Hao Wang,
Qinglan Zhao,
Xianyou Wang*,
Youwei Zhang,
Jiao Gao,
Yanqing Fu,
Xiukang Yang and
Hongbo Shu*
Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, Hunan Province Key Laboratory of Electrochemical Energy Storage and Conversion, School of Chemistry, Xiangtan University, Xiangtan 411105, Hunan, China. E-mail: wxianyou@yahoo.com; hongboshuxtu@gmail.com; Fax: +86 731 58292061; Tel: +86 731 58292060
First published on 8th September 2014
The rational design and facile synthesis of transition metal oxides are necessary to improve their application in supercapacitors. Herein three kinds of hierarchical mesoporous structured transition metal oxides, which are composed of a β-MnO2 nanorod core and one of three different nanosheet hybrid (Ni, Co, Mn) oxide shells, are facilely synthesized via a novel in situ nucleation and growth of transition metal oxides on the surface of the β-MnO2 nanorods. The crystallographic analyses demonstrated that the three kinds of hybrid oxide shells consisted of cobalt manganese oxide (CMO), nickel manganese oxide (NMO), and nickel cobalt manganese oxide (NCMO). These transition metal oxides are evaluated as electrodes for high performance supercapacitors (SCs). The results reveal that β-MnO2@CMO exhibits a good rate capability of 35% capacity retention even at 20 A g−1, while β-MnO2@NMO displays a high pseudocapacitance of 560 F g−1 at 1 A g−1. However, β-MnO2@NCMO combined the advantages of both β-MnO2@CMO and β-MnO2@NMO, and exhibits a high specific capacitance of 675 F g−1 at 1 A g−1 with excellent rate performance (about 30% capacity retention at 20 A g−1) and cycling stability (83% capacity retention after 3000 cycles). The improved electrochemical performance can be attributed to the unique hierarchical architecture and the synergistic effect of different components.
Among transition metal oxides, Ni, Co, and Mn oxides, have been extensively studied as promising candidates for SCs because of their variety of oxidation states for charge transfer and high mass density.6,7 For example, MnO2 has a high theoretical specific capacitance of about 1370 F g−1, Co3O4 has a theoretical specific capacitance of about 890 F g−1.8 However, the NiO2 and Co3O4 suffer from poor capacity retention and rate capacity, and the specific capacitance of MnO2 is still limited due to the low conductivity (10−5 to 10−6 S cm−1) and low diffusion coefficient of inserted cations.9,10 Undoubtedly, it has been reported that the rational design of transition metal oxide composite can provide a promising solution to achieve excellent supercapacitive performance due to the synergistic effects of different components, which can effectively improve the electrochemical performances of the materials.11 Thus, to address these problems, one strategy is to design MnO2-based composite oxides with highly conductive transition metal compounds. For example, NiCo2O4@MnO2 core–shell nanowire arrays reported by Xu et al. present an improved capacitance (3.31 F cm−2 at 2 mA cm−2).12 However, the MnO2 is generally designed as the shell among most of the MnO2-based composite transition metal oxides, which is still low conductivity.13–15 Even though, many scholars try to prepare MnO2 as the core of the composites, the shell covered on the surface of the MnO2 may be not uniform.16,17
In this work, a facile hydrothermal method has been designed to synthesize three hierarchical nanocomposites with a β-MnO2 nanorod core and hybrid nanosheet shell. As KMnO4 is reduced by low valence metal ions (Ni2+, Co2+) under the hydrothermal process, the resultant MnO2 and Ni–Co hydroxides are in situ nucleation on the surface of the β-MnO2 nanorods prepared as the template. The hybrid shell offers several advantages: one is that the β-MnO2, with stable crystal structure in both nanosheet shells and cores can form a stable framework owing to its high surface areas and interconnected porous nanostructures which could better accommodate structural changes;18,19 the other is that the NiO2, Co3O4 or NiCo2O4 embedded in the framework can improve the conductivity of the composites. Besides, the morphologies and electrochemical properties of the as-prepared three novel composites are discussed in details.
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Fig. 1 Schematic illustration of the formation of nanoflaky Ni, Co, Mn composite oxides in situ growing on the surface of the MnO2 nanorod. |
The morphology of the as-synthesized samples was examined with scanning electron microscopy (SEM). As shown in Fig. 2a–c, Co, Ni and Co–Ni hydroxides grow uniformly on the surface of β-MnO2 templates in company with MnO2. The yield of the shell is so high that the β-MnO2 nanorods with a smooth surface are not observed in the nanocomposites. Obviously, among all composite oxides, the slice of the hybrid shell for β-MnO2@CMO-precursor is the thickest and tightest. Besides, for β-MnO2@NMO-precursor and β-MnO2@NCMO-precursor, the flower texture layer with highly porous structure is uniformly deposited on the surface of the β-MnO2 nanorods. It also can be seen that the diameter of the β-MnO2@CMO-precursor is the least, the β-MnO2@NMO-precursor has the most abundant pores, and β-MnO2@NCMO-precursor shows the largest pore size and diameter. In addition, these nanoflaky shells are not changed after high temperature annealing as shown in Fig. 2d–f. The stability of those architectures can probably be attributed to the crystal structure of β-MnO2, which has the most stable structure among all crystals of MnO2. As a typical example, the low-magnification SEM image of β-MnO2@CMO is displayed in Fig. 2g, it can be seen that the nanocomposites represent uniform distribution. The β-MnO2 nanorod templates are also displayed in Fig. 2h, where the nanorods show a smooth surface and uniform diameter. The porous structures are characterized by BET measurements. As shown in Fig. 2i, the nitrogen adsorption–desorption isotherms of all prepared samples are the combination of type II and IV according to the IUPAC classification. The β-MnO2@NMO possesses the highest BET surface area of 130.9 m2 g−1 with a pore volume of 0.293 cm3 g−1. The specific surface area and pore volume of β-MnO2@NCMO are 102 m2 g−1 and 0.216 cm3 g−1, respectively. Besides, the β-MnO2@CMO also possesses a high BET surface area are 88.3 m2 g−1 with a pore volume of 0.185 cm3 g−1. The BJH pore size distributions of β-MnO2@CMO, β-MnO2@NMO and β-MnO2@NCMO are shown in Fig. 2i insert. It can be found that the pore sizes of β-MnO2@CMO, β-MnO2@NMO and β-MnO2@NCMO are concentrated at 7.0 nm, 5.5 nm and 6.0 nm, respectively. Obviously, the pore size of β-MnO2@NCMO is intermediate between those of β-MnO2@CMO and β-MnO2@NMO.
The hierarchical hybrid structure is further illustrated from the TEM images from Fig. 3. The low-magnification TEM images show that the surface of β-MnO2 nanorods is uniformly covered with Ni, Co, Mn composite oxide nanosheets and formed a core–shell structure. In addition, the thickness of the different nanosheets and layers can be contrasted in the TEM images (Fig. 3b, e and h). It can be found that the layers of the β-MnO2@CMO, β-MnO2@NMO and β-MnO2@NCMO are about 75 nm, 95 nm and 97 nm thick, respectively. Besides, the slice of the β-MnO2@NMO is thinner than that of two others. The HR-TEM is used to investigate the lattice of surface layer hybrid metal oxide. The lattice spacing of 0.467 nm and 0.31 nm were observed in Fig. 3c, which are in a good agreement with the theoretical interplanar spacing of Co3O4 (111) and β-MnO2 (110) planes. As shown in Fig. 3f and i, the interplanar spacing of 0.31 nm corresponds well to the (110) plane of β-MnO2, the 0.48 nm and 0.47 nm can correspond to the (003) plane of NiO2 and (111) plane of NiCo2O4, respectively. Therefore, it can be concluded that the shells of those nanocomposites are consisted of β-MnO2 and Ni/Co composite oxides.
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Fig. 3 Low-magnification and high-magnification TEM images of (a–c) β-MnO2@CMO; (d–f) β-MnO2@NMO; (g–i) β-MnO2@NCMO. |
The crystallographic structures of the as-prepared β-MnO2 nanorod, β-MnO2@NCMO-precursor and crystalline β-MnO2@NCMO nanocomposite obtained by annealing the precursors at 350 °C for 2 h were verified by XRD, as shown in Fig. 4a. The diffraction peaks of the MnO2 nanorod represent all of the characteristic peaks of pure pyrolusite-type MnO2 (JCPDS 24-0735) phase without any impurities. And the three broad diffraction peaks are located at around 29°, 37°and 57° in the XRD pattern of MnO2, which correspond to the diffraction bands (110), (101) and (211) of β-MnO2, respectively. Besides, the diffraction peaks of β-MnO2@NCMO-precursor can be well indexed of (Co, Ni)O(OH) (JCPDS 29-0491) apart from the dominant peak of β-MnO2. There is no contaminant detected, indicating that the precursor is composed of β-MnO2 and Ni–Co hydroxide. The patterns of β-MnO2@NCMO reveal that the nickel cobalt oxide (NiCo2O4) (JCPDS 20-0781) can be found after the precursor was calcined at 350 °C for 2 h. Furthermore, Fig. 4b shows that all the diffraction peaks are assigned to pure cobalt oxide (Co3O4) (JCPDS 43-1003) and nickel oxide (NiO2) (JCPDS 85-1977) without any impurity apart from β-MnO2.
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Fig. 4 XRD patterns of (a) β-MnO2, β-MnO2@NCMO-precursor and crystalline β-MnO2@NCMO nanocomposites; (b) β-MnO2, β-MnO2@CMO and β-MnO2@NMO nanocomposites. |
Galvanostatic charge-discharge measurements were conducted between −0.45 to 0.5 V (vs. Hg/HgO) at different current densities ranging from 1 to 20 A g−1 to further evaluate the properties of the nanocomposites. As shown in Fig. 5b–d, it can be observed that there are two voltage plateaus at around −0.35 and 0.25. The specific capacitance is calculated by the formula, , where I is the discharge current, Δt is the discharge time, ΔV is the voltage range and m is the mass of the active material. The specific capacitance of β-MnO2@CMO is as high as 437, 395, 250, 200 and 149 F g−1; for β-MnO2@NMO is about 560, 510, 220, 132 and 70 F g−1, and for β-MnO2@NCMO is about 675, 593, 357, 254 and 190 F g−1 at the current density of 1, 2, 5, 10, 20 A g−1, respectively. The specific capacitances of the three nanocomposite electrodes derived from the discharging curves at different current densities were compared, as shown in Fig. 5e. The β-MnO2@CMO electrode delivered a specific capacitance of 437 F g−1 at the current density of 1 A g−1, which is lower than those of β-MnO2@NMO and β-MnO2@NCMO electrode. However, at the high current density of 20 A g−1, the β-MnO2@CMO electrode still delivered a high specific capacitance of 149 F g−1, indicating much better rate capability compared to the β-MnO2@NMO electrode which has a high specific capacitance of 560 F g−1 but decay fast to merely 70 F g−1 when the current density increased from 1 A g−1 to 20 A g−1. It also can be seen clearly that the β-MnO2@NCMO electrode exhibits best pseudocapacitance and rate capability among three nanocomposites.
The cycling stability of the nanocomposites and β-MnO2 nanorod electrodes are evaluated by the repeated charging-discharging measurement at constant current density of 5 A g−1, as shown in Fig. 5f. A significant specific capacitance loss can be seen for β-MnO2 nanorod electrode that only 51% of the initial capacitance is retained after 3000 cycles. While for β-MnO2@NMO and β-MnO2@NCMO electrodes, the specific capacitances are about 220 and 360 F g−1 in the 1st cycle, and remained 173 and 305 F g−1 after 3000 cycles, the corresponding capacitance loss is 21% and 17%, respectively. In addition, the cycling stability of β-MnO2@CMO is slightly better than that of the other two electrodes, which the capacitance loss is about 14% after 3000 cycles. It can be known that the nanocomposite electrodes show a much better cycling stability than pure β-MnO2.
The results reveal that β-MnO2@CMO exhibits a good rate capability, while β-MnO2@NMO displays a high pseudocapacitance. These differences in performances can be mainly attributed to the morphological characteristics of the mesoporous and the nanosheets architectures, as well as the composition of constituents. Evidently, the mesoporous and nanosheets within those architectures can provide more active sites and path for efficient electrolyte ions transportation.24 Therefore, the fact that β-MnO2@NMO displays a much higher pseudocapacitance can be dependent on its much higher BET surface area than β-MnO2@CMO. Besides, compared with the other two nanocomposites electrodes, β-MnO2@CMO exhibits the best rate capability. It may be attributed to its largest BJH pore sizes which can facilitate the diffusion of ions and improve chare accumulation. Moreover, β-MnO2@NCMO combines the advantages of both β-MnO2@CMO and β-MnO2@NMO, exhibiting a high specific capacitance and cycling stability. Those good performances can be attributed to not only full utilization of individual constituents, but also a strong synergistic effect of different components. However, since the pure transition metal oxide composites still suffer from low conductivity, some efforts remain to be done to further improve the cycling performance and depress the capacity decay of β-MnO2@NCMO, such as combination with high conductivity graphene, carbon or conducting polymer.
Fig. 6a shows the CV curves of β-MnO2@NCMO at various scan rates of 5–40 mV s−1. It can be found that with the increase of the scan rate, the shapes of CV curves show slight distortion and the areas surrounded by the CV curves are not significantly influenced, implying good rate capability of the β-MnO2@NCMO electrode. Electrochemical impedance spectroscopy (EIS) was further employed to detect the properties of ion diffusion and electron transfer in the three kinds of nanocomposite electrodes in the frequency range of 100 kHz to 10 mHz with an AC voltage of 5 mV. The Nyquist plots are shown in Fig. 6b and the inset shows the expanded plots at high frequency region. The impedance spectra of three electrodes are almost similar in form with a semicircle at the high frequency and an inclined line following at the low frequency, which corresponds to the interfacial charge-transfer impendence and the diffusive impendence of OH− ion within the electrode. The slopes of the semicircle at high frequency of the three electrodes are also almost the same, indicating nearly equal value of charge-transfer impendence. The diffusive impendence of OH− ion within the β-MnO2@CMO and β-MnO2@NCMO are much lower than that of β-MnO2@NMO.
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Fig. 6 (a) The CV curves of the β-MnO2@NCMO at different scan rates; (b) Nyquist plots for the three nanocomposite electrodes, the inset is the expanded plots at high frequency region. |
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