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γ-MnOOH composites on carbon cloth
Chemical Physics Letters 738 (2020) 136859
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Effect of annealing temperature on the compositions and electrochemical performance of Mn3O4/γ-MnOOH composites on carbon cloth
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Shujuan Menga,b, Zunli Moa, , Zhenliang Lia, Ruibin Guoa, Nijuan Liua a Research Center of Gansu Military and Civilian Integration Advanced Structural Materials, Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education of China, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China b School of Chemical Engineering, Northwest Minzu University, Lanzhou 730030, China
H I GH L IG H T S
grown on carbon cloth transforms into Mn O between 200 °C and 300 °C. • γ-MnOOH of carbon cloth at high temperature help Mn O convert into MnO. • Reducibility • Increasing annealing temperature reduces the capacitance of Mn O /γ-MnOOH. 3
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A B S T R A C T
Keywords: Mn3O4/γ-MnOOH Carbon cloth Annealing temperature Supercapacitor
In this paper, the effect of annealing temperature on the compositions and corresponding electrochemical performance of Mn3O4/γ-MnOOH composites on the carbon cloth fabricated by a simple hydrothermal method is investigated. The results show that the γ-MnOOH converts into Mn3O4 completely between 200 °C and 300 °C and further increasing the annealing temperature to 400 °C and 500 °C leads to the transformation of partial Mn3O4 into MnO. The electrochemical measurement results for products annealed at different temperatures suggest that increasing the annealing temperature causes the decreased capacitance performance.
1. Introduction Manganese oxides and oxyhydroxides exhibit various stoichiometries and rich crystal structures because of the various oxidation states and variable coordination environments of manganese ions [1,2]. Many of them, including MnO2, Mn3O4, Mn2O3, MnOx and MnOOH [3–8], attract much attention in the supercapacitor electrode design due to their high capacitance, long time stability, low cost, easy preparation and nontoxcity. Different phases of manganese oxides and/or oxyhydroxides can be interconverted when some external factors are applied on them [9–18]. One of the most used strategies to realize this interconversion is annealing. For example, γ-MnOOH nanorods tend to transform into MnO2, Mn2O3, or Mn3O4 nanorods when annealed under different temperatures and atmospheres [10]. This triggers scientists to prepare different phases of manganese oxides with rich morphologies by annealing of various morphologies of γ-MnOOH precursor. Although single component of manganese oxides or oxyhydroxides shows exciting results when used as supercapacitor electrodes, the low conductivity of both manganese oxides and oxyhydroxides limits their
⁎
further improved performance. Some strategies were proposed to address this issue, such as construction of metal oxide heterostructures and doping [19–21]. Combining with high conductivity of carbonbased materials is one of effective ways to increase the conductivity of manganese oxides or oxyhydroxides and thus improves their electrochemical performance [22,23]. In a previous work [24], we demonstrated that growth of Mn3O4/γ-MnOOH composites on carbon cloth (Mn3O4/γ-MnOOH@CC) is beneficial for the improved electrochemical performance due to the formation of heterojunctions at Mn3O4/γMnOOH interface as well as the high conductivity of carbon cloth. Previous works also demonstrated that direct annealing of MnOOH would lead to the formation of different phases of maganese oxides, like MnO2 [25,26], Mn3O4 [10,12] and Mn2O3 [10] depending on the annealing temperature and atmosphere used. Carbon cloth possesses moderate reducibility at high temperature [27]. It is expected that annealing of γ-MnOOH grown on the carbon cloth will present different results when compared with direct annealing of γ-MnOOH. In this work, we investigated the effect of annealing temperature on the compositions and corresponding electrochemical performance of
Corresponding author. E-mail address: [email protected] (Z. Mo).
https://doi.org/10.1016/j.cplett.2019.136859 Received 31 August 2019; Received in revised form 22 September 2019; Accepted 14 October 2019 Available online 14 October 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
Chemical Physics Letters 738 (2020) 136859
S. Meng, et al.
Fig. 1. XRD patterns of Mn3O4/γ-MnOOH@CC, S200, S300, S400 and S500.
Fig. 2. SEM images of S200 with different magnifications.
2. Experimental section
Mn3O4/γ-MnOOH@CC. The results demonstrated that increasing annealing temperature from 200 °C to 500 °C in N2 atmosphere leads to the conversion of γ-MnOOH into MnO via Mn3O4. The electrochemical measurement results suggest that increasing the annealing temperature leads to the decreased capacitance performance, which is due to the destruction of Mn3O4/γ-MnOOH heterostructures and the formation of insulating MnO species at high temperature.
2.1. Synthesis of Mn3O4/γ-MnOOH@CC We firstly synthesized Mn3O4/γ-MnOOH@CC by a simple hydrothermal method as described in the previous work [24]. Briefly, 500 mg solid KMnO4 (Alfa Asera, 99.0%) was dissolved in 40 mL deionized water and the KMnO4 solution was stirred magnetically for 20 min at room temperature. Then the solution was transferred into a 50-mL-capacity Teflon-lined autoclave and three pieces of carbon cloth (Shanghai Lishuo Composite Technology Co., Ltd. China, 220 g m−2) 2
Chemical Physics Letters 738 (2020) 136859
S. Meng, et al.
Fig. 3. Elemental analysis of S200. (b) Energy dispersive spectrum of selected in the rectangular area in (a). EDS mapping for (d) Mn, (e) O and (f) C measured from the corresponding area in (c).
with 2.5 × 4 cm2 area were immersed into the solution. The autoclave was then heated at 180 °C for 15 h. After cooling down to room temperature, the carbon cloth covered with Mn3O4 particles/γ-MnOOH nanowires composites were picked up and dried at 100 °C for 12 h.
2.3. Materials characterizations The composition/phase, microstructures and morphologies of the annealed samples were characterized by X-ray powder diffraction (XRD, X'TRA, ARL, Switzerland) using a 40 kW advanced X-ray diffractometer with Cu Ka radiation (λ = 1.54056 Å), Field-emission scanning electron microscopy (FE-SEM, FEI, Inspect F50). The elemental composition and distribution in samples were characterized by energy-dispersive X-ray spectroscopy (EDS) and elemental mapping techniques with X-Max Extreme detector (Oxford Instruments).
2.2. Annealing of Mn3O4/γ-MnOOH@CC The obtained Mn3O4/γ-MnOOH@CC was placed in a porcelain boat and then was transferred to a quartz tube. A protective gas of high purity N2 (99.99%) was passed through the quartz tube at a rate of 400 standard cubic centimeters per minute (sccm) for 10 min to purge the air in the tube. The system was then heated to expected temperatures at a heating rate of 5 °C /min and held at that temperature for 2 h before it cooled down to room temperature in the protective gas with 400 sccm. The Mn3O4/γ-MnOOH@CC annealed at 200 °C, 300 °C, 400 °C and 500 °C are labelled as S200, S300, S400 and S500, respectively.
2.4. Electrochemical measurements The electrochemical properties of S200, S300, S400 and S500 were measured using a three-electrode system in fresh 1 M Na2SO4 aqueous solution as electrolyte. The working electrode was a piece of carbon 3
Chemical Physics Letters 738 (2020) 136859
S. Meng, et al.
Fig. 4. CV curves of S200, S300, S400 and S500 recorded at various scan rates.
cloth covering with corresponding products (1 cm2 in area), which was cut from S200, S300, S400 and S500 directly. A platinum plate and Ag/ AgCl electrode (saturated with KCl (aq)) were used as the counter and reference electrode, respectively. The electrochemical performance of the supercapacitor electrode was evaluated by the cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS) using a CHI660E electrochemical workstation (CHI660E, Shanghai Chenhua Device Company, China) at room temperature. EIS was done at the open circuit voltage with an alternate voltage amplitude of 5.0 mV in the frequency range of 0.01 –100 kHz. Here, specific areal capacitance is chosen to evaluate the electrode performance in three-electrode system. The specific areal capacitance (Cs, F cm−2) was calculated based on Eq. (1):
Cs =
Icons S ΔV /Δt
S200 indicate that the main compositions of S200 are Mn3O4 (hausmannite, JCPDS No. 24-0734) and γ-MnOOH (manganite, JCPDS No. 41-1379), which are the same with that of Mn3O4/γ-MnOOH@CC. For S300, only peaks of Mn3O4 are observed, indicating that γ-MnOOH transformed into Mn3O4 completely between 200 °C and 300 °C. When the temperature increased to 400 °C and 500 °C, the main composition of S400 and S500 is Mn3O4, but several peaks belonging to MnO (manganosite, JCPDS No. 07-0230) appeared and its content is enhanced with the increasing temperature. This suggests that partial Mn3O4 converted into MnO at higher temperature (400 °C and 500 °C). It is reported that γ-MnOOH would transfrom into Mn3O4 at ~272 °C when calcined under argon atmosphere with release of water and oxygen via Eq. (2) [12]: 12γ-MnOOH → 4Mn3O4 + 6H2O + O2
(1)
(2)
Mn3O4 particles can be reduced by graphite oxide sheets and H2 when heated at 400 °C because of the reducibility of the graphite sheets and H2 [28]. Based on the above reports and our experimental facts, we proposed a conversion process from γ-MnOOH to MnO. At ~272 °C, γMnOOH transformed into Mn3O4 via Eq. (2) and Mn3O4 contacting with carbon cloth closely was further reduced into MnO because of the enhanced reducibity of carbon cloth at high temperature via Eq. (3):
where Icons (A), ΔV/Δt (V/s), and S (cm2) corresponds to the applied constant discharge current, the slope of the discharge curve excluding IR drop and the projected area of the electrode material, respectively. 3. Results and discussion 3.1. Materials characterizations
2Mn3O4 + C → 6MnO + CO2
XRD was used to confirm the compositions of the annealed products. The XRD patterns of Mn3O4/γ-MnOOH@CC, S200, S300, S400 and S500 are displayed in Fig. 1. A broad peak at 2θ = ~26° corresponding to carbon cloth can be observed for all samples. The peaks of
The electrochemical performance of the materials is closely related with their morphologies. The SEM images of Mn3O4/γ-MnOOH@CC are displayed in Fig. S1. It can be seen that the products covering on the
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(3)
Chemical Physics Letters 738 (2020) 136859
S. Meng, et al.
Fig. 5. GCD curves of S200, S300, S400 and S500 recorded at different current densities.
Fig. 6. (a) Rate performance and (b) Nyquist curves of S200, S300, S400 and S500.
respectively. Fig. 2 shows the SEM images of S200, and it is found that the morphologies of products are the same with that of Mn3O4/γMnOOH@CC. The SEM images of S300, S400 and S500 are shown in Fig. S2, Fig. S3 and Fig. S4, respectively. From careful observation of
carbon cloth surface displayed two different morphologies: one is particles contacting with carbon cloth closely and the other is nanowires covering on the particles. In the previous work [24], it is confirmed that the composition of particles and nanowires is Mn3O4 and γ-MnOOH, 5
Chemical Physics Letters 738 (2020) 136859
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increased charge transfer resistance, leading to the decreased capacitance of S400 and S500.
the morphologies of S300, S400 and S500, it is also found that the morphologies of products are nearly unchanged when compared with that of Mn3O4/γ-MnOOH@CC and S200, indicating that increasing annealing temperature only changes the compositions but not the morphologies of products. Elemental analysis was carried out to confirm the elemental distribution for S200. The energy dispersive spectrum of S200 exhibited in Fig. 3b suggests that the main elements of S200 are Mn, O and C. The minor amount of K elements may come from the residual unreacted KMnO4 species. The EDS mapping for Mn, O and C displayed in Fig. 3d, e and f measured from corresponding area in Fig. 3c demonstrates that both Mn and O distributed uniformly.
4. Conclusions In this work, the effect of annealing temperature on the compositions and corresponding electrochemical performance of Mn3O4/γMnOOH@CC is investigated. It is found that the γ-MnOOH grown on the carbon cloth synthesized by one step hydrothermal method converts into Mn3O4 between 200 °C and 300 °C. When the annealing temperature increases to 400 °C and 500 °C, partial Mn3O4 transforms into MnO because of the enhanced reducibility of carbon cloth at high temperature. The electrochemical measurement results indicate that increasing the annealing temperature leads to decreased capacitance performance for Mn3O4/γ-MnOOH@CC, which can be explained by that the destruction of Mn3O4/γ-MnOOH heterostructures for S300 and that the formation of insulating MnO species increased the charge transfer resistance for S400 and S500.
3.2. Electrochemical performance of products The CV curves of S200, S300, S400 and S500 recorded at different scan rates are displayed in Fig. 4a, b, c and d, respectively. All curves show a quasi-rectangular shape, which indicates that all samples exhibit ideal capacitive behavior. No redox peaks are observed from the CV curves, which suggests that the Faradic redox reaction occurred on the products surface is very fast and reversible. The GCD curves of S200, S300, S400 and S500 recorded at various current densities are displayed in Fig. 5. The discharge time of these samples recorded at fixed current density (for example, 1 mA cm−2) is decreased with the increasing annealing temperature from S200 to S500 and the IR drop increased from S200 to S500, indicating that the internal resistance gradually increased with the increasing annealing temperature. The specific capacitance calculated based on Eq. (1) at different current densities for S200, S300, S400 and S500 is exhibited in Fig. 6a. The specific areal capacitance of S200 is 2878.7, 2501.1, 2183.2, 1786.9, 1503.5, 1165.8, 974.8, 191.3 and 117.6 mF cm−2 at current density of 1, 2, 3, 5, 7, 9, 10, 15 and 20 mA cm−2, respectively. These values are higher than many other metal oxides or oxyhydroxides grown on carbon cloth, such as Co3O4 nanowires@α-Fe2O3 nanorod@CC (245 mF cm−2 at 1 mA cm−2 in 1 M Na2SO4) [29], α-Fe2O3@ MnCo2O4@CC (1073 mF cm−2 at 1 mA cm−2 in 1 M KOH) [30], βFeOOH@CC (1120 mF cm−2 at 1.0 mA cm−2 in 2 M KOH) and NiMoO4@CC (1270 mF cm−2 at 5.0 mA cm−2 in 2 M KOH) [31,32]. S300 possesses similar capacitance with S200 at low current density, but it decreased faster than that of S200 with the increasing current density. It is known that rational design of heterostructures can enhance the electrochemical performance of the materials due to the enhanced electron transfer and separation [33,34]. The complete conversion of γMnOOH to Mn3O4 leads to the destruction of γ-MnOOH/Mn3O4 heterojunctions, which may reduce the electron conductivity causing reduced rate performance of S300. S400 and S500 showed less capacitance performance than that of S300 at all current densities. The capacitance of S500 is even lost when the current density increased to higher values (20 mA cm−2). All above results demonstrate that the increasing the annealing temperature leads to reduced capacitance performance for Mn3O4/γ-MnOOH@CC. In order to investigate the possible reason for the decreased capacitance with the increasing annealing temperature, EIS was carried out to characterize the electrons/ions conductivity characteristics of these products. Fig. 6b shows the Nyquist curves of S200, S300, S400 and S500, the equivalent series resistance of these samples is 9.1 Ω, 10.6 Ω, 5.6 Ω, and 5.5 Ω, respectively and the charge transfer resistance of these samples is 5.36 Ω, 3.57 Ω, 8.55 Ω and 26.9 Ω, respectively. It can be seen that the charge transfer resistance of S500 and S400 is much larger than that of S200 and S300, which indicates that the Faradic redox reaction occurred on the S400 and S500 is very slow leading to its decreased capacitance performance. The increased charge transfer resistance may be caused by the formation of MnO. Mn3O4 and γ-MnOOH are semiconductors with band gaps of 2.15 eV and 1.3 eV, respectively [35–37], but MnO is insulating with a wide band gap (~3.9 eV) [38,39]. Thus the formation of MnO at high temperature leads to
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 51262027), the Fundamental Scientific Research Funds for the Central Universities (No. 31920130063), and the State Key Laboratory of Solidification Processing in NWPU (SKLSP201754). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cplett.2019.136859. References [1] R. McKenzie, Mineral. Mag. 38 (1971) 493. [2] T. Moore, M. Ellis, P. Selwood, J. Am. Chem. Soc. 72 (1950) 856. [3] Z. Lv, Y. Luo, Y. Tang, J. Wei, Z. Zhu, X. Zhou, W. Li, Y. Zeng, W. Zhang, Y. Zhang, Adv. Mater. 30 (2018) 1704531. [4] B. Li, X. Zhang, J. Dou, C. Hu, Ceram. Int. 45 (2019) 16297. [5] Z.-Y. Li, M.S. Akhtar, P.T. Bui, O.-B. Yang, Chem. Eng. J. 330 (2017) 1240. [6] E. Samuel, H.S. Jo, B. Joshi, H.G. Park, Y.I. Kim, S. An, M.T. Swihart, J.M. Yun, K.H. Kim, S.S. Yoon, Appl. Surf. Sci. 423 (2017) 210. [7] Y.O. Yesilbag, F.N. Tuzluca, M. Ertugrul, J. Mater. Sci. Mater. Electron. 30 (2019) 8201. [8] S.K. Jana, V.P. Rao, S. Banerjee, Chem. Phys. Lett. 593 (2014) 160. [9] D. Dubal, D. Dhawale, R. Salunkhe, C. Lokhande, J. Electroanal. Chem. 647 (2010) 60. [10] L. Lan, Q. Li, G. Gu, H. Zhang, B. Liu, J. Alloys Compd. 644 (2015) 430. [11] D. Dubal, D.S. Dhawale, R. Salunkhe, C. Lokhande, J. Alloys Compd. 496 (2010) 370. [12] B. Folch, J. Larionova, Y. Guari, C. Guérin, C. Reibel, J. Solid State Chem. 178 (2005) 2368. [13] F. Li, J. Wu, Q. Qin, Z. Li, X. Huang, J. Alloys Compd. 492 (2010) 339. [14] Y.C. Zhang, T. Qiao, X.Y. Hu, W.D. Zhou, J. Cryst. Growth 280 (2005) 652. [15] C. Wei, C. Xu, B. Li, D. Nan, J. Ma, F. Kang, Mater. Res. Bull. 47 (2012) 1740. [16] C.J. Lind, Environ. Sci. Technol. 22 (1988) 62. [17] S. Rabiei, D. Miser, J. Lipscomb, K. Saoud, S. Gedevanishvili, F. Rasouli, J. Mater. Sci. 40 (2005) 4995. [18] D.P. Dubal, D.S. Dhawale, R.R. Salunkhe, C.D. Lokhande, J. Electrochem. Soc. 157 (2010) A812. [19] Y. Wang, Y. Lu, K. Chen, S. Cui, W. Chen, L. Mi, Electrochim. Acta 283 (2018) 1087. [20] A. Kumar, D. Sarkar, S. Mukherjee, S. Patil, D. Sarma, A. Shukla, ACS Appl. Mater. Interfaces 10 (2018) 42484. [21] Q. Gao, J. Wang, B. Ke, J. Wang, Y. Li, Ceram. Int. 44 (2018) 18770. [22] W. Du, X. Wang, J. Zhan, X. Sun, L. Kang, F. Jiang, X. Zhang, Q. Shao, M. Dong, H. Liu, Electrochim. Acta 296 (2019) 907. [23] A. Paleo, P. Staiti, A. Brigandì, F. Ferreira, A. Rocha, F. Lufrano, Energy Storage
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Energy 11 (2015) 119. [33] H.-E. Wang, X. Zhao, X. Li, Z. Wang, C. Liu, Z. Lu, W. Zhang, G. Cao, J. Mater. Chem. A 5 (2017) 25056. [34] Y. Cai, H.-E. Wang, X. Zhao, F. Huang, C. Wang, Z. Deng, Y. Li, G. Cao, B.-L. Su, ACS Appl. Mater. Interfaces 9 (2017) 10652. [35] G.A.E. Oxford, A.M. Chaka, Phys. Rev. B 84 (2011) 205453. [36] J. Zhao, J. Nan, Z. Zhao, N. Li, J. Liu, F. Cui, Appl. Catal. B-Environ. 202 (2017) 509. [37] N. Li, Y. Tian, J. Zhao, J. Zhang, J. Zhang, W. Zuo, Y. Ding, Appl. Catal. B-Environ. 214 (2017) 126. [38] F. Tran, P. Blaha, Phys. Rev. Lett. 102 (2009) 226401. [39] C. Rödl, F. Fuchs, J. Furthmüller, F. Bechstedt, Phys. Rev. B 79 (2009) 235114.
Mater. 12 (2018) 204. [24] S. Meng, Z. Mo, Z. Li, R. Guo, N. Liu, J. Solid State Chem. 274 (2019) 134. [25] G. Xi, Y. Peng, Y. Zhu, L. Xu, W. Zhang, W. Yu, Y. Qian, Mater. Res. Bull. 39 (2004) 1641. [26] D. Zheng, Z. Yin, W. Zhang, X. Tan, S. Sun, Cryst. Growth Des. 6 (2006) 1733. [27] Y. He, S. Du, H. Li, Q. Cheng, V. Pavlinek, P. Saha, J. Solid State Electrochem. 20 (2016) 1459. [28] S.-Y. Liu, J. Xie, Y.-X. Zheng, G.-S. Cao, T.-J. Zhu, X.-B. Zhao, Electrochim. Acta 66 (2012) 271. [29] F. Yang, K. Xu, J. Hu, J. Alloys Compd. 729 (2017) 1172. [30] X. Zheng, Z. Han, F. Chai, F. Qu, H. Xia, X. Wu, Dalton Trans. 45 (2016) 12862. [31] D. Guo, Y. Luo, X. Yu, Q. Li, T. Wang, Nano Energy 8 (2014) 174. [32] L.-F. Chen, Z.-Y. Yu, J.-J. Wang, Q.-X. Li, Z.-Q. Tan, Y.-W. Zhu, S.-H. Yu, Nano
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Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Effect of annealing temperature on the compositions and electrochemical performance of Mn3O4/γ-MnOOH composites on carbon cloth
T
⁎
Shujuan Menga,b, Zunli Moa, , Zhenliang Lia, Ruibin Guoa, Nijuan Liua a Research Center of Gansu Military and Civilian Integration Advanced Structural Materials, Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education of China, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China b School of Chemical Engineering, Northwest Minzu University, Lanzhou 730030, China
H I GH L IG H T S
grown on carbon cloth transforms into Mn O between 200 °C and 300 °C. • γ-MnOOH of carbon cloth at high temperature help Mn O convert into MnO. • Reducibility • Increasing annealing temperature reduces the capacitance of Mn O /γ-MnOOH. 3
4
3
4
3
4
A R T I C LE I N FO
A B S T R A C T
Keywords: Mn3O4/γ-MnOOH Carbon cloth Annealing temperature Supercapacitor
In this paper, the effect of annealing temperature on the compositions and corresponding electrochemical performance of Mn3O4/γ-MnOOH composites on the carbon cloth fabricated by a simple hydrothermal method is investigated. The results show that the γ-MnOOH converts into Mn3O4 completely between 200 °C and 300 °C and further increasing the annealing temperature to 400 °C and 500 °C leads to the transformation of partial Mn3O4 into MnO. The electrochemical measurement results for products annealed at different temperatures suggest that increasing the annealing temperature causes the decreased capacitance performance.
1. Introduction Manganese oxides and oxyhydroxides exhibit various stoichiometries and rich crystal structures because of the various oxidation states and variable coordination environments of manganese ions [1,2]. Many of them, including MnO2, Mn3O4, Mn2O3, MnOx and MnOOH [3–8], attract much attention in the supercapacitor electrode design due to their high capacitance, long time stability, low cost, easy preparation and nontoxcity. Different phases of manganese oxides and/or oxyhydroxides can be interconverted when some external factors are applied on them [9–18]. One of the most used strategies to realize this interconversion is annealing. For example, γ-MnOOH nanorods tend to transform into MnO2, Mn2O3, or Mn3O4 nanorods when annealed under different temperatures and atmospheres [10]. This triggers scientists to prepare different phases of manganese oxides with rich morphologies by annealing of various morphologies of γ-MnOOH precursor. Although single component of manganese oxides or oxyhydroxides shows exciting results when used as supercapacitor electrodes, the low conductivity of both manganese oxides and oxyhydroxides limits their
⁎
further improved performance. Some strategies were proposed to address this issue, such as construction of metal oxide heterostructures and doping [19–21]. Combining with high conductivity of carbonbased materials is one of effective ways to increase the conductivity of manganese oxides or oxyhydroxides and thus improves their electrochemical performance [22,23]. In a previous work [24], we demonstrated that growth of Mn3O4/γ-MnOOH composites on carbon cloth (Mn3O4/γ-MnOOH@CC) is beneficial for the improved electrochemical performance due to the formation of heterojunctions at Mn3O4/γMnOOH interface as well as the high conductivity of carbon cloth. Previous works also demonstrated that direct annealing of MnOOH would lead to the formation of different phases of maganese oxides, like MnO2 [25,26], Mn3O4 [10,12] and Mn2O3 [10] depending on the annealing temperature and atmosphere used. Carbon cloth possesses moderate reducibility at high temperature [27]. It is expected that annealing of γ-MnOOH grown on the carbon cloth will present different results when compared with direct annealing of γ-MnOOH. In this work, we investigated the effect of annealing temperature on the compositions and corresponding electrochemical performance of
Corresponding author. E-mail address: [email protected] (Z. Mo).
https://doi.org/10.1016/j.cplett.2019.136859 Received 31 August 2019; Received in revised form 22 September 2019; Accepted 14 October 2019 Available online 14 October 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
Chemical Physics Letters 738 (2020) 136859
S. Meng, et al.
Fig. 1. XRD patterns of Mn3O4/γ-MnOOH@CC, S200, S300, S400 and S500.
Fig. 2. SEM images of S200 with different magnifications.
2. Experimental section
Mn3O4/γ-MnOOH@CC. The results demonstrated that increasing annealing temperature from 200 °C to 500 °C in N2 atmosphere leads to the conversion of γ-MnOOH into MnO via Mn3O4. The electrochemical measurement results suggest that increasing the annealing temperature leads to the decreased capacitance performance, which is due to the destruction of Mn3O4/γ-MnOOH heterostructures and the formation of insulating MnO species at high temperature.
2.1. Synthesis of Mn3O4/γ-MnOOH@CC We firstly synthesized Mn3O4/γ-MnOOH@CC by a simple hydrothermal method as described in the previous work [24]. Briefly, 500 mg solid KMnO4 (Alfa Asera, 99.0%) was dissolved in 40 mL deionized water and the KMnO4 solution was stirred magnetically for 20 min at room temperature. Then the solution was transferred into a 50-mL-capacity Teflon-lined autoclave and three pieces of carbon cloth (Shanghai Lishuo Composite Technology Co., Ltd. China, 220 g m−2) 2
Chemical Physics Letters 738 (2020) 136859
S. Meng, et al.
Fig. 3. Elemental analysis of S200. (b) Energy dispersive spectrum of selected in the rectangular area in (a). EDS mapping for (d) Mn, (e) O and (f) C measured from the corresponding area in (c).
with 2.5 × 4 cm2 area were immersed into the solution. The autoclave was then heated at 180 °C for 15 h. After cooling down to room temperature, the carbon cloth covered with Mn3O4 particles/γ-MnOOH nanowires composites were picked up and dried at 100 °C for 12 h.
2.3. Materials characterizations The composition/phase, microstructures and morphologies of the annealed samples were characterized by X-ray powder diffraction (XRD, X'TRA, ARL, Switzerland) using a 40 kW advanced X-ray diffractometer with Cu Ka radiation (λ = 1.54056 Å), Field-emission scanning electron microscopy (FE-SEM, FEI, Inspect F50). The elemental composition and distribution in samples were characterized by energy-dispersive X-ray spectroscopy (EDS) and elemental mapping techniques with X-Max Extreme detector (Oxford Instruments).
2.2. Annealing of Mn3O4/γ-MnOOH@CC The obtained Mn3O4/γ-MnOOH@CC was placed in a porcelain boat and then was transferred to a quartz tube. A protective gas of high purity N2 (99.99%) was passed through the quartz tube at a rate of 400 standard cubic centimeters per minute (sccm) for 10 min to purge the air in the tube. The system was then heated to expected temperatures at a heating rate of 5 °C /min and held at that temperature for 2 h before it cooled down to room temperature in the protective gas with 400 sccm. The Mn3O4/γ-MnOOH@CC annealed at 200 °C, 300 °C, 400 °C and 500 °C are labelled as S200, S300, S400 and S500, respectively.
2.4. Electrochemical measurements The electrochemical properties of S200, S300, S400 and S500 were measured using a three-electrode system in fresh 1 M Na2SO4 aqueous solution as electrolyte. The working electrode was a piece of carbon 3
Chemical Physics Letters 738 (2020) 136859
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Fig. 4. CV curves of S200, S300, S400 and S500 recorded at various scan rates.
cloth covering with corresponding products (1 cm2 in area), which was cut from S200, S300, S400 and S500 directly. A platinum plate and Ag/ AgCl electrode (saturated with KCl (aq)) were used as the counter and reference electrode, respectively. The electrochemical performance of the supercapacitor electrode was evaluated by the cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS) using a CHI660E electrochemical workstation (CHI660E, Shanghai Chenhua Device Company, China) at room temperature. EIS was done at the open circuit voltage with an alternate voltage amplitude of 5.0 mV in the frequency range of 0.01 –100 kHz. Here, specific areal capacitance is chosen to evaluate the electrode performance in three-electrode system. The specific areal capacitance (Cs, F cm−2) was calculated based on Eq. (1):
Cs =
Icons S ΔV /Δt
S200 indicate that the main compositions of S200 are Mn3O4 (hausmannite, JCPDS No. 24-0734) and γ-MnOOH (manganite, JCPDS No. 41-1379), which are the same with that of Mn3O4/γ-MnOOH@CC. For S300, only peaks of Mn3O4 are observed, indicating that γ-MnOOH transformed into Mn3O4 completely between 200 °C and 300 °C. When the temperature increased to 400 °C and 500 °C, the main composition of S400 and S500 is Mn3O4, but several peaks belonging to MnO (manganosite, JCPDS No. 07-0230) appeared and its content is enhanced with the increasing temperature. This suggests that partial Mn3O4 converted into MnO at higher temperature (400 °C and 500 °C). It is reported that γ-MnOOH would transfrom into Mn3O4 at ~272 °C when calcined under argon atmosphere with release of water and oxygen via Eq. (2) [12]: 12γ-MnOOH → 4Mn3O4 + 6H2O + O2
(1)
(2)
Mn3O4 particles can be reduced by graphite oxide sheets and H2 when heated at 400 °C because of the reducibility of the graphite sheets and H2 [28]. Based on the above reports and our experimental facts, we proposed a conversion process from γ-MnOOH to MnO. At ~272 °C, γMnOOH transformed into Mn3O4 via Eq. (2) and Mn3O4 contacting with carbon cloth closely was further reduced into MnO because of the enhanced reducibity of carbon cloth at high temperature via Eq. (3):
where Icons (A), ΔV/Δt (V/s), and S (cm2) corresponds to the applied constant discharge current, the slope of the discharge curve excluding IR drop and the projected area of the electrode material, respectively. 3. Results and discussion 3.1. Materials characterizations
2Mn3O4 + C → 6MnO + CO2
XRD was used to confirm the compositions of the annealed products. The XRD patterns of Mn3O4/γ-MnOOH@CC, S200, S300, S400 and S500 are displayed in Fig. 1. A broad peak at 2θ = ~26° corresponding to carbon cloth can be observed for all samples. The peaks of
The electrochemical performance of the materials is closely related with their morphologies. The SEM images of Mn3O4/γ-MnOOH@CC are displayed in Fig. S1. It can be seen that the products covering on the
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(3)
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Fig. 5. GCD curves of S200, S300, S400 and S500 recorded at different current densities.
Fig. 6. (a) Rate performance and (b) Nyquist curves of S200, S300, S400 and S500.
respectively. Fig. 2 shows the SEM images of S200, and it is found that the morphologies of products are the same with that of Mn3O4/γMnOOH@CC. The SEM images of S300, S400 and S500 are shown in Fig. S2, Fig. S3 and Fig. S4, respectively. From careful observation of
carbon cloth surface displayed two different morphologies: one is particles contacting with carbon cloth closely and the other is nanowires covering on the particles. In the previous work [24], it is confirmed that the composition of particles and nanowires is Mn3O4 and γ-MnOOH, 5
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increased charge transfer resistance, leading to the decreased capacitance of S400 and S500.
the morphologies of S300, S400 and S500, it is also found that the morphologies of products are nearly unchanged when compared with that of Mn3O4/γ-MnOOH@CC and S200, indicating that increasing annealing temperature only changes the compositions but not the morphologies of products. Elemental analysis was carried out to confirm the elemental distribution for S200. The energy dispersive spectrum of S200 exhibited in Fig. 3b suggests that the main elements of S200 are Mn, O and C. The minor amount of K elements may come from the residual unreacted KMnO4 species. The EDS mapping for Mn, O and C displayed in Fig. 3d, e and f measured from corresponding area in Fig. 3c demonstrates that both Mn and O distributed uniformly.
4. Conclusions In this work, the effect of annealing temperature on the compositions and corresponding electrochemical performance of Mn3O4/γMnOOH@CC is investigated. It is found that the γ-MnOOH grown on the carbon cloth synthesized by one step hydrothermal method converts into Mn3O4 between 200 °C and 300 °C. When the annealing temperature increases to 400 °C and 500 °C, partial Mn3O4 transforms into MnO because of the enhanced reducibility of carbon cloth at high temperature. The electrochemical measurement results indicate that increasing the annealing temperature leads to decreased capacitance performance for Mn3O4/γ-MnOOH@CC, which can be explained by that the destruction of Mn3O4/γ-MnOOH heterostructures for S300 and that the formation of insulating MnO species increased the charge transfer resistance for S400 and S500.
3.2. Electrochemical performance of products The CV curves of S200, S300, S400 and S500 recorded at different scan rates are displayed in Fig. 4a, b, c and d, respectively. All curves show a quasi-rectangular shape, which indicates that all samples exhibit ideal capacitive behavior. No redox peaks are observed from the CV curves, which suggests that the Faradic redox reaction occurred on the products surface is very fast and reversible. The GCD curves of S200, S300, S400 and S500 recorded at various current densities are displayed in Fig. 5. The discharge time of these samples recorded at fixed current density (for example, 1 mA cm−2) is decreased with the increasing annealing temperature from S200 to S500 and the IR drop increased from S200 to S500, indicating that the internal resistance gradually increased with the increasing annealing temperature. The specific capacitance calculated based on Eq. (1) at different current densities for S200, S300, S400 and S500 is exhibited in Fig. 6a. The specific areal capacitance of S200 is 2878.7, 2501.1, 2183.2, 1786.9, 1503.5, 1165.8, 974.8, 191.3 and 117.6 mF cm−2 at current density of 1, 2, 3, 5, 7, 9, 10, 15 and 20 mA cm−2, respectively. These values are higher than many other metal oxides or oxyhydroxides grown on carbon cloth, such as Co3O4 nanowires@α-Fe2O3 nanorod@CC (245 mF cm−2 at 1 mA cm−2 in 1 M Na2SO4) [29], α-Fe2O3@ MnCo2O4@CC (1073 mF cm−2 at 1 mA cm−2 in 1 M KOH) [30], βFeOOH@CC (1120 mF cm−2 at 1.0 mA cm−2 in 2 M KOH) and NiMoO4@CC (1270 mF cm−2 at 5.0 mA cm−2 in 2 M KOH) [31,32]. S300 possesses similar capacitance with S200 at low current density, but it decreased faster than that of S200 with the increasing current density. It is known that rational design of heterostructures can enhance the electrochemical performance of the materials due to the enhanced electron transfer and separation [33,34]. The complete conversion of γMnOOH to Mn3O4 leads to the destruction of γ-MnOOH/Mn3O4 heterojunctions, which may reduce the electron conductivity causing reduced rate performance of S300. S400 and S500 showed less capacitance performance than that of S300 at all current densities. The capacitance of S500 is even lost when the current density increased to higher values (20 mA cm−2). All above results demonstrate that the increasing the annealing temperature leads to reduced capacitance performance for Mn3O4/γ-MnOOH@CC. In order to investigate the possible reason for the decreased capacitance with the increasing annealing temperature, EIS was carried out to characterize the electrons/ions conductivity characteristics of these products. Fig. 6b shows the Nyquist curves of S200, S300, S400 and S500, the equivalent series resistance of these samples is 9.1 Ω, 10.6 Ω, 5.6 Ω, and 5.5 Ω, respectively and the charge transfer resistance of these samples is 5.36 Ω, 3.57 Ω, 8.55 Ω and 26.9 Ω, respectively. It can be seen that the charge transfer resistance of S500 and S400 is much larger than that of S200 and S300, which indicates that the Faradic redox reaction occurred on the S400 and S500 is very slow leading to its decreased capacitance performance. The increased charge transfer resistance may be caused by the formation of MnO. Mn3O4 and γ-MnOOH are semiconductors with band gaps of 2.15 eV and 1.3 eV, respectively [35–37], but MnO is insulating with a wide band gap (~3.9 eV) [38,39]. Thus the formation of MnO at high temperature leads to
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 51262027), the Fundamental Scientific Research Funds for the Central Universities (No. 31920130063), and the State Key Laboratory of Solidification Processing in NWPU (SKLSP201754). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cplett.2019.136859. References [1] R. McKenzie, Mineral. Mag. 38 (1971) 493. [2] T. Moore, M. Ellis, P. Selwood, J. Am. Chem. Soc. 72 (1950) 856. [3] Z. Lv, Y. Luo, Y. Tang, J. Wei, Z. Zhu, X. Zhou, W. Li, Y. Zeng, W. Zhang, Y. Zhang, Adv. Mater. 30 (2018) 1704531. [4] B. Li, X. Zhang, J. Dou, C. Hu, Ceram. Int. 45 (2019) 16297. [5] Z.-Y. Li, M.S. Akhtar, P.T. Bui, O.-B. Yang, Chem. Eng. J. 330 (2017) 1240. [6] E. Samuel, H.S. Jo, B. Joshi, H.G. Park, Y.I. Kim, S. An, M.T. Swihart, J.M. Yun, K.H. Kim, S.S. Yoon, Appl. Surf. Sci. 423 (2017) 210. [7] Y.O. Yesilbag, F.N. Tuzluca, M. Ertugrul, J. Mater. Sci. Mater. Electron. 30 (2019) 8201. [8] S.K. Jana, V.P. Rao, S. Banerjee, Chem. Phys. Lett. 593 (2014) 160. [9] D. Dubal, D. Dhawale, R. Salunkhe, C. Lokhande, J. Electroanal. Chem. 647 (2010) 60. [10] L. Lan, Q. Li, G. Gu, H. Zhang, B. Liu, J. Alloys Compd. 644 (2015) 430. [11] D. Dubal, D.S. Dhawale, R. Salunkhe, C. Lokhande, J. Alloys Compd. 496 (2010) 370. [12] B. Folch, J. Larionova, Y. Guari, C. Guérin, C. Reibel, J. Solid State Chem. 178 (2005) 2368. [13] F. Li, J. Wu, Q. Qin, Z. Li, X. Huang, J. Alloys Compd. 492 (2010) 339. [14] Y.C. Zhang, T. Qiao, X.Y. Hu, W.D. Zhou, J. Cryst. Growth 280 (2005) 652. [15] C. Wei, C. Xu, B. Li, D. Nan, J. Ma, F. Kang, Mater. Res. Bull. 47 (2012) 1740. [16] C.J. Lind, Environ. Sci. Technol. 22 (1988) 62. [17] S. Rabiei, D. Miser, J. Lipscomb, K. Saoud, S. Gedevanishvili, F. Rasouli, J. Mater. Sci. 40 (2005) 4995. [18] D.P. Dubal, D.S. Dhawale, R.R. Salunkhe, C.D. Lokhande, J. Electrochem. Soc. 157 (2010) A812. [19] Y. Wang, Y. Lu, K. Chen, S. Cui, W. Chen, L. Mi, Electrochim. Acta 283 (2018) 1087. [20] A. Kumar, D. Sarkar, S. Mukherjee, S. Patil, D. Sarma, A. Shukla, ACS Appl. Mater. Interfaces 10 (2018) 42484. [21] Q. Gao, J. Wang, B. Ke, J. Wang, Y. Li, Ceram. Int. 44 (2018) 18770. [22] W. Du, X. Wang, J. Zhan, X. Sun, L. Kang, F. Jiang, X. Zhang, Q. Shao, M. Dong, H. Liu, Electrochim. Acta 296 (2019) 907. [23] A. Paleo, P. Staiti, A. Brigandì, F. Ferreira, A. Rocha, F. Lufrano, Energy Storage
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