2b). For comparison, we also synthesized different content of PPy on MoO3 nanobelts by controlling the reaction time, as shown in Fig. S1. Obviously, it can be found that with the increase of reaction time, the surface of MoO3@PPy becomes much rougher, indicating that more PPy is coated on the surface of MoO3 nanobelts. After annealing in 700 °C at the Ar atmosphere, it can be found that the MoO3-700 becomes more bulky, as shown in Fig. 2c. However, after coating PPy as shell, MoO2@NCT maintains the nanobelt morphologies after high-
temperature pyrolysis (Fig. 2d). Further, it is worth noting that the solid MoO3 nanobelt core converts into hierarchically assembled MoO2 nanosheets. Fig. S2 shows the SEM images of MoO2@NCT-10–60, where all obtained samples maintain the nanobelt morphologies after hightemperature pyrolysis. In addition, we also investigated the effect of pyrolysis temperature on the MoO2@NCT, as shown in Fig. S3. It can be found that MoO2@NCT-800 are aggregated with each other, which may be due to the low melting points of MoO3 [47]. The TEM and high-resolution TEM (HRTEM) images of MoO3 and MoO2@NCT-700 are shown in Fig. 3. As shown in Fig. 3a, MoO3 shows the nanobelt structure with a smooth surface. The lattice spacing of ∼0.381 nm could be assigned to the (110) plane of α-MoO3 (Fig. 3b). After the pyrolysis process, the overall nanobelt morphology of MoO2@ NCT-700 (Fig. 3c) is well maintained with a width of 200–300 nm. During the pyrolysis process, the PPy shell changes into NCT hollow shell and the shell thickness of the NCT was about 10 nm (Fig. 3d). At the same time, the solid MoO3 nanobelt core has been converted into assembled MoO2 nanosheets. The elemental mappings in Fig. 3e show that the element C and N are mainly distributed in hollow carbon walls, while Mo and O show a uniform distribution in the MoO2 core. It clearly indicates that N is successfully doped into the carbon shells.
Fig. 2. SEM images of (a) MoO3, (b) MoO3@PPy, (c) MoO3-700 and (d) MoO2@NCT-700. 181
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Fig. 3. (a) TEM and (b) HRTEM images of MoO3 nanobelts, (c) TEM, (d) HRTEM images and corresponding EDS element mapping images of Mo, O, C, and N of MoO2@NCT-700.
Fig. 4. SEM images of (a) MnO2 nanowire and (b) MnO@NCT. (c) TEM and (d) HRTEM images of hollow MnO2 nanowire, (e) TEM, (f) HRTEM images and corresponding EDS element mapping images of Mn, O, C, and N of MnO@NCT.
and are shown in Fig. S7. From the GCD curves at 2 A g−1, the optimum parameters for MoO2@NCT are that the oxidative polymerization process lasts 30 min and pyrolysis temperature is 700 °C. According to the above results, the following discussion is mainly focused on the MoO2@ NCT obtained at 700 °C with 30 min polymerization process. Fig. 5a shows the CV curves of MoO3, MoO3@PPy, MoO3-700 and MoO2@NCT with voltage window from −1.0 to 0.0 V (vs. SCE) at 100 mV s−1. Notably, MoO2@NCT shows the largest enclosed CV curve area among four samples, demonstrating the highest chare-storage capacitance. Fig. 5b shows the CV curves of MoO2@NCT at various scan rates, and all of the CV curves exhibit an approximately rectangular shape without any obvious redox peaks, indicating the fast, reversible successive surface redox reactions [48–50]. In addition, Fig. 5c shows GCD curves for the as-fabricated MoO2@NCT at different current densities. From calculations of the discharge time from the GCD curves, the specific capacitances for all the samples were calculated and are shown in Fig. 5d. A desirable specific capacitance of 254.8 F g−1 is obtained for the MoO2@NCT electrode at a current density of 2 A g−1, compared to 109.5, 98.6 and 148.7 F g−1 for the pure MoO3, MoO3@PPy and MoO3700 electrodes, respectively. Even at 20 A g−1, the specific capacitance of the MoO2@NCT still remains at 124.9 F g−1, demonstrating the good
At the same time, we also synthesized MnO@NCT by the similar process, as shown in Fig. 4. It can be found that pure MnO2 shows a regular nanowire structure with an average dimeter of about 30 nm (Fig. 4a), while MnO@NCT becomes much thicker after the growth of NCT as shell (Fig. 4b). The XRD patterns in Fig. S5 show that both samples match well MnO2 (JCPDS no. 44–0141) and MnO (JCPDS no. 75–0626). Further, the TEM image in Fig. 4c shows the hollow morphology of MnO2 nanowire, and the lattice fringe of ∼0.692 nm in Fig. 4d corresponds to the (110) plane of MnO2. Fig. 4e shows the TEM image of MnO@NCT. During the high-temperature pyrolysis, the solid MnO2 nanowire core converts into hierarchically assembled MnO nanosheets, which is encapsulated by NCT shells. And the shell thickness of the NCT was estimated to be ∼10 nm (Fig. 4f). In addition, the elemental mappings of MnO@NCT in Fig. 4g confirm the coexistence and the dispersion of Mn, O, C and N elements. We evaluate the electrochemical performances by testing cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) measurements in a three-electrode cell using 1 M Na2SO4 as electrolyte. In order to investigate the effect of PPy content and pyrolysis temperature, electrochemical performances were evaluated with different PPy content and pyrolysis temperature, and the corresponding GCD analysis 182
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Fig. 5. (a) CV curves of MoO3, MoO3@PPy, MoO3-700 and MoO2@NCT at 100 mV s−1. (b) CV and (c) GCD curves of MoO2@NCT. (d) The corresponding specific capacitance, (e) Nyquist plots and (f) cycling performances of MoO3 and MoO2@NCT. (g) CV curves of MnO2, MnO, MnO2@PPy and MnO@NCT at 100 mV s−1. (h) CV curves of MnO@NCT. (i) The corresponding specific capacitance of MnO2, MnO, MnO2@PPy and MnO@NCT.
delivers a high capacitance up to 306.4 F g−1 at 2 A g−1, and still holds a capacitance up to 191.2 F g−1 at 20 A g−1, confirming the good rate capability. Additionally, the synthesized MnO@NCT electrode exhibits a capacitance retention of about 94.9% after 10000 cycles (Fig. S10). Therefore, we believe that the as-synthesized MnO@NCT electrode is an excellent positive electrode for the design of hybrid supercapacitors. In addition, we fabricated hybrid supercapacitors by using MnO@ NCT as positive electrode and MoO2@NCT as negative electrode. Fig. 6a shows the CV curves of MnO@NCT//MoO2@NCT in different potential windows at 100 mV s−1. It can be found that the potential window of 0–1.8 V is the most suitable potential window. Fig. 6b and c shows the CV and GCD curves of MnO@NCT//MoO2@NCT at different scan rates and different current densities. As shown in Fig. 6b, the CV curves maintain the quasi-rectangular shapes as the scan rate increases from 10 to 100 mV s−1, implying the ideal capacitive behaviors and high rate capabilities. The almost symmetric sharp of GCD curves in Fig. 6c demonstrates the good coulombic efficiency and excellent reversibility of MnO@NCT//MoO2@NCT. Based on the GCD curves, the calculated specific capacitance of MnO@NCT//MoO2@NCT is shown in Fig. 6d. The MnO@NCT//MoO2@NCT device exhibited maximum capacitance of 99.6 F g−1 at 1 A g−1. As expected, a remarkable rate capability has also been achieved by the MnO@NCT//MoO2@NCT, which retained about 56.2% of the initial capacitance as the current density increased from 1 to 10 A g−1. Fig. 6e shows the cycling performance of MnO@NCT//MoO2@NCT device tested by GCD at 5 A g−1. After 10000 cycling tests, the capacitance retains about 92.5%, suggesting the good cycling stability. Fig. 6f shows the Ragone plot of
rate capability. Consequently, it is apparent that incorporation of NCT as shell is beneficial and effective for improving the capacitive performance. Further, the enhanced electron transfer from both highly conductive NCT and MoO2 is beneficial for improved rate capability. As for the long term cycle stability, as-synthesized MoO2@NCT achieved a good cycling stability with 96.3% capacitance retention after 10000 cycles (Fig. 5e). Electrochemical impedance spectrum (EIS) tests were carried out to investigate the electrochemical performances of obtained samples, as shown in Fig. 5f. In the low-frequency region, the slope of the MoO2@NCT samples is much closer to 90° than that of pure MoO3 sample, suggesting the better capacitive behavior and faster ion transportation [51,52]. The charge transfer resistance (Rct) could be estimated by the semicircle in the high-frequency region [53]. It can be found that MoO2@NCT shows a Rct value of about 0.4 Ω, which is much lower than that of pure MoO3 (about 0.9 Ω). It indicates that MoO2@ NCT faster electron transfer reaction kinetics [53,54], which may be due to the combination with conductive NCT layers. The electrochemical performances of MnO@NCT electrode were measured in a three-electrode cell, as shown in Figs. 5g–6i. As shown in Fig. 5g, the CV integral area of MnO@NCT electrode is the largest among four samples at 100 mV s−1, suggesting the superior capacitance. As for MnO@NCT, all CV curves in Fig. 5h maintained a quasirectangular and symmetric shape, suggesting the fast reversible faradaic reactions. Further, as the scan rate increases from 2 to 50 mV s−1, the CV sharp shows no obvious changes, indicates the excellent rate capability. Based on the discharge times, the specific capacitances of obtained samples are shown in Fig. 5i. The as-fabricated MnO@NCT 183
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Fig. 6. (a) CV curves of the obtained hybrid supercapacitor at different voltage windows. (b) CV and (c) GCD curves and (d) the corresponding specific capacitance of the obtained hybrid supercapacitor. (e) The cycling performance of the obtained hybrid supercapacitor tested using GCD at 5 A g−1. (f) Ragone plot of the obtained hybrid supercapacitor.
MnO@NCT//MoO2@NCT device. The MnO@NCT//MoO2@NCT device could achieve a high energy density of 44.82 Wh kg−1 at a power density of 900 W kg−1, which is better than previously reported device, such as MoO3@CNT//MnO2@CNT (27.8 Wh kg−1 at the power density of 524 W kg−1) [21], β-MnO2//activated graphene oxide (40.4 Wh kg−1 at the power density of 300 W kg−1) [36], CNTs-MnO2//MoO3PPy (21.03 Wh kg−1 at the power density of 220 W kg−1) [54], and RGO/MnO2//RGO/MoO3 (42.6 Wh kg−1 at the power density of 276 W kg−1) [55]. The good electrochemical properties of MnO@NCT and MoO2@NCT can be contributed to following advantages. (1) One-dimensional nanostructures in MnO@NCT and MoO2@NCT with a high aspect ratio could provide efficient electron and ion transport. (2) The well-defined inner voids in yolk-shelled nanostructures could not only serve as “electrolyte container” to create sufficient electron paths, but also relax the electrode strains during cycles. (3) The N-doped carbon as shell could not only improve the electrical conductivity, but also buffer the volume changes during cycling tests.
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4. Conclusion In summary, we have successfully developed a simple strategy to fabricate the N-doped carbon-confined MoO2 and MnO with yolkshelled nanostructures for hybrid supercapacitors. Based on SEM, TEM and Raman results, N-doped carbon coating has been successfully prepared on MoO2 and MnO. The N-doped carbon as shell could not only effectively improve the electron transfer, but also maintain the structural integrity. Consequently, electrochemical tests demonstrated the yolk-shelled MoO2 and MnO showed the enhanced electrochemical performances, when compared to pristine MoO3 and MnO2. Further, the as-fabricated hybrid supercapacitor of MnO@NCT//MoO2@NCT showed a high energy density up to 44.82 Wh kg−1 at a power density of 900 W kg−1, as well as good cycling performance. Acknowledgements The support from the National Natural Science Foundation of China 184
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