SDS vesicle templating for high performance supercapacitors

Accepted Manuscript Synthesis of Mn3O4 nano-materials via CTAB/SDS vesicle templating for high performance supercapacitors Yuqing Qiao, Qujiang Sun, Ou Sha, Xiaoyu Zhang, Yongfu Tang, Tongde Shen, Lingxue Kong, Weimin Gao PII: DOI: Reference:

S0167-577X(17)31348-4 http://dx.doi.org/10.1016/j.matlet.2017.09.006 MLBLUE 23120

To appear in:

Materials Letters

Received Date: Revised Date: Accepted Date:

1 December 2016 23 August 2017 2 September 2017

Please cite this article as: Y. Qiao, Q. Sun, O. Sha, X. Zhang, Y. Tang, T. Shen, L. Kong, W. Gao, Synthesis of Mn3O4 nano-materials via CTAB/SDS vesicle templating for high performance supercapacitors, Materials Letters (2017), doi: http://dx.doi.org/10.1016/j.matlet.2017.09.006

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Synthesis of Mn3O4 nano-materials via CTAB/SDS vesicle templating for high performance supercapacitors Yuqing Qiao a,b, , Qujiang Sun b, Ou Sha b, Xiaoyu Zhang b, Yongfu Tang b, Tongde Shen a, Lingxue Kong c and Weimin Gao b,c, * a

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

066004, PR China b

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

c

Institute for Frontier Materials, Deakin University, Geelong, VIC 3217, Australia

Abstract: In this work, nano-structured Mn3O4 particles with spongy-morphology were synthesized through vesicle templating, where hexadecyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) were used as cationic-anionic surfactants. The Mn3O4 electrode materials exhibited a high specific capacitance (451 F g-1) at a current density of 0.5 A g-1 in 1 mol L-1 Na2SO4 electrolyte, and retained 333 F g-1 when the current density was increased to 5 A g-1. Its capacitance retention is also high (up to 92%) after 10000 cycles at a current density of 5 A g-1.

Keywords: Manganese oxide; Energy storage and conversion; Supercapacitor 1 Introduction Improving electrochemical capacitance, cyclic stability and rate dischargeability of electrode materials used in energy storage field are of practical significance [1-5]. Supercapacitors, where energy is stored electrostatically on the surface of the constitutive material, have attracted much attention due to their quick energy burst and longer lifespan [6-10], compared to other energy storage devices, such as Li-ion secondary batteries [7]. According to the mechanisms of ions-accumulation and electron transfer, supercapacitors are divided into two categories: the electrical double layer capacitors (EDLCs) and the pseudocapacitors (PCs). So far, a number of materials such as carbon [11, 12] and metal oxide/hydroxyl [13-15] have been recognized as promising electrode materials. The Mn3O4 electrode material has been characterized by higher theoretical capacitance, inexpensive cost and nontoxicity along with a good security [16-19]. However, its low conductivity and high resistivity during charge-discharge electrochemical



Corresponding authors. Tel.: +86 335 8061569; fax: +86 335 8061569.

E-mail addresses: [email protected] (Y. Qiao); [email protected] (W. Gao) 1

reaction hinder its commercial applications. Various approaches, such as reducing the particle size [16], and forming porous structure [20] were then developed to tackle these problems and most of the Mn3O4 electrode materials exhibited a good electrochemical performance. In our previous work [20-22], porous Mn3O4 electrode materials with high specific capacitance were fabricated by controlling the micelle of the surfactant, where hexadecyltrimethylammonium bromide (CTAB) was used and CH3CH2OH/H2O were used as the co-solvent achieved synchronically with the decomposing of the source material containing crystalline hydrate. In order to decrease the critical micelle concentration (CMC), CTAB and sodium dodecyl sulfate (SDS) with a ratio of 1:1 were used as cationic-anionic surfactant mixture in the present work. 2. Experimental Mn3O4 electrode materials were prepared with a solvothermal method. CTAB of 0.005 mol was first dissolved in 40 ml CH 3CH2OH at ambient temperature. SDS (x=0.005 mol) was then added to the CTAB ethanol solution and stirred for 1 h at 50℃ to prepare a complex solution. Mn(CH3COO)2.4H2O of 0.01 mol was added to the CTAB and SDS ethanol solution slowly, followed by adding carbamide of 0.01 mol. The precursor was transferred into a 100 mL PTFE-lined stainless container and, subsequently, heated at 100℃ for 12 h in a vacuum drying oven, followed by filtrating, washing and drying to prepare Mn3O4 sample. The crystal structure of the sample was characterized on a Rigaku D/max 2500pc X-ray Diffractometer. The morphology was characterized on an S-4800 scanning electron microscope. Analysis of N 2 (77 K) adsorption-desorption isotherms of the sample was performed with ASAP-2020e system. The X-ray photoelectron spectroscopy (XPS) was carried by an ESCALAB 250Xi. The electrochemical performance was examined by the cyclic voltammetry (CV) and galvanostatic charging-discharging (GCD) measurements with a three-electrode system in 1 mol L-1 Na2SO4 electrolyte solution where 70 wt% Mn3O4, 20 wt% acetylene black and 10 wt % polovinylidene fluorde (PVDF) were mixed to prepare the working electrode with an active material mass loading of about 2 mg cm-2. Pt electrode ( 1×1 cm2 ) and Hg/Hg2Cl2 electrode were used as the counter electrode and reference electrode, respectively. In addition, a symmetric Mn3O4/ Mn3O4 two-electrode cell was also constructed in 1 mol L-1 Na2SO4 electrolyte solution. Electro-chemical impedance spectroscopy (EIS) was deduced on a CHI 660E electrochemical workstation with alternating current amplitude of ±5 mV in the frequency range from 105 Hz to 2

10-2 Hz. 3. Results and discussion 3.1 Microstructure Fig. 1 a is the XRD patterns of the Mn3O4 prepared in the CTAB/SDS cationic-anionic surfactant mixture, showing a body centered tetragonal structure Mn3O4 phase (JCPDS no. 01-1127, space group: I41/amd). A slight signal of MnCO3 (JCPDS no. 44-1472) was also observed. The isotherm curve determined by the N2 adsorption-desorption measurement of the Mn3O4 presents a characteristic of mesoporous material with a specific surface area of 87.6 m2 g-1 and a pore diameter of about 3.6 nm, as shown in Fig. 1 b. Mn3O4 can be identified from the multiple splitting of the Mn 2p region with a splitting width of 11.46 eV (see Fig. 1 c-d), where

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the binding energy of Mn 2p2/3 and Mn 2p1/2 are 641.62eV and 653.08 eV, respectively.

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Fig. 1 (a) XRD, (b) N2 adsorption-desorption isotherms and pore size distribution (inset), (c) XPS survey scan spectra and (d) Mn 2p region of the Mn3O4 sample. 3.2 Morphology and formation mechanisms Figs. 2 a-b are the SEM and TEM micrographs of the Mn3O4 samples synthesized in the 3

cationic-anionic surfactant mixture. It can be seen that the Mn3O4 samples synthesized in CTAB:SDS =1:1 system present nano-particles of about 15 nm. The formation mechanisms of the Mn3O4 synthesized in the cationic-anionic surfactant mixture is illustrated in Figs. 2 c and d). First, CTAB solution is obtained using CH3CH2OH solvent at 25℃ without micelle existed. Second, CTAB and SDS solution are obtained using CH3CH2OH as the solvent at 50℃ without micelle existed. Third, CH3CH2OH/H2O solvent is obtained along with the addition of Mn(CH3COO)2.4H2O and CTAB and SDS are mixed in CH3CH2OH/H2O synchronously. As the CMC value of the CTAB/SDS mixture is small, vesicles are formed easily, which will act as the templates of the final products (see Fig. 2d) [23, 24].

Fig. 2 (a) SEM, (b) TEM and (c and d) schematic illustration of the growth mechanism of the Mn3O4 sample. As a typical surfactant, CTAB is composed of a large hydrophilic group (quaternary ammonium salt ion) and a long hydrophobic group (hexadecyl: -(CH2)15CH 3)), so the structure-model of CTAB can be described as pyramidal. On the other hand, SDS is composed of a small hydrophilic group (sulfate ion) and short hydrophobic group (dodecyl: -(CH2)11CH3)), the structure-model of SDS can be described as cylindric. In the cationic and anionic surfactant

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mixture, CTAB molecules couple with SDS molecules, forming vesicles which act as the templates for the growth of nano-structured Mn3O4 particles (Fig. 2d). 3.4 Electrochemical properties Fig. 3 a shows the CV curves of the Mn3O4 electrodes at different rates from 5 mV s-1 to 100 mV s-1, which exhibit an approximate rectangular shape even at high scan rate of 100 mV s-1. At the scan rates of 5, 10, 25, 50 and 100 mV -1, the corresponding specific capacitances were 420, 374, 275, 187 and 114 F g-1, respectively. Fig. 3 b shows the charge-discharge curves of the Mn3O4 electrodes at different current densities. It can be seen that the Mn3O4 electrodes have a linear dependence of the charge stored on the width of the potential window, suggesting pseudocapacitance behavior. 0.03

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Fig. 3 (a) Cyclic voltammetry curves, (b) Charge-discharge curves, (c) Rate dischargeability and (d) EIS with the inset of equivalent fitting circuit and impedance spectrum at high frequency region of the Mn3O4 electrode material. 5

The Mn3O4 fabricated in the present work exhibited a high specific capacitance of 451 F g-1 at a current density of 0.5 A g-1 and the specific capacitance still retains at 333 F g-1 when the discharge current density is increased by 10 times (to 5 A g-1), indicating a good high-rate dischargeability (Fig. 3 c). In addition, the cycle stability of the Mn3O4 electrodes were carried out at current density of 5 A g-1 and the result is shown in the inset of Fig. 3 c. The specific capacitance is 306 F g-1 after 10000 cycles, which is about 92% of the maximum specific capacitance (333 F g-1), indicating a good cycle stability of the spongy Mn3O4 electrodes. EIS was used to detect the charge transfer resistance (Rct) of the electrochemical reaction. It can be seen that the EIS curve consists of a semicircle in the high-frequency region, followed by a straight line in the low-frequency region (Fig. 3d). The semicircle reflects the impedance of electrochemical reaction, while the straight line indicates diffusion of the electroactive species. An equivalent circuit model (inset in Fig. 3d) is used to analyze the impedance spectra and the parameters in the equivalent circuit are fitted using least-square method with ZVIEW electrochemical impedance software. The fitting result shows that the Rct is only 0.21 Ω for the Mn3O4 electrode synthesized in CTAB:SDS=1:1 mixture (Chi-square: 2.2×10-4). Fig. 4 and Table 1 summarize the specific capacitance and cycle stability of previously studied Mn3O4 nano materials [8,10,17,18,20]. After 1,000 cycles, preciously studied Mn3O4 nano materials have a cycle stability between approx. 46 % and 90 % whereas the Mn3O4 have a cycle stability of 92 % after 10000 cycles. Although the new method that was used in the present work is an effective way to improve the electrochemical performance of Mn3O4, the high dissipative losses characterized were high (Fig. 3d) and the approximate rectangular shape (Fig. 3a) is unlike the one of good supercapacitors. This may due to the nature of the pseudocapacitive, which was used to designate electrode materials that had the electrochemical signature of a capacitive electrode (as observed with activated carbon), i.e., exhibiting a linear dependence of the charge stored with the width of the potential window, but where the charge storage originated from different reaction mechanisms. In addition, the capacitance and the cycle stability of the Mn3O4 fabricated were evaluated by constructing a symmetrical supercapacitor, and the high specific capacitance 94 F g-1 (0.5 A g-1) and cycle stability (100 % after 10000 cycles) also confirm the excellent electrochemical performance of the Mn3O4 nano-materials synthesized. 6

Table 1. Electrochemical performances of various Mn3O4 nano materials. Mn3O4

Capacitance(F g-1)

Electrolyte

Nano-spheres Nano-particles Nano-fiber Nano-sheets Micro-nano Nano-particles

0.5 M Na2SO4 1M Na2SO4 1M Na2SO4 1M Na2SO4 1M Na2SO4 1M Na2SO4

235 322 210 398 302 451

Cycle stability(%)

Reference

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[8] [10] [17] [18] [20] This work

90, 1000 77, 1000th 100, 500th 82, 2000th 89, 5000th 92, 10000th

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This work 90 80

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Fig. 4 Summarized specific capacitance and cycle stability values of various Mn3O4 nano materials. 4. Conclusions Nano-structured Mn3O4 particles were fabricated through vesicle templating. As the CMC value of the CTAB/SDS mixture is small, vesicles are formed easily and act as the templates of the final products. The structure characteristic of the vesicles is more beneficial for the formation of the uniform nanoparticles. The nano-structured Mn3O4 particles synthesized exhibit a higher specific capacity, rate capability and cycle stability when used as electrode materials of supercapacitors. Acknowledgement We gratefully acknowledge the financial support by the Natural Science Foundation of Hebei Province (B2016203391), the Natural Science Research Keystone Program of Universities in

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Hebei Province (ZD2016075), the High-level Talents Research Program of the Yanshan University (606001101), and the Foundation of the Key Technology Research and Development Program of Qinhuangdao (201501B008). References [1] A.M. Abioye, F.N. Ani, Renew. Sust. Energ. Rev. 52 (2015) 1282-1293. [2] Y.T. Li, S. Zhang, H.H. Song, X.H. Chen, J.S Zhou, S. Hong, Electrochim. Acta 180 (2015) 879-886. [3] Y.Q. Qiao, L.X. Pan, P. Jia, H.P. Wang, L. X. Kong, W.M. Gao, X.H. Wang, Materials Letters 137 (2014) 432-434. [4] Y.J. Li, G.L. Wang, T. Wei, Z.J. Fan, P. Yan, Nano Energy 19 (2016) 165-175 [5] Y.F. Tang, Y.Y. Liu, S.X. Yu, Y.F. Zhao, S.C. Mu, F.M. Gao, Electrochim. Acta 123 (2014) 158-166. [6] Y.F. Lee, K.H. Chang, C.C. Hu, Y.H. Chu, J. Power Sources 206 (2012) 469-475. [7] Y.Q. Qiao, H.P. Wang, X.Y. Zhang, P. Jia, T.D. Shen, X.F. Hao, Y.F. Tang, X.H. Wang,W.M. Gao, L. X. Kong, Materials Letters.184(2016)252-256. [8] H.B. Feng, H. Hu, H.W. Dong, Y. Xiao , Y.J. Cai , B.F. Lei, Y.L. Liu, M.T. Zheng, J. Power Sources 302 (2016) 164-173. [9] H. Zhu, J. Yin, X.L. Wang, H.Y. Wang, X.R. Yang, Adv. Funct. Mater. 23 (2013) 1305-1312. [10] C.L. Long, X. Chen, L.L. Jiang, L.J. Zhi, Z.J. Fan, Nano Energy 12 (2015) 141-151. [11] J.S. Zhou, J. Lian, L. Hou, J.C. Zhang, H.Y. Gou, M.R. Xia , Y.F. Zhao, T.A. Strobel, L. Tao, F.M. Gao, Nat. Commun. 6 (2015) 8503-8510. [12] 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. [13] Z.Y. Li, M.S. Akhtar, O.B. Yang, J. Alloy Compd. 653 (2015) 212-218. [14] Y.F. Zhao, W. Ran, D.B. Xiong, L. Zhang, J. Xu, F.M. Gao, Materials Letters 118 (2014) 80-83. [15] Y.Z. Luo, H.M. Zhang, L.W. Wang, M. Zhang, T.H. Wang, Electrochim. Acta 180 (2015) 983-989. [16] B.G.S. Raj, R.N.R. Ramprasad, A.M. Asiri, J. J. Wu, S. Anandan, Electrochim. Acta 156 (2015) 127-137. 8

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Highlights 1. Nano-structured Mn3O4 were synthesized through vesicle templating system. 2. The Mn3O4 electrode exhibits an higher specific capacitance up to 451 F g-1. 3. The Mn3O4 electrode displays superior cycle stability of 92% after 10000 cycles.

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