Author's Accepted Manuscript

In situ synthesis of SWNTs@MnO2/PolypyrroleHybrid film as binder-free supercapacitor elec-trode

Kun Liang, Taoli Gu, Zeyuan Cao, XianzhongTang, Wencheng Hu, Bingqing Wei

PII: S2211-2855(14)20076-0DOI: http://dx.doi.org/10.1016/j.nanoen.2014.07.017Reference: NANOEN444

To appear in: Nano Energy

Received date: 23 July 2014Accepted date: 25 July 2014

Cite this article as: Kun Liang, Taoli Gu, Zeyuan Cao, Xianzhong Tang, WenchengHu, Bingqing Wei, In situ synthesis of SWNTs@MnO2/Polypyrrole Hybrid film asbinder-free supercapacitor electrode, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2014.07.017

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 resultinggalley proof before it is published in its final citable form. Please note that duringthe production process errors may be discovered which could affect the content,and all legal disclaimers that apply to the journal pertain.

www.elsevier.com/nanoenergy

1��

In situ synthesis of SWNTs@MnO2/Polypyrrole hybrid film as binder-free supercapacitor electrode Kun Lianga,b, Taoli Gub, Zeyuan Caob, Xianzhong Tanga, Wencheng Hua*, Bingqing Weib

a State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science & Technology of China, Chengdu, 610054, P. R. China . Fax: 86-28-83202550; Tel: +86-28-83201171; E-mail: huwc@uestc.edu.cn b Department of Mechanical Engineering, University of Delaware, Newark, DE 19716, USA. Abstract: In this study, a flexible hybrid film based on single-wall carbon nanotubes (SWNTs) was fabricated. The SWNTs@MnO2/Polypyrrole (PPy) film was used as a supercapacitor electrode without binders to achieve high capacitance. The binder-free electrode with SWNT and PPy layers improved the conductivity of the electrode materials, as well as the ion diffusion rate and charge-transfer resistance, thus achieving excellent electrochemical performance compared with SWNTs@MnO2 electrodes. The specific capacity was 351 F g-1 based on the total weight of the electrodes with energy density of 39.7 Wh kg-1 and power density of 10 kW kg-1. Our study could provide a novel and facile strategy for the development of high-performance energy storage devices. Keywords: hybrid film, binder-free, supercapacitor 1. Introduction

The rapid development of electronic devices and equipment has resulted in the growing for high-power density devices in the modern electronic industry, especially for application in electric and hybrid electric vehicles. Thus, supercapacitors have attracted considerable attention in recent years. Supercapacitors have higher power density than lithium-ion batteries and conventional dielectric capacitors. In addition, supercapacitors have other advantages, such as low cost, short charging time, and long life cycle, compared with secondary batteries. For these reasons, supercapacitors have become the most promising candidate for next-generation power devices from backup power for memory and portable electronics to electric vehicles1–4.

Although the energy density of supercapacitors is lower than that of secondary batteries, more applications can be developed if such energy density is improved while maintaining power density. Supercapacitors can achieve higher energy density in an organic electrolyte than in an aqueous electrolyte. However, the power density of organic electrolyte-based supercapacitors is usually lower than that based on aqueous electrolytes because of poor electronic conductivity (approximately 100 times lower than aqueous electrolytes). Thus, Na2SO4 aqueous solution is used as the electrolyte in this study. For supercapacitor electrode materials, transition metal oxides and conducting polymers in pseudocapacitors can achieve higher energy density than that of carbon materials with a capacitive electrochemical double layer 5.

MnO2 is an environment-friendly and low-cost supercapacitor electrode material with

2��

theoretical capacitance that can reach 1370 F g-1 in an aqueous electrolyte6. However, the kinetic features of ion and electron transport are limited between a MnO2 electrode and electrode/electrolyte interface. Therefore, excellent electrical conductivity and large specific surface area are important to enhance electrochemical performance. The incorporation of MnO2 into conductive materials, such as carbon nanotubes, graphene, or conducting polymers, to form hybrid electrodes improves the specific capacitance7-10. The addition of organic binders during the preparation of composite electrodes generally decreases the ion diffusion and electronic conductivity of electrodes. Thus, composite coatings on a conductive substrate without binders can improve electrode performance.

Single-wall carbon nanotubes (SWNTs) have excellent potential application in energy storage devices for their high specific surface area, superior electrical conductivity, and excellent chemical stability. Free-standing SWNT macro film is an excellent choice as a substrate to incorporate active materials and as flexible and stretchable supercapacitors11, 12.

Combining MnO2 and SWNTs to produce macro films has been proven effective to improve specific capacitance13–15. However, the dissolution of MnO2 cannot be prevented during experiments. In this study, a facile and in situ method was developed to synthesize SWNTs@MnO2/PPy hybrid films as supercapacitor electrodes without binders at room temperature. The produced film provides a method to resolve the above two problems. A commercial coin cell system (CR 2032) was assembled with SWNTs@MnO2/PPy as electrodes. Electrochemical performances, including specific capacitance, cycling stability, energy, and power densities of the electrodes, were investigated, and we found that the SWNTs@MnO2/PPy films have better electrochemical properties than SWNT@ MnO2 films.

2. Experimental 2.1 Synthesis of SWNT macro films

The SWNT macro films were synthesized through chemical vapor deposition, as previously reported by our group.16 The films were annealed and then washed with diluted hydrochloric acid. The films were then washed with deionized water until pH = 7. 2.2 Preparation of SWNTs@MnO2/PPy hybrid films

Free-standing SWNT films were immersed in a mixed solution with 10 mL of ethanol and 2 mL of pyrrole monomer. Next, 0.1 M KMnO4 aqueous solution was added dropwise. A layer of nanostructured MnO2/PPy was co-deposited on the surface of SWNT film within 5 min. The as-prepared SWNTs@MnO2/PPy hybrid film was rinsed with deionized water and ethanol thrice. Finally, the films were dried at room temperature for 12 h. Then, we obtained the binder-free electrode. 2.3 MnO2/PPy coating measurement

A Mettler Toledo XP6 microbalance with an accuracy of 0.001 mg was used to measure the mass of the MnO2/PPy films. Briefly, 0.5 in of SWNT macro film plate was measured. After the co-deposition reaction, the SWNTs@MnO2/PPy hybrid film was weighed, after which the mass of the MnO2/PPy was determined. On average, 10% of MnO2/PPy precipitated on the surface of SWNT films, as determined by weighing several SWNT films before and after coating. 2.4 Structural characterization

The structure of the obtained hybrid film was characterized by using a Philips X’Pert X-ray diffractomer with Cu-K� radiation operating at 0.15406 nm. The data were recorded

3��

from 2� = 10° to 80° at a scan rate of 0.02° per step and 0.2 s per point. The infrared (IR) spectra were obtained using an 8400S Fourier-transform IR spectrometer. Morphological characterizations were performed using scanning electron microscopy (SEM, JEOL JSM-7400F) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2010F). The Mn contents in SWNTs@MnO2/PPy and SWNTs@MnO2/PPy electrode before/after cycling were measured by inductively coupled plasma mass spectrometer. 2.5 Electrochemical characterization

Electrochemical characterizations were performed using a standard symmetric 2032 coin cell. Copper foil was used as current collector. The working electrode was a free-standing SWNTs@MnO2/PPy hybrid film, and 1 M Na2SO4 aqueous solution was used as the electrolyte. Wattman glass microfiber paper was punched and used as the separator. Cyclic voltammetry (CV) and electrochemical impedance scanning (EIS) were conducted using a PARSTAT 2273 potentiostat/galvanostat. EIS spectra were obtained with frequencies ranging from 100 kHz to 10 mHz. Galvanostatic charge/discharge (GCD) tests were conducted using an Arbin BT4+ test system. All of the electrochemical measurements were performed at room temperature. 3 Results and discussion 3.1 Schematic preparation for SWNTs@MnO2/PPy hybrid film

Scheme 1 shows that KMnO4 aqueous solution was added to the mixture of ethanol and pyrrole monomers. Upon mixing, Mn7+ was reduced to Mn4+ by the pyrrole monomers, and the as-prepared MnO2 was impregnated and deposited on the surface of the SWNT films. Meanwhile, the pyrrole monomers formed the conductive polypyrrole through in situ oxidation polymerization at room temperature. Finally, a hybrid structured SWNTs@MnO2/PPy hybrid film was obtained. The SWNT@MnO2 film was obtained through a previously reported method by our group15. 3.2 Morphological properties of SWNTs@MnO2/PPy hybrid film

IR spectroscopy was performed to confirm the presence of PPy. The bands shown in Figure 1a are consistent with the results reported in the literature17–19. The main characteristic peaks of PPy are located at 1574, 1470 cm-1 and can be attributed to the antisymmetric and symmetric pyrrole ring vibration, respectively. The band at 1235 cm-1 is assigned to the C-N stretching vibration. The peak at 1045 cm-1 indicates the in-plane deformation vibration of N-H. The peaks at 932 and 805 cm-1 correspond to the out-of-plane deformation vibration of C-H. Compared with the spectral pattern of SWNT@MnO2 film, the presence of PPy is clearly observed, which indicates the formation of the SWNTs@MnO2/PPy hybrid film.

Figure 1b shows that the as-prepared nanocomposite thin films were successfully characterized using X-ray diffraction (XRD). The peaks at 2� = 37.1°, 45.1°, 49.4°, and 65.6° corresponded to the (311), (400), (331), and (440) planes, respectively. These peaks agreed with the standard pattern of �-MnO2 (JCPDS card No. 44-0992). The broad peak around 2� = 26° was a characteristic peak of amorphous PPy19. No other peaks were observed, indicating that the synthesized thin films were purely made of SWNTs@MnO2/PPy hybrid composites.

Figures S1, 2a, and 2b show SEM images of the SWNTs and SWNTs@ MnO2 films without/with PPy. The images evidently show that the surfaces became rough after MnO2 deposition, and then turned smooth with PPy. PPy served as a bridge between the MnO2 and SWNTs, thus increasing the electron transport efficiency. The inset of Figure 2b shows a

4��

digital image of flexible hybrid film. HRTEM was performed to obtain a more detailed morphological and structural information. Figures 2c and 2d show that the MnO2 is located at the SWNT bundles. Moreover, the lattice structures can be clearly observed. The lattice feature is approximately 0.21 nm and corresponds to the interplanar distance of the (400) MnO2 planes and with the (400) peak in the XRD spectrum. Figure 2d indicates that a core-shell structure with an amorphous PPy layer of about 1 nm to 2 nm formed on the SWNTs/MnO2 surface, which agrees with the IR result. 3.3 Electrochemical characterization

CV was performed to study the electrochemical performance of the as-prepared samples in 1 M Na2SO4 aqueous solution. Figures 3a and b show the CV curves of the SWNTs@MnO2, and SWNTs@MnO2/PPy electrodes with a potential window of 0 V to 0.8 V. With a scan rate of 5 mV s-1, the SWNTs@MnO2 electrode was quasi-rectangular in shape, while a pair of redox peaks were observed at 0.1 V and 0.45 V, which correspond to the faradaic redox reaction of PPy and agree with the result reported by Zhao20. When the scan rate increased to 50 mV s-1, the two composite electrodes showed substantially better capabilities than those at 5 mV s-1. Figure 3c shows the SWNTs@MnO2/PPy electrodes at different scan rates, and the figure also shows the faradaic redox peaks. In addition, the curves were quasi-rectangular in shape, indicating a remarkable capacitive behavior. Figures S2a and S2b show the CV curves of the SWNTs@MnO2 electrodes at scan rates from 1 mV s-1to 2500 mV s-1. The shapes were rectangular, indicating the ideal electrochemical double-layer capacitive behavior.

The specific capacitances were determined from the CV curves using Equation S1 in the supporting information and as presented in Figure S2d. The specific capacitance of the SWNTs@MnO2/PPy and SWNTs@MnO2 electrodes at the scan rate of 1 mVs-1 were 351 and 186 F g-1, respectively, suggesting that PPy enhanced the specific capacitance as PPy provided pseudocapacitance during the test. The specific capacitance of the SWNTs@MnO2/PPy electrodes was higher than those of similar electrodes or systems15, 18,

21–23. At the high scan rate of 2500 mVs-1, the specific capacitance of the SWNTs@MnO2/PPy electrodes decreased to 186 F g-1, which is still about 53% of the capacitance at the scan rate of 1 mV s-1. The capacitance of the other electrodes is only 39% under the same conditions.

To further study the electrochemical performance of the SWNTs@MnO2/PPy electrodes, GCD tests were performed in 1 M Na2SO4 aqueous solution at room temperature. GCD curves with different charge/discharge current densities for the as-prepared electrode were obtained from 0 V to 0.8 V, as shown in Figures 3d and S2e. At low current densities of 0.1, 0.5, and 1 A g-1, the GCD curves show a pseudocapacitance behavior. However, at the higher current densities of 5, 10, and 25 A g-1, a small internal resistance (IR) drop was observed at the initial discharge. In addition, the charging and discharging curves are symmetric, indicating a reversible electrochemical behavior. Figure S2f presents the specific capacitance determined based on the GCD curves. The pattern in Figure S2f is similar with that calculated from the CV curves. The highest specific capacitance was 353 F g-1, and about 33% of the capacitance was retained when the current density increased from 0.1 A g-1 to 25 A g-1.

The energy and power densities of the SWNTs@MnO2/PPy electrodes were determined from the CV curves at the scan rate of 12500 mV s-1 to 2500 mV s-1, as exhibited in the Ragone plat shown in Figure 4a. The highest energy density was 39.7 Wh kg-1 at the scan rate

5��

of 1 mV s-1, and the highest power density is 10 kW kg-1 at the scan rate of 2500 mV s-1. Cycling stability and capacitance retention were examined in the potential window from 0

V to 0.8 V with the current density of 1 Ag-1. Figure 4b shows the specific capacitance retention of SWNTs@MnO2/PPy and SWNTs@MnO2 electrodes after 10000 cycles. Comparing with the images of SWNTs@MnO2/PPy before/after 10000 cycles, as presented in Fig. 2(b) and Fig. S3, it can be observed that the surfaces haven’t any significant change, suggesting that the electrode showed excellent cycling stability. From the ICP analysis, as shown in Table 1, we can find that the Mn content in SWNTs@MnO2/PPy was very similar, but the Mn content in SWNTs@MnO2 was decreased after 10000 cycles. It is noted that PPy layer can keep MnO2 stable during cycling. The retention of SWNTs@MnO2/PPy electrode became almost stable after 100 cycles, and for SWNTs@MnO2 electrodes, the retention became stable after 200 cycles. The capacitance losses of the two electrodes were 5.6% and 25.8%, respectively.

EIS was conducted at room temperature on the cell after the 1st and 10000th cycles to investigate the electrochemical performance of the electrodes. Figures 4c and d shows the Nyquist plots for SWNTs@MnO2and SWNTs@MnO2/PPy, respectively. The EIS spectrum has a semicircle in the high-frequency region and a straight line in the low-frequency region, indicating a capacitive behavior. The equivalent electrical circuit in Figure S4 was obtained by fitting the impedance data and was similar to the circuit reported in the literature24-26. The intersections of the curves at the real part (Z�) in high-frequency area were the bulk solution resistances (Rs), which include the whole resistance of the electrolyte, separator, and collector. The diameters of the semi-circles corresponded to the charge-transfer resistances (Rct),which are related to the interface between the electrode and electrolyte, and the electrical charge transfer in Faradic process of the electrode materials27. A constant phase element was used to account for the double-layer capacitance and pseudocapacitance. The Warburg impedance corresponds to the straight line in low-frequency area, which is associated with the ion diffusion in the electrode. Table 2 in supporting information shows the fitting EIS parameters derived from the equivalent circuit. The Rs of the SWNTs@MnO2 and SWNTs@MnO2/PPy were 4.54 � and 2.37 � before cycling, respectively. In addition, the Rct of the SWNTs@MnO2/PPy electrode slightly increased from 11.53 � to 11.89 �, and one of the SWNTs@MnO2 electrodes evidently increased from 15.51 � to 19.36 � after 10000 cycles. The charge-transfer resistance significantly decreased after PPy coating. The PPy layer generally improved the conductivities of the electrode materials, ion diffusion rate and charge-transfer, and stability of the electrode to enhance the cycle performance. 4. Conclusion

In summary, a MnO2/PPy hybrid film was fabricated as a supercapacitor electrode by using SWNTs as flexible substrates. The as-prepared hybrid film presented ideal capacitive behavior with excellent electrochemical performance. The specific capacity was 353 F g-1 on the basis of the total weight of the electrodes with energy density of 39.7 Wh kg-1 and power density of 10 kW kg-1. The flexible composite film is a promising binder-free electrode for energy storage devices.

6��

Acknowledgement:

This work was supported by the Fundamental Research Funds for the Central Universities (ZYGX2012YB013). K. Liang acknowledges the financial support of China Scholarship Council (CSC). BQW is grateful to the US national science foundation (NSF) for the financial support under the contract of 1067947. References: [1] P. Simon, Y. Gogotsi, Nat. Mater. 7 (2008) 845-854. [2] J.R. Miller, P. Simon, Science, 321 (2008) 651-652. [3] G. Wang, L. Zhang, J. Zhang, Chem. Soc. Rev. 41 (2012) 797-828. [4] M. Winter, R.J. Brodd, Chem. Rev. 104 (2004) 4245-4270. [5] C. Liu, F. Li, L.P. Ma, H.M. Cheng, Adv. Mater. 22 (2010) E28-E62. [6] M. Toupin, T. Brousse, D. Belanger, Chem. Mater. 16 (2004) 3184-3190. [7] Y. Hou, Y. Cheng, T. Hobson, J. Liu, Nano Lett. 10 (2010) 2727-2733. [8] W. Chen, R. Rakhi, L. Hu, X. Xie, Y. Cui, H. Alshareef, Nano Lett. 11 (2011) 5165-5172. [9] G. Yu, L. Hu, N. Liu, H. Wang, M. Vosgueritchian, Y. Yang, Y. Cui, Z. Bao, Nano Lett. 11 (2011) 4438-4442. [10] Z. Chen, V. Augustyn, J. Wen, Y. Zhang, M. Shen, B. Dunn, Y. Lu, Adv. Mater., 23 (2011) 791-795. [11] X. Li, T. Gu, B. Wei, Nano Lett. 12 (2012) 6366-6371. [12] C. Masarapu, L.P. Wang, X. Li, B. Wei, Adv. Energy Mater. 2 (2012) 546-552. [13] Z. Cao, B. Wei, J. Power Sources 241 (2013) 330-340. [14] J. Qin, Q. Zhang, Z. Cao, X. Li, C. Hu, B. Wei, Nano Energy 2 (2013) 733-741. [15] X. Li, B. Wei, Nano Energy 1 (2012) 479-487. [16] H. Zhu, B. Wei, Chem. Commun. 29 (2007) 3042-3044. [17] F. Han, D. Li, W.C. Li, C. Lei, Q. Sun, A.H. Lu, Adv. Funct. Mater. 23 (2012) 1692-1700. [18] J. Zang, X. Li, J. Mater. Chem. 21 (2011) 10965-10969. [19] D. Zhang, X. Zhang, Y. Chen, P. Yu, C. Wang, Y. Ma, J. Power Sources, 196 (2011) 5990-5996. [20] L.L. Zhang, S. Zhao, X.N. Tian, X. Zhao, Langmuir, 26 (2010) 17624-17628. [21] Q. Qu, Y. Zhu, X. Gao, Y. Wu, Adv. Energy Mater. 2 (2012) 950-955. [22] S. Biswas, L.T. Drzal, Chem. Mater. 22 (2010) 5667-5671. [23] J. Zang, S.J. Bao, C.M. Li, H. Bian, X. Cui, Q. Bao, C.Q. Sun, J. Guo, K. Lian, J. Phys. Chem. C, 112 (2008) 14843-14847. [24] K. Liang, X. Tang, W. Hu, J. Mater. Chem. 22 (2012) 11062-11067. [25] C. Zhou, Y. Zhang, Y. Li, J. Liu, Nano Lett. 13 (2013) 2078-2085. [26] J. Kang, J. Wen, S.H. Jayaram, X. Wang, S.K. Chen, J. Power Sources, 234 (2013) 208-216. [27] Z. Niu, W. Zhou, J. Chen, G. Feng, H. Li, W. Ma, J. Li, H. Dong, Y. Ren, D. Zhao, S. Xie, Energy Environ. Sci. 4 (2011) 1440-1446.

7��

�

� �

� Scheme 1. Schematic illustration of MnO2 and MnO2/PPy coating on the surface of SWNTs films.

Figure 1. a) IR spectra of SWNTs@MnO2/PPy hybrid film and SWNTs@MnO2 hybrid film. b) XRD pattern of as-prepared SWNTs@MnO2/PPy hybrid film.

8��

Figure 2. (a) FESEM images of SWNTs@MnO2. (b) FESEM images of SWNTs@MnO2/PPy. Inset shows a digital image of the flexible hybrid film. (c) TEM and (d) HRTEM images of SWNTs@MnO2 after coating PPy.

9��

Figure 3. (a, b) CV curves of the SWNTs@MnO2 and SWNTs@MnO2/PPy electrodes at scan rate of 5 and 50 mVs-1, respectively. (c) CV curves of the SWNTs@MnO2/PPy electrodes at different scan rates. (d) Charge-discharge curves of the SWNTs@MnO2/PPy electrodes at different current densities.

10��

Figure 4. (a) Ragone plot of the SWNTs@MnO2/PPy electrode. Energy density-power density map for all existing energy storage systems were added; (b) Cycling performance of the nanocomposites showing specific capacitance retention after 10000 cycles with the current density of 1 Ag-1. (c, d) Nyquist plots of the SWNTs@MnO2 and SWNTs@MnO2/PPy electrodes before/after 10000 cycles, respectively.