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Hydrothermal synthesis of reduced graphene oxide-Mn3O4 nanocomposite as an eﬃcient electrode materials for supercapacitors ⁎
Hidayat Ullah Shaha, Fengping Wanga, , Muhammad Sufyan Javedb,c, Nusrat Shaheenb,c, Muhammad Saleemd, Yan Lia a
Department of Physics, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083, PR China Department of Applied Physics, Chongqing University, Chongqing 400044, PR China c Department of Physics, COMSATS Institute of Information Technology, Lahore 54000, Pakistan d Department of Physics, Khwaja Freed University of Engineering and Information Technology, Rahim Yar-Khan 64200, Pakistan b
A R T I C L E I N F O
A B S T R A C T
Keywords: Mn3O4 Graphene oxide Nanocomposites Nanoparticles Supercapacitors
Mn3O4 nanoparticles (NPs) are decorated with reduced graphene oxide nanosheets (rGO-Mn3O4) through a facile and eco-friendly hydrothermal method. The as-synthesized composite was characterized by XRD, SEM, TEM and Raman spectroscopy. The electrochemical properties of (rGO-Mn3O4) nanocomposite were studied as electrode materials for supercapacitors. The rGO-Mn3O4 nanocomposite exhibit high speciﬁc capacitance of 457 Fg−1 at 1.0 A/g in 1 M Na2SO4 aqueous electrolyte. The rGO-Mn3O4 exhibits good capacitance retention by achieving 91.6% of its initial capacitance after 5000 cycles. The excellent electrochemical performance is attributed to the increased electrode conductivity in the presence of graphene network.
1. Introduction During the past decades, numerous eﬀorts have been made to explore new energy storage devices with high energy and high power density that can be used in electric vehicles. Supercapacitor emerges as a promising energy storage device, which exhibits various advantages such as high power density, long cycle life, fast charge/discharge ability and high stability [1,2]. However, its energy density is not so high as compared to conventional batteries. So the hot topic of supercapacitors is improving its energy density without apparent damage to its power performance . Graphene, a unique single layer of carbon atoms tightly packed into 2-dimensional honeycomb sp2 carbon lattice has been attracted considerable attention due to their wonderful properties and potential application in many technological ﬁelds such as sensors, energy storage and electronic devices. Graphene also exhibits excellent chemical and physical properties such as high surface area, high electronic conductivity and excellent mechanical strength [4,5]. As compared to other carbon matrixes such as graphite, carbon black, and carbon nanotubes; graphene is an emerging as one of the most appealing carbon materials because of these unique properties . It has been suggested that graphene is an excellent candidate as an electrode material for energy conversion/storage systems as a result of the abovementioned characteristics. Reduced graphene oxide (rGO) is one of the exciting topics in many research ﬁelds especially in the ﬁeld of
nanotechnology during the last few years [7,8]. rGO, a kind of graphene derivative, exhibits excellent electrical, thermal, mechanical properties, a ﬂexible porous structure with mesopores and microspores, high surface area, high conductivity, high energy density and excellent electrochemical stability [9,10]. As a result of these features, graphene nanosheets can acts better as matrices for hosting active nanomaterials to improve the electrochemical properties of the composite materials . Transition metal oxides are usually considered the best candidates for electrode materials in supercapacitors owing to their large speciﬁc capacitance and fast redox kinetics. Hausmannite Mn3O4 has drawn particularly colossal research attention due to its distinctive structural features combined with fascinating physicochemical properties, which are of great interest in magnetic, energy storage and catalyst applications . Several studies have been done with Mn3O4 as the supercapacitor electrode material. However, the poor conductivity of MnOx hinders the practical applications. One useful approach to enhance electrical conductivity is to make composites with highly conductive materials, such as carbon/graphene . Combining Mn3O4 with other conducting substrates such as carbon nanotubes and activated carbons would lead to enhance the electrochemical performance of the resulting electrode . Therefore, the development of a convenient and feasible method to prepare Mn3O4 based composite with improved electrochemical performance will be a great signiﬁcance.
Corresponding author. E-mail address: [email protected]
https://doi.org/10.1016/j.ceramint.2017.11.062 Received 16 October 2017; Received in revised form 8 November 2017; Accepted 9 November 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Shah, H.U., Ceramics International (2017), https://doi.org/10.1016/j.ceramint.2017.11.062
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electron microscopy (HRTEM), energy dispersive spectroscopy (EDS, OXFORD 55-XMX) and Raman spectroscopy to examine the structural properties.
Herein, we report a simple and robust approach for the synthesis of rGO-Mn3O4 nanocomposite with enhanced electrochemical performance as a supercapacitor electrode. The as-synthesized composite was characterized by XRD, SEM, TEM and Raman spectroscopy. The rGOMn3O4 delivered high capacitance of 457 Fg−1 at 1 A/g with high rate capability and good retention after 5000 charge/discharge cycles.
2.3. Electrode preparation and electrochemical characterization The rGO-Mn3O4 based electrodes were prepared as follows; a conducting ﬁller of acetylene black (20 wt%) and a binder of polyvinylidene ﬂuoride (10 wt%) was mixed with rGO-Mn3O4 (70 wt%) as an active material. The mixture was milled to a homogeneous gel and pasted into a nickel foam (1 × 1 cm2) current collector substrate. All electrochemical measurements were carried out with a three-electrode cell with Pt foil (1 × 1 cm2) as the counter electrode, silver/silver chloride electrode (Ag/AgCl) as the reference electrode and the rGOMn3O4 as working electrode. The mass loading density of the active material on the current collector substrate is 1.25 mg/cm2. Potential sweep cyclic voltammetric (CV), galvanostatic charge-discharge (GCD) measurements and electrochemical impedance performance spectroscopy (EIS, 100 kHz-0.01 Hz) were studied respectively.
2. Experimental section 2.1. Synthesis of GO, rGO and rGO-Mn3O4 2.1.1. Synthesis of graphite oxide (GO) Graphite oxide was synthesized from graphite powder by modiﬁed Hummer's method . In brief, 2.5 g of graphite powder was ﬁrst added into 150 mL concentrated H2SO4 at room temperature. After 2 h, 6 g of KMnO4 was added gradually to the above solution while keeping the temperature less than 10 °C to prevent overheating and explosion. 100 mL distilled water was added to the mixture, stirred for 1 h and further diluted to approximately 300 mL with distilled water. Aſter 6 h stirring, 20 mL of 30% H2O2 was added to the mixture to reduce the residual KMnO4. The resulting GO powder was collected after washing the solid material successively with 10% HCl, ethanol and water to remove metal ions until the pH was 6. The GO was dried at 45 °C for 24 h.
3. Results and discussion Fig. 1(b) shows the XRD patterns of pure Mn3O4-NPs, rGO and rGOMn3O4 nanocomposite. All the diﬀraction peaks can be quickly indexed to Hausmannite Mn3O4 (JCPDS no. 80–0382). No additional peaks were detected, which indicates the high purity of the samples. Furthermore, EDX analysis has proved that the product is mainly comprised of three elements: C, O and Mn as shown in Fig. 1(c). Raman spectroscopy is one of the most common and eﬀective techniques for analyzing the structure changes of graphene-based materials, including disorder and defect structures, defect density and doping levels . Two prominent features are usually observed in the Raman spectra of graphene, namely the G band (~1580 cm−1) and the D band (1270–1450 cm−1, depending on laser wavelength ). G band relating to the graphite carbon structure is corresponding to the ﬁrst order scattering of E2g phonon of sp2 C atoms at the Brillouin zone center, while the D band, indicating typical defects attributed to the structural edge eﬀects, is arising from a breathing mode of rings or Kpoint photons of A1g symmetry . Raman spectra of the rGO-Mn3O4 nanocomposite are shown in [Fig. 1(c) inset]. Four characteristic peaks at 302, 353, 372 and 642 cm−1 were observed which are attributed to crystalline Mn3O4 and corresponds to the skeletal vibrations . Two broad peaks were observed at 1337 and 1593 cm−1, which are assigned to the graphene D and G bands, respectively [17,18]. These results suggest that the rGO-Mn3O4 nanocomposite is composed of pure graphene nanosheets and crystalline Mn3O4-NPs. Fig. 2(a) shows the FE-SEM image of Mn3O4-NPs, which indicate the homogenous and uniform size of NPs. Fig. 2(b-c) shows the FE-SEM images rGO-Mn3O4 nanocomposite, and it can be observed that Mn3O4NPs are well ordered distributed on the surface of graphene nanosheets. TEM and HRTEM images (Fig. d, e, f) reveal the existence of Mn3O4-NPs
2.1.2. Synthesis of reduced graphite oxide (rGO) rGO was prepared by the reduction of the as-prepared GO with ascorbic acid. 2 g GO with 50 mg L-ascorbic acid was ultrasonicated in 40 mL H2O for 30 min. Then, the mixture solution was transferred into a 40-mL Teﬂon autoclave and kept in a muﬄe furnace at 180 °C for 6 h. The product was washed with a water and ethanol mixture and dried at 60 °C. The colors of rGOs changed from brown to black, which is the evidence of the reduction process converting GO into rGO . 2.1.3. Synthesis of rGO-Mn3O4 nanocomposite rGO-Mn3O4 was achieved by dissolving 1.5 g MnCl2, 1.5 g GO powder and 1.0 g NaOH in 40 mL deionized water. The mixture was magneticlally stirred for 2 h. Finally, the mixture was sealed in a Teﬂonlined stainless steel autoclave for hydrothermal reaction at 180 °C for 18 h. The ﬁnal product was washed several times with water and ethanol and then dried at 90 °C for 6 h. Mn3O4 nanoparticles were also prepared under the same conditions but without the presence of rGO. The synthesis procedure for the synthesis of the rGO-Mn3O4 nanocomposite is schematically shown in Scheme 1. 2.2. Characterization The structural properties of the materials were characterized by Xray diﬀractometer (X′Pert MPD-XRD). The surface morphology of the as-prepared products were studied by ﬁeld emission scanning electron microscopy (FESEM, ZEISS SUPRA™ 55), high-resolution transmission
Scheme 1. The schematic diagram of the synthesis process of the rGO-Mn3O4 nanocomposite.
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Fig. 1. (a) Standard XRD patterns of Mn3O4 NPs (red), rGO-Mn3O4 composite (blue), and rGO (black); (b) EDS and Raman spectra (inset Fig. b). (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)
computed from the GCD curves using the following equation:
on the surface of thin graphene layers (nanosheets) and make the porous structure. Such a porous composite of metal and graphene allows easy access for electrolyte ions and also with robust conduction paths for electrons and therefore improve the electrochemical performance . Also, it has been observed that some of the Mn3O4-NPs are dark than others, which suggest that they are enveloped in thin rGO ﬁlm. Lattice fringes with a separation of 0.24 nm were clearly observed in the pattern shown in Fig. 2(f) and can be indexed as the  planes of the tetragonal Hausmannite Mn3O4. The electrochemical behaviour of the rGO-Mn3O4 electrode was checked by CV and GCD tests in 1.0 M Na2SO4 aqueous electrolyte using a three-electrode system. Typical CV curves of the rGO-Mn3O4 electrode are shown in Fig. 3(a,b) in a potential window of −0.1–0.9 V at diﬀerent scan rates ranging from 10 to 200 mV s−1. All the CV curves exhibit rectangular shapes even at high scan rate (200 mV s−1) without displaying additional redox peaks, indicating the superior electrochemical features and ideal rate capability. The GCD curves of the rGOMn3O4 electrode are illustrated in Fig. 3(c), which indicates the high columbic eﬃciency of the electrode. The speciﬁc capacitance was
I ∆t ∆Vm
where, C is the speciﬁc capacitance, I is the discharge current, Δt is the discharge time, ΔV is the potential window, and m is the active mass of the material. The speciﬁc capacitance of 457, 396, 300, 272 and 230 F g−1 was calculated at a current density of 1, 2, 4, 5 and 6 A/g, respectively and are shown in Fig. 3(d). These speciﬁc capacitance values are higher than those of the graphene/Mn3O4 reported previously [11,20–22]. The speciﬁc capacitance decreased from 457 to 230 F g−1 when the current density increased from 1 to 6 A/g, demonstrating the high rate capability. The speciﬁc capacitance has a lower value at a higher current density because the electrolyte ions have not enough time to diﬀuse into the interior surface of the active material; hence, it can be concluded that the limited diﬀusion reduces the speciﬁc capacitance of electrode material . The cycling stability of the rGO-Mn3O4 electrode was investigated by the repeating charge-discharge process for 5000 times, and the results are shown in Fig. 3(e). The speciﬁc capacitance decreased to
Fig. 2. (a) FE-SEM image of Mn3O4-NPs and (b, c) rGO-Mn3O4; (d, e) TEM and (f) HRTEM images of the rGO-Mn3O4 composite.
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Fig. 3. (a, b) CV curves for rGO-Mn3O4 electrode; (c) GCD curves; (d) speciﬁc capacitance versus current density; (e) Capacitance retention versus cycle numbers (inset last 500 cycles at a current density of 4 A/g); (f) EIS spectra (Nyquist plots) for rGO-Mn3O4 electrode before and after the rate performance test.
91.6% of the initial capacitance after the completion of 5000 cycles. It can be seen that the speciﬁc capacitance slowly reduced for 2092 cycles, and after that became stable and attained 91.6% after the completion of 5000 cycles. The inset of the ﬁgure Fig. 3(e) demonstrates the last 500 GCD cycles. EIS test was measured to know the conductivity and diﬀusion performance of the rGO-Mn3O4 electrode. Fig. 3(f) illustrate the Nyquist plots of the rGO-Mn3O4 electrode, which consists of a quasi-semicircle and vertical diagonal line. For rGO-Mn3O4 the semicircle at a highfrequency range is not very clear, which indicated the low pseudocharge transfer resistance. The verticle line at low-frequency nearly parallel to the y-axis, show the excellent capacitive features. The intercation with x-axis at high frequency rage indicate the total resistance (Rs) of the electrochemical system, which is only 1.458 Ω and slightly increases to 1.822 Ω after completion of 5000 repeated cycles. The negligible diﬀerence in the value of Rs (~0.364 Ω) demonstrates the excellent stability of the rGO-Mn3O4 electrode in aqueous electrolyte [24,25]. These results suggested the superior electrical conductivity and fast reaction kinetics in the rGO-Mn3O4 electrode and rGO-Mn3O4 is promosing electrode material for high-performance supercapacitors.
In summary, a simple hydrothermal method was used to fabricate rGO-Mn3O4 nanocomposite, and the electrochemical properties were investigated in details in 1 M Na2SO4 aqueous electrolyte. The as-prepared samples were characterized by XRD, FE-SEM, TEM and Raman spectroscopy. A high speciﬁc capacitance of 457 F g−1 has been obtained for the rGO-Mn3O4 electrode at 1 A/g, good retention and cycle stability lead it to be a promising electrode materials for supercapacitors. Acknowledgements We appreciate the ﬁnancial support of the National Natural Science Foundation of China (Grant No. 61373072). References  Y.G.J. Chmiola, C. Largeot, P.L. Taberna, P. Simon, Monolithic carbide-derived carbon ﬁlms for micro-supercapacitors, Science 328 (80-.) (2010) 480–483, http:// dx.doi.org/10.1126/science.1184126.  S. Dai, W. Xu, Y. Xi, M. Wang, X. Gu, D. Guo, C. Hu, Charge storage in KCu7S4 as
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