Controllable synthesis of different microstructured MnO2 by a facile hydrothermal method for supercapacitors

Journal of Alloys and Compounds 692 (2017) 26e33

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Controllable synthesis of different microstructured MnO2 by a facile hydrothermal method for supercapacitors Na Li, Xiaohong Zhu*, Caiyun Zhang, Liuqin Lai, Rong Jiang, Jiliang Zhu College of Materials Science and Engineering, Sichuan University, Chengdu 610064, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 April 2016 Received in revised form 15 August 2016 Accepted 30 August 2016 Available online 31 August 2016

Different microstructured MnO2 was synthesized as a pseudocapacitive material by using a facile hydrothermal method. MnSO4$H2O and (NH4)2S2O8 were reacted at 120
C for 12 h to form MnO2, for which the morphology and phase composition of the MnO2 formed could be well controlled. The obtained MnO2 samples were comparatively characterized in terms of their crystallographic structure, microstructure and electrochemical performance. The results revealed that the MnO2 was controllably formed in different microstructures (nanorod, hollow urchin and smooth ball) with different crystalline phase compositions. The average specific capacitance of MnO2 nanorod, hollow urchin and smooth ball at a scan rate of 5 mV/s was 317, 204 and 276 F/g, respectively, indicating that MnO2 nanorod displayed the best electrochemical capacity. All the synthesized MnO2, however, showed a good reversibility and cycling stability, roughly 70% of the initial capacitance retained after 2000 charge/discharge cycles. This work sheds light on a facile approach for synthesis of high-performance metal oxide electrode materials with controllable microstructure. © 2016 Elsevier B.V. All rights reserved.

Keywords: Manganese dioxide Hydrothermal method Microstructure Supercapacitors

1. Introduction With the depletion of fossil fuels, it is critical to develop sustainable and renewable energy resources to deal with the growing global demand of energy together. Developing relevant energy storage systems is essential to utilizing sustainable and renewable energy resources [1,2]. A supercapacitor is highly beneficial in storing renewable energy [3]. According to the energy storage mechanism and the nature of electrode materials, supercapacitors can be classified into three types: (i) electrical double-layer capacitors (EDLCs), in which the electrostatic charge accumulates at the interface between the electrode surface and the electrolyte, (ii) pseudocapacitors with fast and reversible redox reactions occurring on the surface of electrodes and (iii) hybrid capacitors in which both the EDLC and pseudocapacitor work together in a single device [4e7]. Various carbon materials exhibit EDLC [8] while a series of transition and noble metal oxides show the pseudocapacitive behavior [9]. Typical active pseudocapacitive materials of transition metal oxides include RuO2 [10], Fe3O4, Co3O4 [11], NiO [12], MnO2 [13e15], and so on. MnO2 is one of the most promising

* Corresponding author. E-mail address: [email protected] (X. Zhu). http://dx.doi.org/10.1016/j.jallcom.2016.08.321 0925-8388/© 2016 Elsevier B.V. All rights reserved.

pseudocapacitive materials due to its low cost, high energy density, natural abundance, being environmentally friendly, and most importantly, high theoretical specific capacitance [1]. Several routes have been reported for the synthesis of MnO2 [16]. For instance, Fe-doped MnO2 was synthesized by a facile hydrothermal method with the introduction of iron ions. The presence of Fe3þ ions leads to a transformation of phase structure from a-MnO2 to a mixture of ε-MnO2 and a-MnO2 [17]. Xiaowu Sun and Mengyu Gan et al. [18] synthesized honeycomb-like MnO2 nanospheres via microemulsion method involving KMnO4 and oleic acid. MnCO3 was readily oxidized by KMnO4 to MnO2 by Jinbo Fei and Yue Cui et al. [19]. In addition, a-MnO2 hollow urchins were previously prepared by Li et al. through a low-temperature mild reaction in an acidic solution and the addition of a Cu foil was found to be necessary [20]. In this work, we describe the use of a simple hydrothermal method [21]. Hydrothermal reaction is a cheap and environmentally friendly method to prepare materials in different nanostructures such as nanospheres, nanowires and nanorods [6,22]. In the previous reports, the researchers mainly focused on the synthesis of MnO2 with a single morphology or use of different reaction times and temperatures that afford control over the morphology and phase composition of MnO2 [23e25]. Few articles have been reported with regard to the influences of the reactant concentration and surfactant under the condition of a certain

N. Li et al. / Journal of Alloys and Compounds 692 (2017) 26e33

reaction temperature and time. Hence, with the aim of combining the advantages of MnO2 as a pseudocapacitive material with the unique characteristics of hydrothermal technique to fabricate interesting nanostructures of MnO2 with excellent electrochemical properties, our main task is to study the changes of morphology and phase composition in different reactant concentrations and by adding of surfactant; meanwhile, the electrochemical properties of MnO2 electrode material in different morphologies will be comparatively explored. The chemical reaction can be formulated as

MnSO4 þ ðNH4 Þ2 S2 O8 þ 2H2 O/MnO2 þ ðNH4 Þ2 SO4 þ 2H2 SO4

(1)



Mn2þ þ 2H2 O/MnO2 þ 4Hþ þ 2e E ¼ 1:23 V

(2)

S2 O8 2 þ 2e /2SO4 2 ðE+ ¼ 2:01 VÞ

(3)

E
,

On the basis of the values of the standard Gibbs free energy change DG
of reaction (1) could be estimated to be 151 kJ/mol, implying a very strong tendency for reaction (1) to progress toward the right-hand side [26]. Through this method, we have synthesized three different types of microstructures for manganese dioxide, namely, MnO2 nanorods, hollow urchins and smooth balls, which were compared detailedly by phase composition, morphologies and electrochemical performance. 2. Experimental section 2.1. Synthesis In this work, different microstructures of manganese dioxide were synthesized under hydrothermal conditions. Synthesis of MnO2 was carried out by the oxidation of hydrated manganese sulfate MnSO4$H2O with ammonium persulfate (NH4)2S2O8 [27,28]. All chemical reagents were used without further purification. (1) MnO2 nanorods MnO2 nanorods were synthesized here by using MnSO4$H2O with an equal molar weight of (NH4)2S2O8. Firstly, 0.1366 g MnSO4$H2O and 0.1866 g (NH4)2S2O8 were dissolved in 15 mL of deionized water at room temperature. Secondly, the above solution was kept continuous stirring to form a clear solution. After stirring, the mixture was loaded to Teflon-lined stainless steel autoclave (20 mL). The autoclave was heated at 120
C for 12 h in an oven and then allowed to cool to room temperature naturally. Then, the formed brownish black precipitate was filtered and washed with distilled water and ethyl alcohol for several times to remove any unreacted starting materials during reaction. Finally, the products were dried at 50
C. (2) MnO2 hollow urchins MnO2 hollow urchins were synthesized in the same way, whereas the concentration of mixture was increased. In this case, 1.3318 g MnSO4$H2O and 1.816 g (NH4)2S2O8 were dissolved in 15 mL of deionized water. Then, with the same subsequent processing steps, the MnO2 hollow urchins were achieved. (3) MnO2 smooth balls The synthesis of MnO2 smooth balls was performed similarly under hydrothermal conditions. At first, 1.0245 g MnSO4$H2O and

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1.397 g (NH4)2S2O8 were dissolved in 15 mL of deionized water. Meanwhile, 0.5 g PVP (polyvinyl pyrrolidone K30) was added slowly with vigorous stirring. When the solution was clarified, this mixed solution was transferred into 20 mL Teflon-lined stainless steel autoclaves and allowed to react at 120
C. Then, MnO2 smooth balls were obtained from the reaction. 2.2. Characterization The phase structure was characterized by X-ray diffraction (XRD; DX-1000, Dandong Fangyuan Instrument Co., Ltd., Dandong, China) using Cu Ka radiation (l ¼ 0.154056 nm) at a scanning rate of 0.03
s1 in the 2q range from 8
to 70
. The morphologies and nanostructures of the as-synthesized MnO2 samples were characterized by scanning electron microscopy (SEM; Hitachi S-4800, Hitachi, Ltd., Chiyoda-ku, Japan, operated at 30 kV). A three electrode cell configuration was used to study the cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy by using an electrochemical station (CHI 660E, Chenhua, Shanghai, China) in 6 M KOH alkaline electrolyte solution. 2.3. Preparation of electrodes For the electrochemical measurements of the samples, the working electrodes were prepared with the mixture including the active materials, carbon black and polytetrafluoroethylene (PTFE) with mass ratio of 80:10:10 to make a homogenous mixture in ethanol. The resultant mixture was vacuum-dried at 100
C to eliminate ethanol and then spread onto a nickel foam substrate (about 1  1 cm2) with a loading of 1.5 mg. Finally, the nickel foam was pressed under a pressure of 10 MPa in order that the electrode material can adhere to the current collector much better, and the preparation of electrode slice was finished. Before nickel foam was used, it should be cleaned sequentially by acetone, deionized water, diluted hydrochloric acid, deionized water and ethanol for the sake of removing impurities. All electrochemical experiments were carried out in a traditional three-electrode cell, in which graphite electrode, mercuric oxide electrode (Hg/HgO) and the prepared electrodes were used as counter, reference and working electrodes, respectively. 3. Results and discussion 3.1. Phase composition of the specimens X-ray diffraction was firstly employed to investigate the crystalline structure of the synthesized materials. Fig. 1 shows the XRD patterns of the three different samples obtained. In Fig. 1(a) for MnO2 nanorod, the diffraction peaks observed at 2q values of 28.6
, 37.3
, 40.80
, 42.8
, 46.0
, 56.6
, 59.2
, 64.8
and 67.2
can be respectively indexed to (110), (101), (200), (111), (210), (211), (220), (002), (310) and (411) plane reflections of a tetragonal phase bMnO2 [JCPDS card 024-0735, space group P42/mnm(136)] with lattice constants a ¼ 4.400 Å and c ¼ 2.874 Å. No other phase is detected, manifesting that pure b-MnO2 is successfully prepared [26]. These diffraction peaks of MnO2 nanorod are narrow and intensive. In contrast, there are some changes in the relative peak intensities for MnO2 hollow urchin, as shown in Fig. 1(b). Moreover, the XRD pattern of MnO2 hollow urchin shows that although this material is also composed of crystalline MnO2, two different phases are present: the major phase is a tetragonal phase [space group: I4/ m (87)] of a-MnO2 with lattice constants a ¼ 9.785 Å and c ¼ 2.863 Å (JCPDS 044-0141) and the other is a small amount of gMnO2 [21]. In Fig. 1(c), the diffraction peaks of MnO2 smooth ball are too broad to be detected with low intensity, which is probably

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N. Li et al. / Journal of Alloys and Compounds 692 (2017) 26e33

(SEM) observations with results shown in Fig. 2. From the different magnification SEM images (Fig. 2(a) and (b)), one can see that the panoramic morphology of b-MnO2 powder is constituted with nanorod crystals that are roughly 100 nm in diameter and 3e4 mm in length and are present in high quantity. In contrast, Fig. 2(c) shows a sea urchin-like sphere microstructure with the diameter of 4e7 mm for the second type of MnO2. It is interesting to note that the microspheres of MnO2 hollow urchin are actually constructed by a large number of densely distributed nanorods, which can be manifested by the fact that these 3D microscopic spheres had collapsed into nanorods of varying lengths after 5 months, as illustrated in the upper right corner of the picture. In Fig. 2(d), the MnO2 smooth ball displays a very smooth surface and more uniform particle size distribution with the diameter of 3e5 mm, quite different from that for MnO2 hollow urchin. 3.3. Electrochemical performance in supercapacitors

Fig. 1. XRD patterns of the different microstructured MnO2 materials produced: a) MnO2 nanorod, b) MnO2 hollow urchin, c) MnO2 smooth ball, d) MnO2 smooth ball sintered at 400
C for 3 h.

ascribed to its lower crystallinity and the residuals of PVP. Hence, the MnO2 smooth ball was tentatively sintered at 400
C for 3 h, and the XRD picture is shown in Fig. 1(d). However, PVP still cannot be removed completely and the degree of crystallinity is still very low, indicating that the MnO2 smooth ball presents an amorphous structure.

To evaluate the electrochemical characteristics of MnO2 samples, cyclic voltammetry (CV), galvanostatic charge/discharge (CD) and electrochemical impedance spectroscopy (EIS) measurements were employed to characterize the capacitive properties in a 6 M KOH alkaline electrolyte solution at ambient temperature. The capacitance of MnO2 samples is derived from the contribution of the pseudocapacitance. There are two mechanisms proposed for the charge storage in MnO2-based electrodes: The first involves the intercalation of protons (Hþ) or alkali cations into the bulk of oxide particles upon reduction followed by the deintercalation upon oxidation:

MnO2 þ Mþ þ e 4MnOOH; 3.2. Morphologies and microstructures The micro-/nanostructure and dimension of the as-prepared samples were further examined by scanning electron microscopy

(4)

where Mþ represents the proton Hþ or alkali metal ions such as Kþ. The second is based on the surface adsorption of electrolyte cations (Mþ) on MnO2:

Fig. 2. SEM images of MnO2 at different magnifications: (a)e(b) MnO2 nanorod, (c) MnO2 hollow urchin, (d) MnO2 smooth ball.

N. Li et al. / Journal of Alloys and Compounds 692 (2017) 26e33

MnO2surface þ Mþ þ e 4MnOOHsurface :

(5)

Both the reaction mechanisms involved a redox reaction between the III and IV oxidation states of Mn [4,5,29]. Fig. 3 presents the cyclic voltammetric curves for the supercapacitors based on the as-prepared MnO2 samples, measured at various scan rates from 5 to 50 mV s1 in a potential window between 0 and 0.5 V. The average specific capacitance of MnO2 is calculated from the CV curves according to the following equation [30]:

Z

V

idV Cm ¼

0

nmV

;

(6)

where Cm (A g1) is the average specific capacitance, v (V s1) is the potential scan rate, m (g) is the mass of active material and V (V) is the width of the potential window. The shapes of CV curves are almost the same for those three samples and all exhibit redox peaks of MnO2. The couples of redox peaks are observed, corresponding to the redox transition of III and IV oxidation states of Mn. It is noted that the area of CV curves increases with increasing the scan rate and the CV curves almost maintain the same shape, indicating the MnO2 electrode has good rate capability. The average specific capacitance at a scan rate of 5 mV/s is obtained to be 317, 204 and 276 F/g for MnO2 nanorod, hollow urchin and smooth ball, respectively. When scanned at 10 and 20 mV/s, their average specific capacitance is slightly decreased, as summarized in Table 1. It is noteworthy that compared to the hollow urchin and the smooth ball, the nanorod exhibits the highest specific average capacitance. The good capacitive behavior and low internal resistance have been further confirmed by the galvanostatic chargeedischarge experiment. Fig. 4(aec) illustrates a nearly symmetric shape of the chargeedischarge curves recorded at different current densities (1, 2 and 5 A/g), indicating the fast ion transport within the electrodes, superior reversible redox reaction and good electrochemical capacitive characteristic. Furthermore, at the start of the discharge curve, these chargeedischarge curves show a very small voltage

29

drop, manifesting their very low internal resistance between the current collector and the electrode. The specific capacitance can be also evaluated from chargeedischarge curves according to the equation given below [8]:

C¼

I  Dt m  DV

(7)

where I is the charge/discharge current, Dt is the discharging time, m is the mass of the active material and DV is the voltage window from the deduction of the IR drop. Accordingly, the specific capacitance at a current density of 1 A/g reaches 270, 215 and 242 F/ g, respectively, for MnO2 nanorod, hollow urchin and smooth ball. Similarly, their specific capacitance is slightly decreased with the increase in current density from 1 A/g to 2 and 5 A/g, while the nanorod maintains the highest capacity regardless of which current density is used, as summarized in Table 1. Through combining both the CV and galvanostatic chargeedischarge data, it is concluded that the MnO2 nanorod possesses the best specific capacitance. In order to find more possible impacting factors, the nitrogen adsorption/desorption isotherms and corresponding pore size distributions of MnO2 nanorod, hollow urchin and smooth ball are shown in Fig. 5(aec). The specific surface area and pore size distribution are presented in Table 2. This indicates that MnO2 nanorod, hollow urchin and smooth ball are all mesoporous materials (2 nm < pore size < 50 nm). Surprisingly, the BrunauereEmmetteTeller (BET) surface area of MnO2 hollow urchin is the largest among the three samples, which is probably ascribed to its construction of nanoneedles and its low mass density induced by the hollow microstructure, while the average pore size of MnO2 nanorod is bigger than that of the others. In a faradaic chargetransfer storage mechanism, it is obvious that the capacitance is not due to either pure surface or bulk redox processes. Therefore, the specific capacitance of MnO2 is not a linear dependence between the capacitance and the surface area [31]. In our own experience, it is concluded that the MnO2 nanorod possesses the best specific capacitance despite of a lower surface area. The reason

Fig. 3. Cyclic voltammograms of the MnO2 samples with different sweep rates for a) MnO2 nanorod, b) MnO2 hollow urchin, c) MnO2 smooth ball.

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N. Li et al. / Journal of Alloys and Compounds 692 (2017) 26e33

Table 1 Comparison of average specific capacitance based on MnO2 electrode materials between experimental data and reference data. (Unit of specific capacitance: F/g).

Nanorod Hollow urchin Smooth ball Reference data

Specific capacitance at different scan rates for CV from 1 to 20 mV/s

Specific capacitance at different current densities for CP from 0.8 to 5 A/g

1 mV/s

0.8 A/g

5 mV/s

10 mV/s

20 mV/s

317 204 276

251 187 246

225 173 212

168 [4]

253 [6]

1 A/g

2 A/g

5 A/g

270 215 242 151.5 [28] 197.3 [37]

252 210 229

201 178 202

Fig. 4. Galvanostatic charge/discharge curves of the supercapacitors based on MnO2 electrodes: a) MnO2 nanorod, b) MnO2 hollow urchin, c) MnO2 smooth ball.

may be explained that the particle size of nanorods is nanosized, which can shorten greatly the ion diffusion path and improve effectively the transport properties of electrolytes. On the other hand, the MnO2 hollow urchin shows a much bigger particle size than the nanorod and the structure of a-MnO2 consists of double chains of edge-sharing [MnO6] octahedra, which share corners to form the 1D (2  2) and (1  1) tunnels that extend in a direction parallel to the c axis of the tetragonal unit cell. The size of (2  2) tunnel is suitable for the insertion or extraction of big alkali cations and H2O, which also blocks the diffusion of other ions to a large degree [22,28]. Hence, the specific capacitance of MnO2 hollow urchin is lowest, though showing the largest surface area. Furthermore, it should be pointed out that from previous reports [31e35], we know that many parameters, such as crystal type, crystallinity, microstructure, tunnel structures, preparation methods and pore-size distribution, must be taken into account with caution because all of them can influence the capacitance of a material. The balances of these factors would thus lead to the results obtained in this work. Consequently, we can only say that the highest specific capacitance of MnO2 nanorod is a synergistic effect of the aforementioned factors. The average specific capacitance experimentally obtained in this work is compared among themselves and also compared with the reference data. As shown in Table 1, the capacitance of MnO2 nanorod reduces from 270 F/g (@ 1 A/g current density) to 201 F/g

(@ 5 A/g current density) with 74.4% of the specific capacitance remaining, whereas the capacitance of MnO2 hollow urchin decreases from 215 F/g (@ 1 A/g current density) to 178 F/g (@ 5 A/g current density) with 82.8% remaining and the MnO2 smooth ball also remains 83.5% of the specific capacitance from 242 F/g (@ 1 A/g current density) to 202 F/g (@ 5 A/g current density), suggesting that the MnO2 nanorod has a lower rate capability, though it shows a higher specific capacitance. The larger capacitance fading in MnO2 nanorod is probably because a large part of this electrode surface is inaccessible at high chargeedischarge rates, while the amorphous structure of MnO2 smooth ball remains a bigger surface utilization percentage at high chargeedischarge rates [36]. On the other side, in comparison with the specific capacitance of other related MnO2 electrodes, such as the urchin-like MnO2 electrode (151.5 F/g at a discharge current density of 1 A/g) [28], the hierarchical MnO2 nanoflower electrode (197.3 F/g at a discharge current density of 1 A/g) [37], well-defined MnO2 nanorod electrodes (253 F/g at a discharge current density of 0.8 A/g) [6] and MnO2 with a mixture of nanorods and nanostructured surface with a distinct plate-like morphology (168 F/g at a scan rate of 1 mV/s) [4], it is obvious that the experimental data achieved in this work are comparable to or even much higher than those reported in literature. The EIS analysis is one of the effective methods to examine the fundamental behavior of electrode materials for supercapacitors. Here, it is performed by Nyquist plots in the frequency range from

N. Li et al. / Journal of Alloys and Compounds 692 (2017) 26e33

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Fig. 5. The nitrogen adsorptionedesorption isotherms and the corresponding pore size distribution of a) MnO2 nanorod, b) MnO2 hollow urchin, c) MnO2 smooth ball.

Table 2 Comparison of average specific capacitance, specific surface area and pore size distribution.

Nanorod Hollow urchin Smooth ball

Specific capacitance at 1 A/g (F/g)

BET surface area (m2/g)

Average pore size (nm)

270 215 242

18 71 15

16.05 8.58 3.09

100 kHz to 0.01 Hz. As shown in Fig. 6, all the impedance spectra are almost similar. The intercept at real axis represents the internal resistance (Rs), and the diameter of semicircle indicates the charge transfer resistance (Rct) at the interface [28,37,38]. The typical linear plots in the low frequency region represents the Warburg resistance (RW), which can be ascribed to the ions diffusion/transport from the electrolyte to the electrode surface [37]. For metal oxides, semicircle in theory is not very obvious in the high frequency

region, and a linear part at low frequencies is clear. Accordingly, it can be inferred that ions diffusion is dominating [39,40]. It is observed that the internal resistance (Rs) of MnO2 nanorod, hollow urchin and smooth ball is 0.265, 0.280 and 0.285 U, respectively. The MnO2 nanorod shows the smallest Rs than the others, indicating its outstanding ions transfer nature. Also, the MnO2 nanorod exhibits a smallest Warburg resistance. This implies that the nanosized nanorod structure contributes to the penetration process of electrolyte, which is consistent with the aforementioned galvanostatic chargeedischarge and cyclic voltammetry. To further confirm the electrochemical properties of assynthesized MnO2 materials for potential applications in supercapacitors, excellent cycling stability is a crucial issue. Fig. 7 shows the representative cycling stability of the MnO2 samples, measured at a current density of 5 A/g. All the MnO2 samples exhibit an acceptable cycling stability performance, i.e., 71%, 65% and 67% capacitance is retained over 2000 cycles of charging and discharging for MnO2 nanorod, hollow urchin and smooth ball-based electrodes, respectively, significantly better than that reported for MnO2 (only 44% of the initial capacitance available after 1000 cycles) [6]. 4. Conclusions

Fig. 6. Electrochemical impedance spectra (EIS): a) MnO2 nanorod, b) MnO2 hollow urchin, c) MnO2 smooth ball. The inset shows the plots at high frequencies.

In summary, we have successfully synthesized three different microstructured MnO2 with high performance by a facile hydrothermal method. MnO2 nanorod, hollow urchin and smooth ball all show quite high specific capacitance, reaching 317, 204 and 276 F/g

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N. Li et al. / Journal of Alloys and Compounds 692 (2017) 26e33

Fig. 7. Cycle-life performance of different microstructured MnO2 materials during 2000 cycles at a current density of 5 A/g: a) MnO2 nanorod, b) MnO2 hollow urchin, c) MnO2 smooth ball.

at a scan rate of 5 mV/s, respectively, and they remain respectively at 71%, 65% and 67% capacitance over 2000 cycles at 5 A/g. Among these three different microstructures, however, nanorod exhibits the highest electrochemical properties, confirming that nano-sized MnO2 electrode materials are more promising for supercapacitor applications. This work provides important implications for synthesis of high-performance metal oxide electrode materials with controllable microstructure. Acknowledgments This work was financially supported by the Ministry of Science and Technology of China (MOST) (Grant No. 2013CB934700). References [1] G.H. Yu, L.B. Hu, M. Vosgueritchian, H.L. Wang, X. Xie, J.R. McDonough, X. Cui, Y. Cui, Z.N. Bao, Solution-processed graphene/MnO2 nanostructured textiles for high-performance electrochemical capacitors, Nano Lett. 11 (2011) 2905e2911. [2] J. Chang, M.H. Jin, F. Yao, T.H. Kim, V.T. Le, H.Y. Yue, F. Gunes, B. Li, A. Ghosh, S.S. Xie, Y.H. Lee, Asymmetric supercapacitors based on graphene/MnO2 nanospheres and graphene/MoO3 nanosheets with high energy density, Adv. Funct. Mater 23 (2013) 5074e5083. [3] T. Chen, L.M. Dai, Flexible supercapacitors based on carbon nanomaterials, J. Mater. Chem. A 2 (2014) 10756e10775. [4] V. Subramanian, H.W. Zhu, B.Q. Wei, Nanostructured MnO2: hydrothermal synthesis and electrochemical properties as a supercapacitor electrode material, J. Power Sources 159 (2006) 361e364. [5] Y.C. Xiong, M. Zhou, H. Chen, L. Feng, Z. Wang, X.Z. Yan, S.Y. Guan, Synthesis of honeycomb MnO2 nanospheres/carbon nanoparticles/graphene composites as electrode materials for supercapacitors, Appl. Surf. Sci. 357 (2015) 1024e1030. [6] P. Ragupathy, H.N. Vasan, N. Munichandraiah, Synthesis and characterization of nano-MnO2 for electrochemical supercapacitor studies, J. Electrochem. Soc. 155 (2008) A34eA40. [7] J.D. Wang, H.Y. Xian, T.J. Peng, H.J. Sun, F.X. Zheng, Three-dimensional graphene-wrapped PANI nanofiber composite as electrode material for supercapacitor, RSC Adv. 5 (2015) 13607e13612. [8] J.Y. Dong, Z.Y. Wang, X.H. Kang, The synthesis of graphene/PVDF composite binder and its application in high performance MnO2 supercapacitors, Colloid

Surf. a-physicochem. Eng. Asp. 489 (2016) 282e288. [9] Z.N. Yu, L. Tetard, L. Zhai, J.Y. Thomas, Supercapacitor electrode materials: nanostructures from 0 to 3 dimensions, Energy Environ. Sci. 8 (2015) 702e730. [10] Z.S. Wu, D.W. Wang, W.C. Ren, J.P. Zhao, G.M. Zhou, F. Li, H.M. Cheng, Anchoring hydrous RuO2 on graphene sheets for high-performance electrochemical capacitors, Adv. Funct. Mater 20 (2010) 3595e3602. [11] X.H. Xia, J.P. Tu, Y.J. Mai, X.L. Wang, C.D. Gu, X.B. Zhao, Self-supported hydrothermal synthesized hollow Co3O4 nanowire arrays with high supercapacitor capacitance, J. Mater. Chem. 21 (2011) 9319e9325. [12] J.J. Chen, Y. Huang, C. Li, X.F. Chen, X. Zhang, Synthesis of NiO@MnO2 core/ shell nanocomposites for supercapacitor application, Appl. Surf. Sci. 360 (2016) 534e539. [13] C.Y. Zhang, X.H. Zhu, Z.X. Wang, P. Sun, Y.J. Ren, J.L. Zhu, J.G. Zhu, D.Q. Xiao, Facile synthesis and strongly microstructure-dependent electrochemical properties of graphene/manganese dioxide composites for supercapacitors, Nanoscale Res. Lett. 9 (2014), 490e490. [14] S.S. Falahatgar, F.E. Ghodsi, F.Z. Tepehan, G.G. Tepehan, Turhan, S. Pishdadian, Electrochromic performance of sol-gel derived amorphous MnO2eZnO nanogranular composite thin films, J. Non-Crystalline Solids 427 (2015) 1e9. [15] Z. Fan, J.H. Chen, M.Y. Wang, K.Z. Cui, H.H. Zhou, Y.F. Kuang, Preparation and characterization of manganese oxide/CNT composites as supercapacitive materials, Diam. Relat. Mater. 15 (2006) 1478e1483. [16] W.F. Wei, X.W. Cui, W.X. Chen, D.G. Ivey, Manganese oxide-based materials as electrochemical supercapacitor electrodes, Chem. Soc. Rev. 40 (2011) 1697e1721. [17] Y.P. Duan, H. Jing, Z. Liu, S.Q. Li, G.J. Ma, Controlled synthesis and electromagnetic performance of hollow microstructures assembled of tetragonal MnO2 nano-columns, J. Appl. Phys. 111 (2012) 084109. [18] X.W. Sun, M.Y. Gan, L. Ma, H.H. Wang, T. Zhou, S.Y. Wang, W.Q. Dai, H.N. Wang, Fabrication of PANI-coated honeycomb-like MnO2 nanospheres with enhanced electrochemical performance for energy storage, Electrochim. Acta 180 (2015) 977e982. [19] J.B. Fei, Y. Cui, X.H. Yan, W. Qi, Y. Yang, K.W. Wang, Q. He, J.B. Li, Controlled preparation of MnO2 hierarchical hollow nanostructures and their application in water treatment, Adv. Mater 20 (2008) 452e456. [20] B.X. Li, G.X. Rong, Y. Xie, L.F. Huang, C.Q. Feng, Low-temperature synthesis of a-MnO2 hollow urchins and their application in rechargeable Liþ batteries, Inorg. Chem. 45 (2006) 6404e6410. [21] M. Alhumaimess, Z.J. Lin, Q. He, L. Lu, N. Dimitratos, N.F. Dummer, M. Conte, S.H. Taylor, J.K. Bartley, C.J. Kiely, G.J. Hutchings, Oxidation of benzyl alcohol and carbon monoxide using gold nanoparticles supported on MnO2 nanowire microspheres, Chem. Eur. J. 20 (2014) 1701e1710. [22] K.J. Babu, A. Zahoor, K.S. Nahm, R. Ramachandran, M.A.J. Rajan, G.G. Kumar, The influences of shape and structure of MnO2 nanomaterials over the nonenzymatic sensing ability of hydrogen peroxide, J. Nanopart. Res. 16 (2014)

N. Li et al. / Journal of Alloys and Compounds 692 (2017) 26e33 2250. [23] T.T. Truong, Y.Z. Liu, Y. Ren, L. Trahey, Y.G. Sun, Morphological and crystalline evolution of nanostructured MnO2 and its application in lithium-air batteries, ACS Nano 6 (2012) 8067e8077. [24] A. Zahoor, J.S. Jeon, H.S. Jang, M. Christy, Y.J. Hwang, K.S. Nahm, Mechanistic study on phase and morphology conversion of MnO2 nanostructures grown by controlled hydrothermal synthesis, Sci. Adv. Mater 6 (2014) 2712e2723. [25] G. Cheng, L. Yu, T. Lin, R.N. Yang, M. Sun, B. Lan, L.L. Yang, F.Z. Deng, A facile one-pot hydrothermal synthesis of b-MnO2 nanopincers and their catalytic degradation of methylene blue, J. Solid State Chem. 217 (2014) 57e63. [26] X. Wang, Y.D. Li, Selected-control hydrothermal synthesis of a-and b-MnO2 single crystal nanowires, J. Am. Chem. Soc. 124 (2002) 2880e2881. [27] J.B. Yang, X.D. Zhou, W.J. James, Growth and magnetic properties of MnO2-d nanowire microspheres, Appl. Phys. Lett. 85 (2004) 3160e3162. [28] S.Q. Zhao, T.M. Liu, D.F. Shi, Y. Zhang, W. Zeng, T.M. Li, B. Miao, Hydrothermal synthesis of urchin-like MnO2 nanostructures and its electrochemical character for supercapacitor, Appl. Surf. Sci. 351 (2015) 862e868. [29] P.R. Jadhav, M.P. Suryawanshi, D.S. Dalavi, D.S. Patil, E.A. Jo, S.S. Kolekar, A.A. Wali, M.M. Karanjkar, J.H. Kim, P.S. Patil, Design and electro-synthesis of 3-D nanofibers of MnO2 thin films and their application in high performance supercapacitor, Electrochim. Acta 176 (2015) 523e532. [30] Z.P. Bai, H.J. Li, M.J. Li, C.P. Li, X.F. Wang, C.Q. Qu, B.H. Yang, Flexible carbon nanotubes-MnO2/reduced grapheme oxide-polyvinylidene fluoride films for supercapacitor electrodes, Int. J. Hydrog. Energy 40 (2015) 16306e16315. €l, O. Crosnier, Daniel Be langer, [31] T. Brousse, M. Toupin, R. Dugas, L. Athoue Crystalline MnO2 as possible alternatives to amorphous compounds in electrochemical supercapacitors, J. Electrochem. Soc. 153 (2006) 2171e2180.

33

[32] A. Zolfaghari, F. Ataherian, M. Ghaemi, A. Gholami, Capacitive behavior of nanostructured MnO2 prepared by sonochemistry method, Electrochim. Acta 52 (2007) 2806e2814. [33] W.F. Wei, X.W. Cui, W.X. Chen, D.G. Ivey, Improved electrochemical impedance response induced by morphological and structural evolution in nanocrystalline MnO2 electrodes, Electrochim. Acta 54 (2009) 2271e2275. [34] D.Y. Qu, H. Shi, Studies of activated carbons used in double-layer capacitors, J. Power Sources 74 (1998) 99e107. [35] S. Franger, S. Bach, J. Farcy, J.P. Pereira-Ramos, N. Baffier, Synthsis, structural and electrochemical characterizations of the sol-gel birnessite MnO1.84$0.6H2O, J. Power Sources 109 (2002) 262e275. [36] Y.M. Wang, H.F. Liu, M. Bao, B.J. Li, H.F. Su, Y.X. Wen, F. Wang, Structuralcontrolled synthesis of manganese oxide nanostructures and their electrochemical properties, J. Alloys Compd. 509 (2011) 8306e8312. [37] S.Q. Zhao, T.M. Liua, D.W. Houa, W. Zeng, B. Miao, S. Hussain, X.H. Peng, M.S. Javed, Controlled synthesis of hierarchical birnessite-type MnO2 nanoflowers for supercapacitor applications, Appl. Surf. Sci. 356 (2015) 259e265. [38] L. Wang, Y.L. Zheng, Q.Y. Zhang, L. Zuo, S.L. Chen, S.H. Chen, H.Q. Hou, Y.H. Song, Template-free synthesis of hierarchical porous carbon derived from low-cost biomass for high performance supercapacitors, RSC Adv. 4 (2014) 51072e51079. [39] X. Liu, G. Du, J.L. Zhu, Z.F. Zeng, X.H. Zhu, NiO/LaNiO3 film electrode with binder-free for high performancesupercapacitor, Appl. Surf. Sci. 384 (2016) 92e98. [40] Z.F. Zeng, D.Z. Wang, J.L. Zhu, F.Q. Xiao, Y.D. Li, X.H. Zhu, NiCo2S4 nanoparticles//activated balsam pear pulp for asymmetric hybrid capacitors, CrystEngComm 18 (2016) 2363e2374.