A modified solvothermal synthesis of porous Mn3O4 for supercapacitor with excellent rate capability and long cycle life

Accepted Manuscript A modified solvothermal synthesis of porous Mn3O4 for supercapacitor with excellent rate capability and long cycle life Yuqing Qiao, Qujiang Sun, Jianyi Xi, Haiying Cui, Yongfu Tang, Xianhui Wang PII:

S0925-8388(15)31719-9

DOI:

10.1016/j.jallcom.2015.11.163

Reference:

JALCOM 36018

To appear in:

Journal of Alloys and Compounds

Received Date: 1 September 2015 Revised Date:

20 November 2015

Accepted Date: 22 November 2015

Please cite this article as: Y. Qiao, Q. Sun, J. Xi, H. Cui, Y. Tang, X. Wang, A modified solvothermal synthesis of porous Mn3O4 for supercapacitor with excellent rate capability and long cycle life, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.11.163. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT

A modified solvothermal synthesis of porous Mn3O4 for supercapacitor with excellent rate capability and long cycle life Yuqing Qiao,∗ab Qujiang Sun,a Jianyi Xi,a Haiying Cui,a Yongfu Tang,a and Xianhui Wang*c a

b

RI PT

College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China

State Key Laboratory of Metastable Material Science and Technology, Yanshan University, Qinhuangdao, 066004,

China c

SC

School of Materials Science and Engineering, Xi’an University of Technology, Xi’an, 710048, China

M AN U

Abstract: Porous Mn3O4 with micro/nano-structure were fabricated by a modified solvothermal approach using hexadecyltrimethylammonium bromide as a surfactant and CH3CH2OH/H2O as a co-solvent,

in

which

H2O

derives

from

the

source

material

crystalline

hydrate

Mn(CH3COO)2.4H2O. It is found that a number of micelles form synchronously surrounding the

TE D

crystalline hydrate with the addition of Mn(CH3COO)2.4H2O. The Mn3O4 prepared presents a micro/nano-structure self-assembled by two-dimensional lamella structure with a thickness of approximately 100 nm, and a mesoporous with a specific surface area of 138.5 m2 g-1 and a total

EP

pore volume of 0.273 cm3 g-1. The electrochemical performance of the porous Mn3O4 used as a

AC C

supercapacitor electrode exhibits a relatively high specific capacitance value of 302 F g-1 at a low current density of 0.5 A g-1, and a good high-rate dischargability (246 F g-1, 81% capacity retention when the current densities are increased by 10 times (5 A g-1)), as well as good cycling stability (89% retention after 5000th charge /discharge cycles at 5 A g-1).

Keywords: Porous; Manganese oxide; Supercapacitor; Energy storage and conversion 1 Introduction ∗

Corresponding authors. Tel.: +86 335 8061569; fax: +86 335 8061569. E-mail addresses: [email protected] (Y. Qiao); [email protected] (X. Wang) 1

ACCEPTED MANUSCRIPT Supercapacitors have attracted great attentions due to their power density, high-rate dischargeability and cycling stability [1-3]. Generally, supercapacitors can be divided into electrical double layer capacitors (EDLCs) and pseudocapacitors (PCs) according to ions-accumulation mechanism and electron-transfer mechanism, respectively [4]. In past years,

RI PT

PCs have been extensively studied due to their high capacitance and energy density in comparison with EDLCs [5, 6]. However, poor rate capability and cycling stability hinder their commercial applications. As an electrode material for PCs, Mn3O4 is regarded as a promising electrode material for commercial supercapacitors because of environmental-friendly feature and low cost.

SC

The performance of Mn3O4 depends on its structure, morphology and components, such as porous structure [5, 6], nano-structure [7], or composite with other materials [8-10] and it is critical for

M AN U

Mn3O4 with encouraging electrochemical performances such as high rate capability and long cycle life. Currently, a number of Mn3O4 materials with various morphologies such as nano-particles [11-14], nano-fibers [15], nano-sheets [16], nano-rods [17], have been developed, and most of the Mn3O4

with

nanostructure

present

good

electrochemical

performances.

Though

nanocrystallization can give rise to better electrochemical performances, it also has negative

TE D

effects on the cycling stability and energy density. Subsequently, Mn3O4 materials with micro/nano-structured have been synthesized in order to eliminate its shortcomings [18-23]. In addition, Mn3O4 porous structure is also critical to high rate capability and long cycle life for an

EP

electrode material used in high-power electrochemical capacitors [24-29]. As a typical surfactant, hexadecyltrimethylammonium bromide (CTAB) is composed of a

AC C

hydrophilic group (quaternary ammonium salt ion) and a hydrophobic group (hexadecyl: -(CH2)15CH3), and it is generally used to prepare porous inorganic materials with particular morphologies by utilizing micelle [22, 30]. In the solvent, the hydrophilic group or the hydrophobic group of the surfactant aggregate to form micelle which can act as the template of porous material in the synthesized process. Critical micelle concentration (CMC) is the minimum concentration value for surfactant to form micelle in the solvent, which changes in different solvents. Li et al. [30] reported that the CMC value of CTAB in the aqueous CH3CH2OH solution

2

ACCEPTED MANUSCRIPT increases with increase of CH3CH2OH concentration in the solution. The CMC value of CTAB in water is 0.0009 mol L-1, whereas the CMC value of CTAB in CH3CH2OH solvent is 0.24 mol L-1. They also simulated the aggregation of CTAB in H2O and CH3CH2OH/H2O mixtures utilizing

RI PT

dissipative particle dynamics, and the simulation shows that the hydrophobic groups of CTAB interact more intensively with CH3CH2OH than with water, which suggests that it is difficult for micelles to form in CH3CH2OH solvents. Based on this finding, a modified method was proposed

SC

by the present authors to synthesize porous materials. (1) CTAB solution without micelle can be

M AN U

achieved utilizing CH3CH2OH as the solvent. (2) Sources materials containing crystalline hydrate such as Mn(CH3COO)2.4H2O is added into CTAB solution, which can act as Mn and H2O sources. (3) With the addition of crystalline hydrate, H2O molecules decompose at the surface of crystalline hydrate and are surrounded by CH3CH2OH molecules and CTAB molecules, it is assumed that

TE D

aqueous CH3CH2OH solvent forms in specific micro-area, which is suitable for CTAB to generate micelle, and, thus, micelle can form synchronously with the addition of crystalline hydrate. (4) Crystalline hydrate are surrounded by micelle and micelle will grow and aggregate with the H2O decomposed

from

EP

molecule

the

crystalline

hydrate

continually,

and

resulting

AC C

micro/nano-structured particles can be fabricated with the self assemble of the micelle. In this investigation, CH3CH2OH/H2O are used as co-solvent, in which CH3CH2OH can

act as the solvent of CTAB to get CTAB solution without micelle, and the co-solvent with H2O for CTAB to form micelle in a micro-area of CH3CH2OH/H2O. In addition, the present investigation focuses on the H2O in co-solvent (CH3CH2OH/H2O) which derives from the source material containing crystalline, and the purpose is to achieve micro-area of CH3CH2OH/H2O on the surface of the source material, during which the decomposing of crystalline hydrate and the formation of

3

ACCEPTED MANUSCRIPT micro-area of CH3CH2OH/H2O happen synchronically, resulting in the formation of micelle in the micro-area of CH3CH2OH/H2O and on the surface of the source material. Based on the modified method, porous Mn3O4 electrode material was synthesized in the

RI PT

present work. The phase constituents and morphology of the Mn3O4 microspheres were determined by X-ray diffractometer and scanning electron microscope, and the electrochemical properties of the porous Mn3O4 microspheres fabricated were measured by cyclic voltammetry

on

the

formation

mechanism

of

porous

Mn3O4

electrode

material

with

M AN U

insights

SC

(CV) and galvanostatic charge-discharge test. The purpose of the investigation is to get deep

micro/nano-structure. It is of significance for the preparation on Mn3O4 electrode material with better electronic conductivity and high rate dischargeability. 2 Experimental

TE D

Mn3O4 electrode material was prepared by a modified solvothermal synthesis method using Mn(CH3COO)2·4H2O, CTAB and Carbamide as the precursor and the experimental procedures were as follows: First, CTAB (0.001 mol) was dissolved in 40 ml CH3CH2OH to get CTAB without

micelle,

then

EP

solution

the sources

materials containing

crystalline hydrate

AC C

Mn(CH3COO)2·4H2O (0.01 mol) was added to CTAB solution slowly with a fixed rate of H2O and CH3CH2OH, followed by adding Carbamide (0.01 mol) used as the precipitant, and at last the precursor was transferred into a 100 mL PTFE-lined stainless container, and subsequently heated at 140 ℃ for 4 h.

The structure of the synthesized materials was performed on a Rigaku D/max 2500pc X-ray diffractmeter and an S-4800 field emission scanning electron microscope. Binding energy values of Mn were determined on an ESCALAB 250Xi X-ray photoelectron spectroscopy. The specific

4

ACCEPTED MANUSCRIPT surface area for the synthesized materials was determined by measuring nitrogen adsorption-desorption isotherms at 77 K with ASAP-2020e system. Electrochemical performances were tested on a BTS-5V10 mA system in the range of 0-1V

RI PT

at the current density of 0.5 A g-1 and 5 A g-1 and a three electrode system was used for the electrochemical measurements. Working electrode was prepared as follows: 70 wt% Mn3O4, 20 wt% acetylene black and 10 wt % polovinylidene fluorde (PVDF) was mixed and ground for 30

SC

min; The mass loading of active materials on the current collector was about 2mg cm-2. Pt

M AN U

electrode was used as a counter electrode, and Hg/Hg2Cl2 electrode was used as a reference electrode. The electrochemical measurements were tested in 1 mol L-1 Na2SO4 electrolyte solution. The electrochemical impedance spectroscopy (EIS) and the cyclic voltammeter were conducted on a CHI 660E electrochemical workstation.

TE D

3 Results and discussion 3.1 Structure characterization

Fig. 1 is the XRD pattern of the Mn3O4 sample prepared. As seen from Fig. 1, only a single

EP

Mn3O4 phase presents, which has a body centered tetragonal manganese oxide Mn3O4 (space

AC C

group: I41/amd). The particle size of Mn3O4 calculated from the half-width of (101), (112), (103) and (211) diffraction peaks are 11.1 nm, 11.7 nm, 10.9 nm and 11.6 nm, respectively. X-ray photoelectron spectroscopy spectrum of the Mn3O4 sample prepared is shown in Fig. 2.

With a survey region of 0-1200 eV, manganese, oxygen and carbon can be detected (see Fig. 2a). The manganese oxidation state is identified from the multiple splitting of the Mn 2p peak (see Fig. 2b). The binding energy values of 2p2/3 and Mn 2p1/2 are 641.73 eV and 653.43 eV, respectively, and the splitting width ΔE of the two peaks (Mn 2p2/3 and Mn 2p1/2) is 11.70 eV, which is in

5

ACCEPTED MANUSCRIPT accordance with the spectrum of Mn3O4 [31, 32]. 3.2 Morphology Figs. 3a-b are the overall morphology of the Mn3O4 microspheres at low magnifications.

RI PT

Obviously, most of the Mn3O4 consist of spherical particles with an average diameter of 10 µm. Figs. 3c-d show the surface and the root morphology of a single Mn3O4 microsphere. It is seen from Figs. 3c-d that a single Mn3O4 microsphere presents cauliflower morphology with root and

SC

stem. At higher magnifications, these cauliflower microspheres have micro/nano-structure which

M AN U

is composed of many lamellas with a thickness of approximately 100 nm, see Figs. 3e-f. From above results, it is believed that the micro/nano-structured Mn3O4 electrode materials are self-assembled by two-dimensional nano-lamellas, and pores exist between the nano-lamellas, which are beneficial for the improvement on the electrochemical performance of the Mn3O4

TE D

electrode material.

The porosity of the micro/nano-structured Mn3O4 electrode material was determined by nitrogen adsorption-desorption measurements. The N2 adsorption-desorption isotherm and the

EP

corresponding pore-size distribution curve for Mn3O4 sample are shown in Fig. 4. As seen from

AC C

Fig. 4, the isotherm curve presents the characteristic of mesoporous material, which is classified as type IV. The porous Mn3O4 sample exhibits a specific surface area of 138.5 m2 g-1 and a total pore volume of 0.273 cm3 g-1. The pore-size distribution curve (the inset shown in Fig. 4) shows a single peak around 10 nm. It suggests that Mn3O4 prepared has a uniform mesoporous characteristic. In comparison with the values reported in the literatures [25, 26], the specific surface area of Mn3O4 prepared (138.5 m2 g-1) is increased by about 61% than that of the Mn3O4 electrode (86.172 m2 g-1) using DMF as the solvent [26], and about 34% than that of the Mn3O4

6

ACCEPTED MANUSCRIPT nanoparticles (102 m2 g-1) with tunable microstructures synthesized by using Pluronic F127 as dispersant [25]. Owing to its larger specific surface area and higher porosity, the micro/nano-structured Mn3O4 has numerous potential applications as a catalyst and electrode

RI PT

material with high performance. 3.3 Formation mechanism

Fig. 5 is schematic illustration of a double-deck mechanism of micelle growth mechanism

SC

of porous Mn3O4 microspheres. Based on the morphology of Mn3O4 prepared (Fig. 3), the results

M AN U

of BET (Fig. 4) and the synthesis process, the formation mechanism of the porous Mn3O4 microsphere can be discussed as follows. First, CTAB solution is obtained utilizing ethanol as the solvent. Li et al. [30] reported that the hydrophobic groups of CTAB interacted more strongly with CH3CH2OH rather than H2O, suggesting that it is difficult for CTAB to form micelle in

TE D

CH3CH2OH solvents. Secondly, crystalline hydrate Mn(CH3COO)2·4H2O used as the water source and material source is added in CTAB solution. The purpose of using source material containing crystalline hydrate is to achieve micro-area of CH3CH2OH/H2O on the surface of the source

EP

material. The crystalline hydrate decomposes from the surface of the Mn(CH3COO)2·4H2O source

AC C

material; it can adsorb CH3CH2OH to form CH3CH2OH/H2O interface (double layers), and the CTAB molecules in CH3CH2OH can be adsorbed by H2O synchronically at the interface. The CTAB concentration in CH3CH2OH layer at the interface is about 0.025 mol L-1 (0.001 mol CTAB in 40 mL ethanol), so the CTAB concentration in H2O layer at the interface is also about 0.025 mol L-1, which is big enough for CTAB to form micelles, as the CMC of CTAB in water is as low as 0.0009 mol L-1. Thirdly, the micelles grow into cylinders while the H2O molecules decompose progressively from crystalline hydrate Mn(CH3COO)2·4H2O. As the hydrophobic groups remain

7

ACCEPTED MANUSCRIPT at the outside of the micelles, they attract with each other by Van der Waals force, giving rise to the stack and growth of micelles till the formation of lamellas. Finally, Mn3O4 electrode material with porosity between the lamellas can be synthesized, which is derived from the loss of

RI PT

hydrophobic groups at 140℃. 3.4 Electrochemical properties

The electrochemical properties of the porous Mn3O4 microspheres fabricated were measured

SC

by cyclic voltammetry (CV) and galvanostatic charge-discharge test. Fig. 6a shows the CV curves

M AN U

of the porous Mn3O4 electrode at a scan rate in the range of 5-100 mV s-1. It is evident that most of the curves present a symmetrical shape except at an extremely high rate of 100 mV s-1, suggesting that the porous Mn3O4 electrode fabricated has an excellent capacitive behavior. Fig. 6b shows the effect of the current density on the specific capacitance of the porous

TE D

Mn3O4 electrode. The Mn3O4 prepared has an excellent high-rate performance, and the specific capacitances at the current density of 0.5, 1, 2 and 5 A g-1 are 302, 283, 267 and 246 F g-1, respectively. Compared with the previous values reported by the present authors [22], the specific

EP

capacitance (302 F g-1) is increased by about 5.5% than that of the porous Mn3O4 electrode (286 F

AC C

g-1) at a low current density of 0.5 A g-1, while the specific capacitance (246 F g-1) is increased by about 7% than that of the Mn3O4 electrode (230 F g-1) at a high current density of 5 A g-1. As seen from the inset in Fig. 6b, the porous Mn3O4 electrode has an excellent cycle life.

Especially, the capacitance retention is 100% after 1000th cycles at a current destiny 5 A g-1, and the capacitance retention is as high as 89% after 5000 cycles. Fig. 6c is the galvanostatic charge-discharge curve of the porous Mn3O4 electrode at the current density of 0.5 A g-1. According to the literature [25], the specific surface area of Mn3O4 can provide more

8

ACCEPTED MANUSCRIPT electrochemically active sites for the charge storage and delivery. However, the mesopores or macropores of the materials may affect the ion transportation and electrolyte permeation during the process of charge storage/delivery, which becomes the bottleneck at very high scan rates of

RI PT

CV. Though the specific surface area of Mn3O4 prepared in the present work (138 m2 g-1) is larger than that of the Mn3O4 synthesized with surfactant-assisted methods [25, 26], it is lower than that reported by Yousefi et al. [28]. They synthesized porous Mn3O4 nano spheres by cathodic

SC

electrodeposition on a steel electrode without the help of surfactant. The specific surface area is

M AN U

177.6 m2 g-1, which is larger than that of Mn3O4 synthesized by surfactant-assisted methods [25, 26]. However, the specific capacitance is unsatisfactory, which is only 235.4 F g-1. Though the nano-structure and/or porous-structure material prepared by the method has better electrochemical performance, it also deteriorates the tap density. The tap density value of

TE D

the Mn3O4 electrode material is 4 kg m-3. In addition, the specific energy density and power density of the micro/nano-structure Mn3O4 were evaluated by constructing a symmetrical supercapacitor in Na2SO4 electrolyte, and the Ragone plot is shown in Fig. 6d. The results show

EP

that the specific energy density is 4 Wh kg-1 at a power density of 0.098 kW kg-1 and the highest

AC C

power density is 0.713 kW kg-1 at an energy density of 0.793 Wh kg-1, which is comparable to the nanostructured Mn3O4 with nanoscaled/microscaled porous structures [6]. Fig. 7a is the electrochemical impedance spectra (EIS) of the as-prepared porous Mn3O4

electrode material and Fig. 7b is the fine characteristic of EIS at the high frequency region. As EIS has small amplitude in the range of -5mV - 5mV, the electrochemical reaction can be regarded as a reversible system or a semi-reversible system which can be described by Nyquist eqution. An equivalent circuit was used (see the inset in Fig. 7a), where Rs and Rct represent the internal

9

ACCEPTED MANUSCRIPT resistance and the charge-transfer resistance, respectively. CCPE and Ws are the constant phase element and the Warburg coefficient in the EIS, respectively. The semicircle near the high frequency region (see Fig. 7b) corresponds to the charge transfer resistance Rct at the interface of

RI PT

electrode/electrolyte. The Rct value is 0.52 Ω for the porous Mn3O4 electrode, which is decreased by approximately 4.7 times than that reported in the literature (2.43 Ω) and the charge-transfer resistance 0.52 Ω is decreased by about 1.8 times than that of the Mn3O4 electrode (0.91 Ω)

SC

synthesized in DMF solvent [26].

M AN U

4 Conclusions

Porous Mn3O4 with micro/nano-structure was fabricated by a modified solvothermal synthesis. The porous Mn3O4 microsphere presents cauliflower morphology and a mesoporous characteristic with a specific surface area of 138.5 m2 g-1 and a total pore volume of 0.273 cm3 g-1.

TE D

The micro/nano-structured Mn3O4 used as a supercapacitor electrode exhibits a high specific capacitance of 230 F g-1 and 210 F g-1 at a current density of 5 A g-1 and 10 A g-1, which is respectively about 80% and 73% of the specific capacitance of 286 F g-1 at a current density of 0.5

EP

A g-1. The micro/nano-structured Mn3O4 used as a supercapacitor electrode exhibits larger

AC C

interface of the electrode and electrolyte; higher electrolyte penetration/diffusion rates and larger volumetric energy density. Acknowledgement

This work was supported by the Foundation of the Key Technology Research and Development Program of Qinhuangdao (201501B008), the Pivot Innovation Team of Shaanxi Electrical Materials and Infiltration Technique (2012KCT-25) and Shaanxi Provincial Project of Special Foundation of Key Disciplines (2011HBSZS009).

10

ACCEPTED MANUSCRIPT References [1] Y. F. Zhao, Z. Zhang, Y. Q. Ren, W. Ran, X. Q. Chen, J. S. Wu, F. M. Gao, J. Power Sources 286 (2015) 1-9.

[3] V. C. Bose, V. Biju, Mat. Sci. Semicon. Proc. 35 (2015) 1-9. [4] P. Simon, Y. Gogotsi, B. Dunn, Science 343 (2014) 1210-1211.

RI PT

[2] Z. Y. Li, M. S. Akhatar, O. B. Yang, J. Alloy Compd. 653 (2015) 212-218.

SC

[5] Y. T. Wang, A. H. Lu, W. C. Li, Micropor. Mesopor. Mat. 153 (2012) 247-253.

M AN U

[6] D. Dubal, A. Jagadale and C. Lokhande, Electrochim. Acta, 80 (2012) 160-170. [7] B. G. S. Raj, A. M. Asiri, J. J. Wu, S. Anandan, J. Alloy Compd. 636 (2015) 234-240. [8] Y. F. Zhao, W. Ran, D. B. Xiong, L. Zhang, J. Xu, F. M. Gao, Mater. Lett. 118 (2014) 80-83. [9] G. Z. Jin, X. X. Xiao, S. Li, K. M. Zhao, Y. Z. Wu, D. Sun, F. Wang, Electrochim. Acta 178

TE D

(2015) 689-698.

[10] G. R. Xu, J. J. Shi, W. H. Dong, Y. Wen, X. P. Min, A. P.Tang, J. Alloy Compd. 630 (2015) 266-271.

EP

[11] X. She, X. M. Zhang, J. Y. Liu, L. Li, X. H. Yu, Z. L. Huang, S. M. Shang, Mater. Res. Bull.

AC C

70 (2015) 945-950.

[12] F. Gao, J. Y. Qu, Z. B. Zhao, Q. Zhou, B. B. Li, J. S. Qiu, Carbon, 80 (2014) 640-650. [13] B. G. S. Raj, R. N. R. Ramprasad, A. M. Asiri, J. J. Wu, S. Anandan, Electrochim. Acta 156 (2015) 127-137.

[14] I. Ryu, G. Kim, D. Park, S. Yim, J. Power Sources 297 (2015) 98-104. [15] J. Bhagwan, A. Sahoo, K. L. Yadav, Y. Sharma, Electrochim. Acta 174 (2015) 992-1001.

11

ACCEPTED MANUSCRIPT [16] Y. Z. Luo, H. M. Zhang, L. Wang, M. Zhang, T. H. Wang, Electrochim. Acta 180 (2015) 983-989. [17] L. L. Lan, Q. J. Li, G. R. Gu, H.F. Zhang, B. B. Liu, J. Alloy Compd. 644 (2015)

RI PT

430-437. [18] J. Z. Wang, N. Du, H. Wu, H. Zhang, J. X. Yu, D. R. Yang, J. Power Sources 222 (2013) 32-37.

SC

[19] D. P. Dubal, R. Holze, J. Power Sources 238 (2013) 274-282.

M AN U

[20] T. Nguyen, M. J. Carmezim, M. Boudard, M. F. Montemor, Int. J. Hydrogen Energ. 2015 Available online.

[21] J. Y. Yang, X. F. Yang, Y. L. Zhong, J. Y. Ying, Nano Energy 13 (2015) 702-708. [22] Y.Q. Qiao, Q.J. Sun, H.Y. Cui, D.B. Wang, F.Y. Yang, X.H. Wang, RSC Adv. 5 (2015)

TE D

31942-31946.

[23] J.Y. Liu, Y.H. Ng, M.B. Okatan, R. Amal, K. Bogle, V. Nagarajan, Electrochim. Acta 130 (2014) 810-817.

EP

[24] W.H. Wang, T.L. Yang, G.J. Yan, H.Y. Li, Mater. Lett. 82 (2012) 237-239.

AC C

[25] Y.F. Lee, K.H. Chang, C.C. Hu, Y.H. Chu, J. Power Sources 206 (2012) 469-475. [26] F.Y. Yang, M.S. Zhao, Q.J. Sun, Y.Q. Qiao, RSC Adv. 5 (2015) 9843-9847. [27] S.H. Park, W.J. Lee, J. Power Sources 281 (2015) 301-309. [28] T. Yousefi, A. Golikand, M. Mashhadizadeh, M. Aghazadeh, Curr. Appl. Phys. 12 (2012) 544-549. [29] Y. H. Xiao, Y. B. Cao, Y. Y. Gong, A. Q. Zhang, J. H. Zhao, S. M. Fang, D. Z. Jia, F. Li, J. Power Sourcs 246 (2014) 926-933.

12

ACCEPTED MANUSCRIPT [30] W. Li, M. Zhang, J. Zhang, Y. Han, J. Tianjin University 39(1) (2006) 5-9. [31] S.K. Park, A.H. Jin, S.H. Yu, J.H. Ha, B.C. Jang, S.Y. Bong, S.H. Woo, Y.E. Sung, Y.Z. Piao, Electrochim. Acta 120 (2014) 452-459.

AC C

EP

TE D

M AN U

SC

RI PT

[32] H. Chen, S.X. Zhou, L.M. Wu, ACS Appl. Mater. Interfaces 6 (2014) 8621-8630.

13

ACCEPTED MANUSCRIPT

Figure Captions Fig. 1 XRD pattern of Mn3O4 electrode material Fig. 2 XPS spectra: (a) survey scan, (b) Mn 2p region of Mn3O4 sample

RI PT

Fig. 3 SEM micrographs of the porous Mn3O4 prepared: (a-b) overall morphology of the products at low magnifications; (c-d) a single Mn3O4 microsphere of the surface and the root; (e-f) the constituent detail in the microsphere at high magnifications

SC

Fig. 4 N2 adsorption–desorption isotherms with an inset of pore size distribution curve of Mn3O4

Mn3O4 microspheres

M AN U

Fig. 5 Schematic illustration of the formation of micelle and the growth mechanism of porous

Fig. 6 (a) CV curves, (b) discharge capacity at different current densities and cycling stability profile at a current density of 5 A g-1, (c) charge-discharge curves at 0.5 A g-1 and (d) Ragone plot

TE D

of the symmetrical supercapacitor.

Fig. 7 Electrochemical impedance spectrum of Mn3O4 electrode (a) and the amplification at high

AC C

EP

frequency region (b)

14

RI PT

ACCEPTED MANUSCRIPT

10

(101)

(112)

30

M AN U

20

SC

Intensity / a.u.

(105) (312) (303)

60

(224)

(314)

(413)

80

(315) (415)

Mn3O4

(305)

PDF#01-1127

70

TE D

(220)

50

EP

(211)

40

2 Theta / degree

Fig. 1

90

AC C

(103)

O KLL O 1s

Mn 2p 3/2

Intensity / a.u.

Mn 2p 1/2

C 1s

11.70 eV

Survey 0

200

400

600

800

1200

AC C

EP

Fig. 2

Mn 2p

636 638 640 642 644 646 648 650 652 654 656 658 660

Binding energy / eV

TE D

Binding energy / eV

1000

Mn 2p 1/2

M AN U

Intensity / a.u.

(b)

Mn 2p 3/2

SC

(a)

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Fig. 3

0.004

100

0.003

SC

150

0.002 0.001 0.000 0

50

0.0

0.2

M AN U

dV/dD / cm3 g-1 nm-1

3

RI PT

200

-1

Quanitity adsorbed / m g STP

ACCEPTED MANUSCRIPT

20 40 60 80 100 Pore diameter / nm

0.4

0.6

0.8

AC C

EP

TE D

Ralative pressure / P/P0

Fig. 4

1.0

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Fig. 5

0.06

-0.02 -0.04 0.0

0.2

0.4

0.6

Potential / V

-1

100

200 100

M AN U

0.00

200

300

SC

0.02

(b) 300 Specific capacitance / F g

Current / A

0.04

Specific capacitance / F g-1

-1

5 mV s -1 10 mV s -1 25 mV s -1 50m V s -1 100 mV s

(a)

RI PT

ACCEPTED MANUSCRIPT

0.8

0

0

0

1.0

0

1

1000 2000 3000 4000 5000 Cycle number / n

2

3

4

5

-1

Current density / A g

1.0

(d)

(c)

TE D

Potential / V

0.8 0.6 0.4 0.2

EP

0.0 0

2000

4000

Power density / kW kg-1

1.0

0.6 0.4 0.2

-1

0.5 A g

6000

Time / s

AC C

0.8

Fig. 6

0.0 0

1

2

3

4 -1

Energy density / Wh kg

5

1.0

(b)

CPE2

CPE1

0.8

Rs Rect

3.329 ohm

Ws

-Z'' / ohm

Rct

40 20

(b)

6

9

12

15

TE D

Z' / ohm

EP

Fig. 7

AC C

0.4

Rct

0.2

0 3

0.6

M AN U

-Z'' / ohm

60

SC

80

(a)

RI PT

ACCEPTED MANUSCRIPT

0.0 3.0

3.5

4.0

4.5

Z' / ohm

5.0

5.5

6.0

ACCEPTED MANUSCRIPT

Highlights

Porous Mn3O4 are fabricated by a modified solvothermal synthesis.

2.

The porous Mn3O4 electrode materials present an excellent rate capability.

3.

The formation mechanism of the porous Mn3O4 microspheres is clarified.

AC C

EP

TE D

M AN U

SC

RI PT

1.