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Rational design of microsphere and microcube MnCO3@MnO2 heterostructures for supercapacitor electrodes
Journal of Power Sources 353 (2017) 202e209
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Rational design of microsphere and microcube MnCO3@MnO2 heterostructures for supercapacitor electrodes Hao Chen a, 1, Zhe Yan b, 1, Xiao Ying Liu a, Xiao Long Guo a, Yu Xin Zhang a, *, Zong-Huai Liu b, ** a
State Key Laboratory of Mechanical Transmissions, College of Material Science and Engineering, National Key Laboratory of Fundamental Science of Micro/ Nano-Devices and System Technology, Chongqing University, Chongqing 400044, PR China Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, and School of Materials Science and Engineering, Shaanxi Normal University, Xi'an 710062, PR China
b
h i g h l i g h t s Self-assembly and oxidation mechanism for MnCO3@MnO2 are proposed. MnCO3@MnO2 heterostructures display good electrochemical properties. The synergistic effect results in excellent electrochemical performances.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 January 2017 Received in revised form 22 March 2017 Accepted 6 April 2017 Available online 14 April 2017
MnCO3 microsphere@MnO2 and MnCO3 microcube@MnO2 heterostructures have been synthesized for supercapacitor electrodes through self-assembly and oxidation process. The morphologies of MnCO3 are effectively controlled by adding (NH4)2SO4 solution. The MnCO3 microsphere@MnO2 and MnCO3 microcube@MnO2 heterostructures display high capacitances of 363 F g1 and 290 F g1 at 1 A g1, respectively. In addition, the assembled asymmetric supercapacitor based on MnCO3 microsphere@MnO2 and active grapheme (AG) exhibits a high energy density of 27.4 Wh kg1 at a power density of 271.7 W kg1. The synthetic strategy provides a fine reference for metal oxide structures and the results indicate the MnCO3 microsphere@MnO2 and MnCO3 microcube@MnO2 heterostructures electrodes can be a promising material for energy storage applications. © 2017 Elsevier B.V. All rights reserved.
Keywords: MnCO3 MnO2 Heterostructures Supercapacitors
1. Introduction Supercapacitors (SCs), as electrochemical capacitors, have captured considerable attention due to the features of fast chargedischarge rates, high power densities and long cycle lifetime [1e6]. In generally, morphology, porosity and electrical conductivity have crucial effects on electrochemical performances of the electrodes [7,8]. Among transition metal oxides, manganese dioxide (MnO2) has been taken into consideration as one of the ideal candidates for supercapacitors owing to their wide potential window, high theoretical specific capacitance and natural abundance [9e11]. * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y.X. Zhang), [email protected] (Z.-H. Liu). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.jpowsour.2017.04.020 0378-7753/© 2017 Elsevier B.V. All rights reserved.
Nevertheless, single electrode material can hardly meet the requirements of supercapacitors with outstanding capacitive properties. In consequence, a large variety of composite structures have been developed as promising electrode materials, which could boost the electrochemical properties due to synergistic effects. For example, Ghosh et al. [12] reported that the reduced graphene oxide/manganese carbonate hybrid composite showed good specific capacitance of 368 F g1 and outstanding cycle stability. Hou et al. [13] synthesized hierarchical MnO2 nanospheres/carbon nanotubes/conducting polymer ternary composite, which displayed remarkable charge/discharge rate and excellent cycling stability. Thus it can be seen that each component of these composites could offer distinct and momentous function to obtain good electrochemical performances. Manganese carbonate has been used as desulfurization catalyst, white pigment, fertilizer, etc. [14e16]. Recently, micro- and nano-
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size manganese carbonate has also attracted widespread interest for energy storage device. Accordingly, Devaraj et al. [17] disclosed submicron sized MnCO3 as electrode materials for supercapacitors with good reversibility and superior rate performance. Xia et al. [18] prepared MnCO3 QDs/Ni(HCO3)2-MnCO3 shell-needle composites by one-step hydrothermal method, which exhibited outstanding electrochemical properties. The above results revealed that manganese carbonate is a respectable electrode material for supercapacitors. In consequence, rational design with homogeneous distribution and well-defined architectures is greatly desirable to achieve optimized electrochemical performance. In this work, MnCO3 microsphere@MnO2 and MnCO3 microcube@MnO2 heterostructures are synthesized for supercapacitor electrodes. Results show that these two types of MnCO3@MnO2 electrodes display excellent capacitive properties and good rate capability. It is mainly ascribed to the synergistic effects of MnCO3 and MnO2. Furthermore, the asymmetric supercapacitor based on MnCO3 microsphere@MnO2 and active graphenes (AG) can achieve a high energy density of 27.4 Wh kg1 at a power density of 271.7 W kg1. These results indicated that the as-prepared electrode materials could be used as a perspective candidate for supercapacitors. 2. Experimental 2.1. Materials synthesis All the chemical reagents with reagent grade were used in the experiments without further purification.
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which was equipped with an energy dispersive X-ray spectrometer (EDS). The compositions of the materials were analyzed through powder X-ray diffraction (XRD, D/max 2500, Cu Ka) and Xray photoelectron spectroscopy (XPS, Kratos XSAM800). The structures of the materials were obtained with transmission electron microscopy (TEM, FEI TECNAI G2 F20). The N2 adsorptiondesorption isotherms were tested by a micro-meritics ASAP 2020 sorptometer. 2.3. Electrochemical measurements A three-electrode system was used to measure the electrochemical properties of the as-prepared electrodes in 1 M Na2SO4 electrolyte solution by an electrochemical workstation (CHI 660E). The nickel foam (1 1 cm2) was used as positive electrode, containing the active materials, carbon black and PVDF (8: 1: 1 wt%). The platinum plate and the saturated calomel (SCE) worked as the counter and reference electrode respectively. Nearly 3.1 mg of MnCO3 microsphere@MnO2 and 2.8 mg MnCO3 microcube@MnO2 were used in the measurements. The specific capacitances are calculated according to the following equation [19]:
Cm ¼
I Dt mDV
where I is the constant discharge current (A), Dt is the discharge time (s), m is the weight (g) of active materials, and DV is the discharging potential window (V). 2.4. Assembly of an asymmetric supercapacitor (ASC) device
2.1.1. Synthesis of MnCO3 microsphere and microcube MnSO4$H2O (0.169 g) was firstly mixed with H2O (70 mL), and then ethanol (7 mL) was added to the above solution after vigorous stirring to form a homogeneous solution. NH4HCO3 (0.84 g) was dissolved in H2O (70 mL) to form another uniform solution. The uniform solution of NH4HCO3 was directly added into the homogeneous solution of MnSO4 under stirring, and the obtained mixture was maintained at room temperature for 3 h before washing with deionized water and ethanol. Finally, the obtained MnCO3 microsphere was dried at 60 C for 6 h. In addition, the MnCO3 microcube was synthesized in a similar way compared to the MnCO3 microsphere, the only difference is the final step. For obtaining MnCO3 microcube, additional 1.321 g of (NH4)2SO4 was added into the final mixture, which was dried at 50 C for 7 h. 2.1.2. Synthesis of microsphere and microcube MnCO3@MnO2 heterostructures In a typical synthesis, KMnO4 (0.03 M, 30 mL) solution was added to two beakers containing MnCO3 microsphere (50 mg) and MnCO3 microcube (50 mg) powder, respectively. Then the mixtures were maintained at room temperature for 12 h after soft stirring. Finally, the samples were washed with water, ethanol and then dried at 60 C for 8 h. 2.1.3. Synthesis of the MnO2 nanosheets Briefly, KMnO4 (0.03 M, 30 mL) solution was poured into the Teflon-lined stainless steel autoclave maintained at 160 C for 24 h. In the end, the precipitate was collected, washed and dried at 60 C in a vacuum for 12 h. The morphologies of the MnO2 nanosheets are shown in Fig. S1 (Supporting Information). 2.2. Materials characterization The morphologies of the samples were observed by a fieldemission scanning electron microscopy (FESEM, JEOL JSM-7800F),
The asymmetric supercapacitor (ASC) device was based on MnCO3 microsphere@MnO2 as positive electrode and active graphenes as negative electrode with a separator (80 wt% active materials, 10 wt% carbon black, and 10 wt% PVDF were deposited on the 1 1.5 cm2 nickel foam). A Na2SO4ePVA gel membrane was used as the solid electrolyte between positive electrode and negative electrode. The electrolyte was solidified to form a sandwich structure after hot pressing at 60 C for 5 min. The energy density and power density of the device were calculated using the following equations [20]: E ¼ C$(Vhigh$VhighVlow$Vlow)/7.2, P ¼ 3600 E/Dt. Where C (F g1) is the capacitance of the device, Vhigh (V) and Vlow (V) are the highest and lowest potential of the discharging potential window after IR drop, Dt (s) is the discharge time, and E (Wh kg1) and P (W kg1) are the energy density and power density, respectively. The mass of positive and negative electrodes are based on the charge balance: qþ ¼ q. And the calculation is based on the total mass of MnCO3 microsphere@MnO2 (~4.3 mg) and active graphenes (~7.5 mg). 3. Results and discussion Fig. 1 shows the representative synthetic procedure of the MnCO3@MnO2 heterostructures. There are two steps for synthesizing MnCO3@MnO2 heterostructures. Firstly, MnCO3 microspheres and MnCO3 microcubes are synthesized by the method described in experiment section. MnCO3 microspheres are obtained at room temperature due to their self-assembly growth mechanism. The sulfate ions have an effect on the morphology of manganese (II) carbonate. After adding the (NH4)2SO4, the [SO2 4 ]t/ [Mn2þ]t ratio dramatically increases, deposition of the carbonate particles are retarded due to an excess of sulfate ions [21]. The MnCO3 seeds self-assembled into MnCO3 microcube according to an ‘‘oriented attachment’’ process, and the size of MnCO3 microcube also becomes larger than MnCO3 microspheres under the
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Fig. 1. The representative synthetic procedure of the MnCO3@MnO2 heterostructures.
restricted conditions (50 C for 7 h) [22,23]. The second step is the forming of MnCO3@MnO2 structures. These structures are simply obtained by surface redox reaction of MnCO3 and KMnO4 solution at room temperature. The reaction of MnCO3 and KMnO4 can develop a diffusion pair and the coupled diffusion at the crystal/ solution interface could result in the fast formation of the MnO2 nanosheets on the outside surfaces of the MnCO3 crystals. The chemical reaction is presented as follow [24]: 3MnCO3 þ 2KMnO4 / 5MnO2 þ K2CO3 þ 2CO2 The morphologies of the products are observed by a fieldemission scanning electron microscopy (FESEM). The typical FESEM images of MnCO3 microspheres and microcubes are shown in Fig. 2a and d. The average diameters of which are 2.5 mm and 3 mm. It is clear that the microspheres and microcubes are still maintained after the partial MnCO3 is oxidized to MnO2 (Fig. 2b and e). Naturally, compared to the pure MnCO3, the surfaces of the MnCO3@MnO2 heterostructures become rough and the sizes of which are a little larger. For the details, the surface of MnCO3
Fig. 3. XRD patterns of the as-prepared MnCO3@MnO2 heterostructures.
microcube@MnO2 is laminar (Fig. 2f). Nevertheless, the surface of MnCO3 microsphere@MnO2 is composed of interwoven porous MnO2 nanosheets. In addition, the chemical composition (the mass ratio) of the composites is calculated by etching the MnCO3 in excessive H2SO4 solution (Table S1). The porous and open structure may accelerate a high utilization of the active materials with enhanced capacitive performance. XRD patterns of the as-prepared MnCO3@MnO2 are shown in Fig. 3. It is shown that the major peaks of MnCO3 at about 24.3 , 31.4 , 37.5 , 41.4 , 45.2 , 49.7, 51.5 , 59.2, 60.1, 63.9 and 67.7 can match the standard XRD pattern of MnCO3 phase (JCPDS card No. 44-1472) [25]. After the chemical reaction of MnCO3 and KMnO4, the peaks at 12.3 , 18.7, 36.8 and 54.9 are appeared, which are corresponding to the (002), (101), (006) and (301) planes of hexagonal birnessite-MnO2 (JCPDS card No. 18-0802) [24]. In addition, the elements of C, O and Mn are observed by EDS analysis (Fig. S2).
Fig. 2. SEM images of MnCO3 microsphere (a), MnCO3 microsphere@MnO2 (b, c), MnCO3 microcube (d), MnCO3 microcube@MnO2 (e, f).
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Moreover, typical SEM images and corresponding EDS mapping of the MnCO3@MnO2 structures have been revealed in Fig. S3. The elements of C, O and Mn are uniformly distributed in the whole structure. The phenomenon further confirms that there are no other impurities in the composites. XPS spectra are used to identify the valence states of the elements of the obtained MnCO3@MnO2 composites (Fig. 4). As shown in Fig. 4a, the XPS results strongly support the presence of Mn, C and O. In the carbon 1s spectrum, the two binding energy peaks at 284.4 and 285.9 eV correspond to the characteristic bands of C-C and C-O bonds, respectively. Another binding energy peak of 289.1 eV is attributed to MnCO3 [18]. The Mn 2p spectrum displays two peaks at binding energy of 654.2 and 642.4 eV with a spinenergy separation of 11.8 eV, which is in good agreement with the Mn 2p1/2 and Mn 2p3/2 in MnO2, respectively [26]. Meanwhile, another peak at binding energy of 640.6 eV is assigned to Mn2þ [27]. In addition, the O 1s spectra show three peaks: two peaks at 533.3 and 531.5 eV are attributed to surface-adsorbed oxygen and water molecules, and another one at 529.9 eV manifests the O2 band with Mn [28]. All the characteristics can further identify the compositions of samples. TEM images with different magnification of two kinds of heterostructures are shown in Fig. 5. The outlines between the cores and the shells are observed distinctly (Fig. 5a and d). The MnO2 nanosheets uniformly cover on the surface of MnCO3 microcubes (Fig. 5b). Nevertheless, the MnO2 nanosheets on the surface of
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MnCO3 microspheres show a little heterogeneous (Fig. 5e), corresponding to the SEM images. In addition, the corresponding HRTEM images of two kinds of structures are displayed in Fig. 5c and f. The d-spacing of 0.72 and 0.47 nm correspond to the (002) and (101) planes of hexagonal birnessite-MnO2, corresponding to the previous XRD result. To analyze the specific surface area and porosity of the samples, nitrogen adsorption/desorption isotherms of the as-prepared samples measured at 77 K are shown in Fig. S4. The hysteresis loop behavior indicates that the as-prepared materials possess open mesoporous nature. The Brunauer-Emmett-Teller (BET) surface area of the MnCO3 microsphere@MnO2 and MnCO3 microcube@MnO2 heterostructures are calculated to be 38.1 and 27.8 m2 g1. Moreover, the pore size distributions calculated by the Barrett-Joyner-Halenda (BJH) theory model indicates that the average pore sizes of MnCO3 microsphere@MnO2 and MnCO3 microcube@MnO2 are approximate 27.8 nm and 32.9 nm, respectively. Generally, high specific surface area and unique mesoporous structure contribute efficient ions and electrons transport in the active materials and electrolyte [29,30]. The higher specific surface area of MnCO3 microsphere@MnO2 may have better electrochemical capacity compared to MnCO3 microcube@MnO2 heterostructure sample. The specific capacitances of MnCO3, MnO2 and MnCO3@MnO2 have been measured and compared in Fig. 6. As we can see from the figures, MnCO3@MnO2 electrodes own the best specific capacitance
Fig. 4. XPS spectra of MnCO3@MnO2 composites: (a) XPS survey scan; (b) C 1s spectrum; (c) Mn 2p spectrum; (d) O 1s spectrum.
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Fig. 5. Low-magnification and high-magnification TEM images of two kinds of heterostructures; The corresponding HRTEM images of two kinds of heterostructures.
Fig. 6. Cyclic voltammograms of MnCO3 microsphere@MnO2, MnCO3 microsphere and MnO2 nanosheets at 100 mV s1 (a); Cyclic voltammograms of MnCO3 microcube@MnO2, MnCO3 microcube and MnO2 nanosheets at 100 mV s1 (b); Galvanostatic charge-discharge curves of MnCO3 microsphere@MnO2, MnCO3 microsphere and MnO2 nanosheets at 1 A g1 (c); Galvanostatic charge-discharge curves of MnCO3 microcube@MnO2, MnCO3 microcube and MnO2 nanosheets at 1 A g1.
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Fig. 7. The electrochemical performance of the two kinds of heterostructures: cyclic voltammograms of MnCO3 microsphere@MnO2 (a) and MnCO3 microcube@MnO2 (b); Galvanostatic chargeedischarge curves of MnCO3 microsphere@MnO2 (c) and MnCO3 microcube@MnO2 (d); Specific capacitance of the two kinds of heterostructures measured under different current densities (e); Nyquist plots of the two kinds of heterostructures with inset showing the corresponding equivalent circuit (f).
among them. And specific capacitances (F g1) of the electrodes at 1 A g1 are displayed in Table S2. In addition, the electrochemical impedance spectroscopy (EIS) is shown in Fig. S5. The MnCO3 and MnCO3@MnO2 electrodes have nearly same overall ohmic resistance, but MnCO3@MnO2 electrode has the lowest charge transfer resistance (Rct) compared with MnCO3 and MnO2, which is attributed to unique combination of MnCO3 and interwoven porous MnO2 with a large surface area. Obvious, the excellent electrochemical performances of MnCO3@MnO2 composites are attributed
to the synergistic effect of MnCO3 and MnO2. (i) Highly porous structure of MnO2 nanosheets can't only enhance the specific area to improve the effective utilization of whole active materials, but facilitate the penetration of the electrolyte. (ii) The improved electrical conductivity of MnCO3@MnO2 composites can make for effective charge transfer properties and ion delivery. (iii) MnO2 can facilitate the intercalation and de-intercalation of Naþ ions. Moreover, the redox process of Mn (II) 4 Mn (I) occurs between MnCO3 and Naþ ions. All features indicate that MnCO3@MnO2
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heterostructures have good electrochemical properties because of synergistic effect of each component. Electrochemical behaviors of the two kinds of heterostructures are systematically investigated in a three-electrode system (Fig. 7). Fig. 7a and b presents cyclic voltammograms of MnCO3 microsphere@MnO2 and MnCO3 microcube@MnO2 heterostructures in the potential of 0.2e0.8 V. These curves exhibit nearly regular and symmetric shapes, manifesting ideal capacitive behavior of the electrodes [31,32]. As for the charge storage mechanisms in MnO2, it involves the intercalation/extraction of Naþ together with reduction/oxidation of the Mn ion. In addition, it is also a surface process, including the adsorption/desorption of Naþ ions [33,34]. The redox process of Mn (II) 4 Mn (I) occurs in Na2SO4 electrolyte: MnCO3 þ Naþþ e 4 NaMnCO3 [12]. The shapes of CV curves change litter along with the increment of current density. The cover area of the MnCO3 microsphere@MnO2 is larger than that of the MnCO3 microcube@MnO2 electrode at same current density, which suggests that the specific capacitance of MnCO3 microsphere@MnO2 electrode is larger than the latter. It is consistent with the larger surface area of MnCO3 microsphere@MnO2. Furthermore, galvanostatic charge-discharge curves of the electrodes are shown in Fig. 7c and d. The durations of charging and discharging are almost identical for the two electrodes, suggesting high columbic efficiency. The specific capacitances of MnCO3 microsphere@MnO2 and MnCO3 microcube@MnO2 heterostructures are 363 F g1 and 290 F g1 at a current density of
1 A g1, respectively. And the specific capacitances from 1 to 10 A g1 are addressed in Table S3. The specific capacitances of the two kinds of heterostructures measured under different current densities are revealed in Fig. 7e. The rate capability of the MnCO3 microsphere@MnO2 and MnCO3 microcube@MnO2 is about 61.2% and 59.3% from 1 A g1 to 10 A g1, respectively. It is clearly that the MnCO3 microsphere@MnO2 electrode has more excellent electrochemical properties than the latter. This result is ascribed to the higher specific surface area and extraordinary porous structure for effective redox reaction [26,35]. As shown in Fig. 7f, the internal resistances (Rs) of MnCO3 microsphere@MnO2 and MnCO3 microcube@MnO2 are 1.6 and 2.1 U, indicating the better electrical conductivity of MnCO3 microsphere@MnO2 electrode. Furthermore, the charge-transfer resistance (Rct) values of them are calculated to be 4.5 and 5.3 U, respectively. The lower charge-transfer resistance of MnCO3 microsphere@MnO2 electrode is owing to the unique combination of MnCO3 microsphere and interwoven porous MnO2 nanosheets, giving rise to efficient charge transfer performance. In order to evaluate the practical application of MnCO3 microsphere@MnO2 electrode, the asymmetric supercapacitors are assembled on the basis of MnCO3 microsphere@MnO2 as positive electrode and active graphenes (AG) as negative electrode. Moreover, asymmetric supercapacitors based on MnCO3 microcube@MnO2 and AG are fabricated and measured in Fig. S6. SEM images and the electrochemical performances of active graphenes
Fig. 8. CV curves of MnCO3 microsphere@MnO2//AG asymmetric supercapacitor measured at different potential window at a scan rate of 50 mV s1 (a); CV curves of the asymmetric supercapacitor measured at different scan rates between 0 and 1.8 V (b); Galvanostatic charge-discharge curves at different current densities (c); Ragone plots of the MnCO3 microsphere@MnO2//AG asymmetric supercapacitor (d).
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(AG) have been shown in Figs. S7 and S8. The CV curves of MnCO3 microsphere@MnO2//AG asymmetric supercapacitor measured with different potential window at a scan rate of 50 mV s1 are shown in Fig. 8a. The nearly rectangular and symmetric shapes have no change even at the potential window of 2.0 V, showing good electrochemical properties of the asymmetric supercapacitors [36,37]. The calculated specific capacitance is about 60.8 F g1 at a current density of 0.3 A g1, which is higher than the specific capacitance of asymmetric supercapacitor based on MnCO3 microcube@MnO2 and AG (45.4 F g1). The cycle performance is a significant evaluation factor for asymmetric supercapacitors, and cycling stability of the asymmetric supercapacitors at 1 A g1 is exhibited in Fig. S9. Slight enhancement of the specific capacitance in the first few cycles could be attributed to the activation of the electrode by aggrandizing the contact area between active materials and the electrolyte. It can be seen that the specific capacitance of the ACS device maintains 84.2% of initial capacitance after consecutive 2000 cycles. Additionally, the Ragone plot of the ACS device is presented in Fig. 8d. Our ACS device displays a high energy density of 27.4 Wh kg1 at a power density of 271.7 W kg1. Even at a high power density of 4507.6 W kg1, the asymmetric supercapacitor has an energy density of 14.9 Wh kg1. Nevertheless, asymmetric supercapacitor based on MnCO3 microcube@MnO2 and AG has an energy density of 20.43 Wh kg1 at a power density of 270.1 W kg1 (Fig. S6d). These results indicate that the asymmetric supercapacitor based on MnCO3 microcube@MnO2 and AG displays more excellent electrochemical properties. 4. Conclusion In summary, two kinds of MnCO3@MnO2 heterostructures have been developed by a simple self-assembly method and an oxidation process with excellent electrochemical performances. MnCO3 microsphere@MnO2 and MnCO3 microcube@MnO2 heterostructures display high specific capacitances and fine rate capabilities. The synergistic effect of MnCO3 and MnO2 contributes to the excellent results. In addition, the two-electrode asymmetric supercapacitor on the basis of MnCO3 microsphere@MnO2 and AG further achieves an energy density of 27.4 Wh kg1 at a power density of 271.7 W kg1. This work reveals an effective way for synthesizing a potential electrode material for energy storage devices and it also provides a general approach for designing special hierarchical structures. Acknowledgements The authors sincerely acknowledge the financial supports provided by National Natural Science Foundation of China (Grant no. 21576034), and International S&T Cooperation Projects of Chongqing (CSTC2013gjhz90001). The authors also thank Electron Microscopy Center of Chongqing University for materials characterizations.
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Rational design of microsphere and microcube MnCO3@MnO2 heterostructures for supercapacitor electrodes Hao Chen a, 1, Zhe Yan b, 1, Xiao Ying Liu a, Xiao Long Guo a, Yu Xin Zhang a, *, Zong-Huai Liu b, ** a
State Key Laboratory of Mechanical Transmissions, College of Material Science and Engineering, National Key Laboratory of Fundamental Science of Micro/ Nano-Devices and System Technology, Chongqing University, Chongqing 400044, PR China Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, and School of Materials Science and Engineering, Shaanxi Normal University, Xi'an 710062, PR China
b
h i g h l i g h t s Self-assembly and oxidation mechanism for MnCO3@MnO2 are proposed. MnCO3@MnO2 heterostructures display good electrochemical properties. The synergistic effect results in excellent electrochemical performances.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 January 2017 Received in revised form 22 March 2017 Accepted 6 April 2017 Available online 14 April 2017
MnCO3 microsphere@MnO2 and MnCO3 microcube@MnO2 heterostructures have been synthesized for supercapacitor electrodes through self-assembly and oxidation process. The morphologies of MnCO3 are effectively controlled by adding (NH4)2SO4 solution. The MnCO3 microsphere@MnO2 and MnCO3 microcube@MnO2 heterostructures display high capacitances of 363 F g1 and 290 F g1 at 1 A g1, respectively. In addition, the assembled asymmetric supercapacitor based on MnCO3 microsphere@MnO2 and active grapheme (AG) exhibits a high energy density of 27.4 Wh kg1 at a power density of 271.7 W kg1. The synthetic strategy provides a fine reference for metal oxide structures and the results indicate the MnCO3 microsphere@MnO2 and MnCO3 microcube@MnO2 heterostructures electrodes can be a promising material for energy storage applications. © 2017 Elsevier B.V. All rights reserved.
Keywords: MnCO3 MnO2 Heterostructures Supercapacitors
1. Introduction Supercapacitors (SCs), as electrochemical capacitors, have captured considerable attention due to the features of fast chargedischarge rates, high power densities and long cycle lifetime [1e6]. In generally, morphology, porosity and electrical conductivity have crucial effects on electrochemical performances of the electrodes [7,8]. Among transition metal oxides, manganese dioxide (MnO2) has been taken into consideration as one of the ideal candidates for supercapacitors owing to their wide potential window, high theoretical specific capacitance and natural abundance [9e11]. * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y.X. Zhang), [email protected] (Z.-H. Liu). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.jpowsour.2017.04.020 0378-7753/© 2017 Elsevier B.V. All rights reserved.
Nevertheless, single electrode material can hardly meet the requirements of supercapacitors with outstanding capacitive properties. In consequence, a large variety of composite structures have been developed as promising electrode materials, which could boost the electrochemical properties due to synergistic effects. For example, Ghosh et al. [12] reported that the reduced graphene oxide/manganese carbonate hybrid composite showed good specific capacitance of 368 F g1 and outstanding cycle stability. Hou et al. [13] synthesized hierarchical MnO2 nanospheres/carbon nanotubes/conducting polymer ternary composite, which displayed remarkable charge/discharge rate and excellent cycling stability. Thus it can be seen that each component of these composites could offer distinct and momentous function to obtain good electrochemical performances. Manganese carbonate has been used as desulfurization catalyst, white pigment, fertilizer, etc. [14e16]. Recently, micro- and nano-
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size manganese carbonate has also attracted widespread interest for energy storage device. Accordingly, Devaraj et al. [17] disclosed submicron sized MnCO3 as electrode materials for supercapacitors with good reversibility and superior rate performance. Xia et al. [18] prepared MnCO3 QDs/Ni(HCO3)2-MnCO3 shell-needle composites by one-step hydrothermal method, which exhibited outstanding electrochemical properties. The above results revealed that manganese carbonate is a respectable electrode material for supercapacitors. In consequence, rational design with homogeneous distribution and well-defined architectures is greatly desirable to achieve optimized electrochemical performance. In this work, MnCO3 microsphere@MnO2 and MnCO3 microcube@MnO2 heterostructures are synthesized for supercapacitor electrodes. Results show that these two types of MnCO3@MnO2 electrodes display excellent capacitive properties and good rate capability. It is mainly ascribed to the synergistic effects of MnCO3 and MnO2. Furthermore, the asymmetric supercapacitor based on MnCO3 microsphere@MnO2 and active graphenes (AG) can achieve a high energy density of 27.4 Wh kg1 at a power density of 271.7 W kg1. These results indicated that the as-prepared electrode materials could be used as a perspective candidate for supercapacitors. 2. Experimental 2.1. Materials synthesis All the chemical reagents with reagent grade were used in the experiments without further purification.
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which was equipped with an energy dispersive X-ray spectrometer (EDS). The compositions of the materials were analyzed through powder X-ray diffraction (XRD, D/max 2500, Cu Ka) and Xray photoelectron spectroscopy (XPS, Kratos XSAM800). The structures of the materials were obtained with transmission electron microscopy (TEM, FEI TECNAI G2 F20). The N2 adsorptiondesorption isotherms were tested by a micro-meritics ASAP 2020 sorptometer. 2.3. Electrochemical measurements A three-electrode system was used to measure the electrochemical properties of the as-prepared electrodes in 1 M Na2SO4 electrolyte solution by an electrochemical workstation (CHI 660E). The nickel foam (1 1 cm2) was used as positive electrode, containing the active materials, carbon black and PVDF (8: 1: 1 wt%). The platinum plate and the saturated calomel (SCE) worked as the counter and reference electrode respectively. Nearly 3.1 mg of MnCO3 microsphere@MnO2 and 2.8 mg MnCO3 microcube@MnO2 were used in the measurements. The specific capacitances are calculated according to the following equation [19]:
Cm ¼
I Dt mDV
where I is the constant discharge current (A), Dt is the discharge time (s), m is the weight (g) of active materials, and DV is the discharging potential window (V). 2.4. Assembly of an asymmetric supercapacitor (ASC) device
2.1.1. Synthesis of MnCO3 microsphere and microcube MnSO4$H2O (0.169 g) was firstly mixed with H2O (70 mL), and then ethanol (7 mL) was added to the above solution after vigorous stirring to form a homogeneous solution. NH4HCO3 (0.84 g) was dissolved in H2O (70 mL) to form another uniform solution. The uniform solution of NH4HCO3 was directly added into the homogeneous solution of MnSO4 under stirring, and the obtained mixture was maintained at room temperature for 3 h before washing with deionized water and ethanol. Finally, the obtained MnCO3 microsphere was dried at 60 C for 6 h. In addition, the MnCO3 microcube was synthesized in a similar way compared to the MnCO3 microsphere, the only difference is the final step. For obtaining MnCO3 microcube, additional 1.321 g of (NH4)2SO4 was added into the final mixture, which was dried at 50 C for 7 h. 2.1.2. Synthesis of microsphere and microcube MnCO3@MnO2 heterostructures In a typical synthesis, KMnO4 (0.03 M, 30 mL) solution was added to two beakers containing MnCO3 microsphere (50 mg) and MnCO3 microcube (50 mg) powder, respectively. Then the mixtures were maintained at room temperature for 12 h after soft stirring. Finally, the samples were washed with water, ethanol and then dried at 60 C for 8 h. 2.1.3. Synthesis of the MnO2 nanosheets Briefly, KMnO4 (0.03 M, 30 mL) solution was poured into the Teflon-lined stainless steel autoclave maintained at 160 C for 24 h. In the end, the precipitate was collected, washed and dried at 60 C in a vacuum for 12 h. The morphologies of the MnO2 nanosheets are shown in Fig. S1 (Supporting Information). 2.2. Materials characterization The morphologies of the samples were observed by a fieldemission scanning electron microscopy (FESEM, JEOL JSM-7800F),
The asymmetric supercapacitor (ASC) device was based on MnCO3 microsphere@MnO2 as positive electrode and active graphenes as negative electrode with a separator (80 wt% active materials, 10 wt% carbon black, and 10 wt% PVDF were deposited on the 1 1.5 cm2 nickel foam). A Na2SO4ePVA gel membrane was used as the solid electrolyte between positive electrode and negative electrode. The electrolyte was solidified to form a sandwich structure after hot pressing at 60 C for 5 min. The energy density and power density of the device were calculated using the following equations [20]: E ¼ C$(Vhigh$VhighVlow$Vlow)/7.2, P ¼ 3600 E/Dt. Where C (F g1) is the capacitance of the device, Vhigh (V) and Vlow (V) are the highest and lowest potential of the discharging potential window after IR drop, Dt (s) is the discharge time, and E (Wh kg1) and P (W kg1) are the energy density and power density, respectively. The mass of positive and negative electrodes are based on the charge balance: qþ ¼ q. And the calculation is based on the total mass of MnCO3 microsphere@MnO2 (~4.3 mg) and active graphenes (~7.5 mg). 3. Results and discussion Fig. 1 shows the representative synthetic procedure of the MnCO3@MnO2 heterostructures. There are two steps for synthesizing MnCO3@MnO2 heterostructures. Firstly, MnCO3 microspheres and MnCO3 microcubes are synthesized by the method described in experiment section. MnCO3 microspheres are obtained at room temperature due to their self-assembly growth mechanism. The sulfate ions have an effect on the morphology of manganese (II) carbonate. After adding the (NH4)2SO4, the [SO2 4 ]t/ [Mn2þ]t ratio dramatically increases, deposition of the carbonate particles are retarded due to an excess of sulfate ions [21]. The MnCO3 seeds self-assembled into MnCO3 microcube according to an ‘‘oriented attachment’’ process, and the size of MnCO3 microcube also becomes larger than MnCO3 microspheres under the
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Fig. 1. The representative synthetic procedure of the MnCO3@MnO2 heterostructures.
restricted conditions (50 C for 7 h) [22,23]. The second step is the forming of MnCO3@MnO2 structures. These structures are simply obtained by surface redox reaction of MnCO3 and KMnO4 solution at room temperature. The reaction of MnCO3 and KMnO4 can develop a diffusion pair and the coupled diffusion at the crystal/ solution interface could result in the fast formation of the MnO2 nanosheets on the outside surfaces of the MnCO3 crystals. The chemical reaction is presented as follow [24]: 3MnCO3 þ 2KMnO4 / 5MnO2 þ K2CO3 þ 2CO2 The morphologies of the products are observed by a fieldemission scanning electron microscopy (FESEM). The typical FESEM images of MnCO3 microspheres and microcubes are shown in Fig. 2a and d. The average diameters of which are 2.5 mm and 3 mm. It is clear that the microspheres and microcubes are still maintained after the partial MnCO3 is oxidized to MnO2 (Fig. 2b and e). Naturally, compared to the pure MnCO3, the surfaces of the MnCO3@MnO2 heterostructures become rough and the sizes of which are a little larger. For the details, the surface of MnCO3
Fig. 3. XRD patterns of the as-prepared MnCO3@MnO2 heterostructures.
microcube@MnO2 is laminar (Fig. 2f). Nevertheless, the surface of MnCO3 microsphere@MnO2 is composed of interwoven porous MnO2 nanosheets. In addition, the chemical composition (the mass ratio) of the composites is calculated by etching the MnCO3 in excessive H2SO4 solution (Table S1). The porous and open structure may accelerate a high utilization of the active materials with enhanced capacitive performance. XRD patterns of the as-prepared MnCO3@MnO2 are shown in Fig. 3. It is shown that the major peaks of MnCO3 at about 24.3 , 31.4 , 37.5 , 41.4 , 45.2 , 49.7, 51.5 , 59.2, 60.1, 63.9 and 67.7 can match the standard XRD pattern of MnCO3 phase (JCPDS card No. 44-1472) [25]. After the chemical reaction of MnCO3 and KMnO4, the peaks at 12.3 , 18.7, 36.8 and 54.9 are appeared, which are corresponding to the (002), (101), (006) and (301) planes of hexagonal birnessite-MnO2 (JCPDS card No. 18-0802) [24]. In addition, the elements of C, O and Mn are observed by EDS analysis (Fig. S2).
Fig. 2. SEM images of MnCO3 microsphere (a), MnCO3 microsphere@MnO2 (b, c), MnCO3 microcube (d), MnCO3 microcube@MnO2 (e, f).
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Moreover, typical SEM images and corresponding EDS mapping of the MnCO3@MnO2 structures have been revealed in Fig. S3. The elements of C, O and Mn are uniformly distributed in the whole structure. The phenomenon further confirms that there are no other impurities in the composites. XPS spectra are used to identify the valence states of the elements of the obtained MnCO3@MnO2 composites (Fig. 4). As shown in Fig. 4a, the XPS results strongly support the presence of Mn, C and O. In the carbon 1s spectrum, the two binding energy peaks at 284.4 and 285.9 eV correspond to the characteristic bands of C-C and C-O bonds, respectively. Another binding energy peak of 289.1 eV is attributed to MnCO3 [18]. The Mn 2p spectrum displays two peaks at binding energy of 654.2 and 642.4 eV with a spinenergy separation of 11.8 eV, which is in good agreement with the Mn 2p1/2 and Mn 2p3/2 in MnO2, respectively [26]. Meanwhile, another peak at binding energy of 640.6 eV is assigned to Mn2þ [27]. In addition, the O 1s spectra show three peaks: two peaks at 533.3 and 531.5 eV are attributed to surface-adsorbed oxygen and water molecules, and another one at 529.9 eV manifests the O2 band with Mn [28]. All the characteristics can further identify the compositions of samples. TEM images with different magnification of two kinds of heterostructures are shown in Fig. 5. The outlines between the cores and the shells are observed distinctly (Fig. 5a and d). The MnO2 nanosheets uniformly cover on the surface of MnCO3 microcubes (Fig. 5b). Nevertheless, the MnO2 nanosheets on the surface of
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MnCO3 microspheres show a little heterogeneous (Fig. 5e), corresponding to the SEM images. In addition, the corresponding HRTEM images of two kinds of structures are displayed in Fig. 5c and f. The d-spacing of 0.72 and 0.47 nm correspond to the (002) and (101) planes of hexagonal birnessite-MnO2, corresponding to the previous XRD result. To analyze the specific surface area and porosity of the samples, nitrogen adsorption/desorption isotherms of the as-prepared samples measured at 77 K are shown in Fig. S4. The hysteresis loop behavior indicates that the as-prepared materials possess open mesoporous nature. The Brunauer-Emmett-Teller (BET) surface area of the MnCO3 microsphere@MnO2 and MnCO3 microcube@MnO2 heterostructures are calculated to be 38.1 and 27.8 m2 g1. Moreover, the pore size distributions calculated by the Barrett-Joyner-Halenda (BJH) theory model indicates that the average pore sizes of MnCO3 microsphere@MnO2 and MnCO3 microcube@MnO2 are approximate 27.8 nm and 32.9 nm, respectively. Generally, high specific surface area and unique mesoporous structure contribute efficient ions and electrons transport in the active materials and electrolyte [29,30]. The higher specific surface area of MnCO3 microsphere@MnO2 may have better electrochemical capacity compared to MnCO3 microcube@MnO2 heterostructure sample. The specific capacitances of MnCO3, MnO2 and MnCO3@MnO2 have been measured and compared in Fig. 6. As we can see from the figures, MnCO3@MnO2 electrodes own the best specific capacitance
Fig. 4. XPS spectra of MnCO3@MnO2 composites: (a) XPS survey scan; (b) C 1s spectrum; (c) Mn 2p spectrum; (d) O 1s spectrum.
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Fig. 5. Low-magnification and high-magnification TEM images of two kinds of heterostructures; The corresponding HRTEM images of two kinds of heterostructures.
Fig. 6. Cyclic voltammograms of MnCO3 microsphere@MnO2, MnCO3 microsphere and MnO2 nanosheets at 100 mV s1 (a); Cyclic voltammograms of MnCO3 microcube@MnO2, MnCO3 microcube and MnO2 nanosheets at 100 mV s1 (b); Galvanostatic charge-discharge curves of MnCO3 microsphere@MnO2, MnCO3 microsphere and MnO2 nanosheets at 1 A g1 (c); Galvanostatic charge-discharge curves of MnCO3 microcube@MnO2, MnCO3 microcube and MnO2 nanosheets at 1 A g1.
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Fig. 7. The electrochemical performance of the two kinds of heterostructures: cyclic voltammograms of MnCO3 microsphere@MnO2 (a) and MnCO3 microcube@MnO2 (b); Galvanostatic chargeedischarge curves of MnCO3 microsphere@MnO2 (c) and MnCO3 microcube@MnO2 (d); Specific capacitance of the two kinds of heterostructures measured under different current densities (e); Nyquist plots of the two kinds of heterostructures with inset showing the corresponding equivalent circuit (f).
among them. And specific capacitances (F g1) of the electrodes at 1 A g1 are displayed in Table S2. In addition, the electrochemical impedance spectroscopy (EIS) is shown in Fig. S5. The MnCO3 and MnCO3@MnO2 electrodes have nearly same overall ohmic resistance, but MnCO3@MnO2 electrode has the lowest charge transfer resistance (Rct) compared with MnCO3 and MnO2, which is attributed to unique combination of MnCO3 and interwoven porous MnO2 with a large surface area. Obvious, the excellent electrochemical performances of MnCO3@MnO2 composites are attributed
to the synergistic effect of MnCO3 and MnO2. (i) Highly porous structure of MnO2 nanosheets can't only enhance the specific area to improve the effective utilization of whole active materials, but facilitate the penetration of the electrolyte. (ii) The improved electrical conductivity of MnCO3@MnO2 composites can make for effective charge transfer properties and ion delivery. (iii) MnO2 can facilitate the intercalation and de-intercalation of Naþ ions. Moreover, the redox process of Mn (II) 4 Mn (I) occurs between MnCO3 and Naþ ions. All features indicate that MnCO3@MnO2
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heterostructures have good electrochemical properties because of synergistic effect of each component. Electrochemical behaviors of the two kinds of heterostructures are systematically investigated in a three-electrode system (Fig. 7). Fig. 7a and b presents cyclic voltammograms of MnCO3 microsphere@MnO2 and MnCO3 microcube@MnO2 heterostructures in the potential of 0.2e0.8 V. These curves exhibit nearly regular and symmetric shapes, manifesting ideal capacitive behavior of the electrodes [31,32]. As for the charge storage mechanisms in MnO2, it involves the intercalation/extraction of Naþ together with reduction/oxidation of the Mn ion. In addition, it is also a surface process, including the adsorption/desorption of Naþ ions [33,34]. The redox process of Mn (II) 4 Mn (I) occurs in Na2SO4 electrolyte: MnCO3 þ Naþþ e 4 NaMnCO3 [12]. The shapes of CV curves change litter along with the increment of current density. The cover area of the MnCO3 microsphere@MnO2 is larger than that of the MnCO3 microcube@MnO2 electrode at same current density, which suggests that the specific capacitance of MnCO3 microsphere@MnO2 electrode is larger than the latter. It is consistent with the larger surface area of MnCO3 microsphere@MnO2. Furthermore, galvanostatic charge-discharge curves of the electrodes are shown in Fig. 7c and d. The durations of charging and discharging are almost identical for the two electrodes, suggesting high columbic efficiency. The specific capacitances of MnCO3 microsphere@MnO2 and MnCO3 microcube@MnO2 heterostructures are 363 F g1 and 290 F g1 at a current density of
1 A g1, respectively. And the specific capacitances from 1 to 10 A g1 are addressed in Table S3. The specific capacitances of the two kinds of heterostructures measured under different current densities are revealed in Fig. 7e. The rate capability of the MnCO3 microsphere@MnO2 and MnCO3 microcube@MnO2 is about 61.2% and 59.3% from 1 A g1 to 10 A g1, respectively. It is clearly that the MnCO3 microsphere@MnO2 electrode has more excellent electrochemical properties than the latter. This result is ascribed to the higher specific surface area and extraordinary porous structure for effective redox reaction [26,35]. As shown in Fig. 7f, the internal resistances (Rs) of MnCO3 microsphere@MnO2 and MnCO3 microcube@MnO2 are 1.6 and 2.1 U, indicating the better electrical conductivity of MnCO3 microsphere@MnO2 electrode. Furthermore, the charge-transfer resistance (Rct) values of them are calculated to be 4.5 and 5.3 U, respectively. The lower charge-transfer resistance of MnCO3 microsphere@MnO2 electrode is owing to the unique combination of MnCO3 microsphere and interwoven porous MnO2 nanosheets, giving rise to efficient charge transfer performance. In order to evaluate the practical application of MnCO3 microsphere@MnO2 electrode, the asymmetric supercapacitors are assembled on the basis of MnCO3 microsphere@MnO2 as positive electrode and active graphenes (AG) as negative electrode. Moreover, asymmetric supercapacitors based on MnCO3 microcube@MnO2 and AG are fabricated and measured in Fig. S6. SEM images and the electrochemical performances of active graphenes
Fig. 8. CV curves of MnCO3 microsphere@MnO2//AG asymmetric supercapacitor measured at different potential window at a scan rate of 50 mV s1 (a); CV curves of the asymmetric supercapacitor measured at different scan rates between 0 and 1.8 V (b); Galvanostatic charge-discharge curves at different current densities (c); Ragone plots of the MnCO3 microsphere@MnO2//AG asymmetric supercapacitor (d).
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(AG) have been shown in Figs. S7 and S8. The CV curves of MnCO3 microsphere@MnO2//AG asymmetric supercapacitor measured with different potential window at a scan rate of 50 mV s1 are shown in Fig. 8a. The nearly rectangular and symmetric shapes have no change even at the potential window of 2.0 V, showing good electrochemical properties of the asymmetric supercapacitors [36,37]. The calculated specific capacitance is about 60.8 F g1 at a current density of 0.3 A g1, which is higher than the specific capacitance of asymmetric supercapacitor based on MnCO3 microcube@MnO2 and AG (45.4 F g1). The cycle performance is a significant evaluation factor for asymmetric supercapacitors, and cycling stability of the asymmetric supercapacitors at 1 A g1 is exhibited in Fig. S9. Slight enhancement of the specific capacitance in the first few cycles could be attributed to the activation of the electrode by aggrandizing the contact area between active materials and the electrolyte. It can be seen that the specific capacitance of the ACS device maintains 84.2% of initial capacitance after consecutive 2000 cycles. Additionally, the Ragone plot of the ACS device is presented in Fig. 8d. Our ACS device displays a high energy density of 27.4 Wh kg1 at a power density of 271.7 W kg1. Even at a high power density of 4507.6 W kg1, the asymmetric supercapacitor has an energy density of 14.9 Wh kg1. Nevertheless, asymmetric supercapacitor based on MnCO3 microcube@MnO2 and AG has an energy density of 20.43 Wh kg1 at a power density of 270.1 W kg1 (Fig. S6d). These results indicate that the asymmetric supercapacitor based on MnCO3 microcube@MnO2 and AG displays more excellent electrochemical properties. 4. Conclusion In summary, two kinds of MnCO3@MnO2 heterostructures have been developed by a simple self-assembly method and an oxidation process with excellent electrochemical performances. MnCO3 microsphere@MnO2 and MnCO3 microcube@MnO2 heterostructures display high specific capacitances and fine rate capabilities. The synergistic effect of MnCO3 and MnO2 contributes to the excellent results. In addition, the two-electrode asymmetric supercapacitor on the basis of MnCO3 microsphere@MnO2 and AG further achieves an energy density of 27.4 Wh kg1 at a power density of 271.7 W kg1. This work reveals an effective way for synthesizing a potential electrode material for energy storage devices and it also provides a general approach for designing special hierarchical structures. Acknowledgements The authors sincerely acknowledge the financial supports provided by National Natural Science Foundation of China (Grant no. 21576034), and International S&T Cooperation Projects of Chongqing (CSTC2013gjhz90001). The authors also thank Electron Microscopy Center of Chongqing University for materials characterizations.
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