Manganese hexacyanoferrate derived Mn3O4 nanocubes-reduced graphene oxide nanocomposites and their charge storage characteristics in supercapacitors.

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Physical Chemistry Chemical Physics View Article Online

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Manganese hexacyanoferrate derived Mn3O4 nanocubes/reduced graphene oxide nanocomposites and their charge storage characteristics in supercapacitors K. Subramani, D. Jeyakumar and M. Sathish* Functional Materials Division, CSIR-Central Electrochemical Research Institute, Karaikudi -630 006, India. E-mail: [email protected]; [email protected] Abstract: Mn3O4/reduced graphene oxide (RGO) nanocomposites were prepared by chemical decomposition of manganese hexacyanoferrate (MnHCF) complex directly on graphene surface. XRD studies revealed the formation of crystalline Hausmannite Mn3O4 nanocubes in the asprepared nanocomposites without any heat treatment. The FE-SEM images showed the formation of Mn3O4 nanocubes on graphene surface in the as-prepared nanocomposites. HRTEM studies confirmed the homogeneous dispersion of ~25 nm Mn3O4 nanocubes on graphene nanosheets. The amount of Mn3O4 nanocubes and graphene in the nanocomposites was estimated using TGA analysis from room temperature to 800 °C in air. The FT-IR and Raman spectroscopic analysis confirmed the functional groups in the nanocomposites and defects in graphene nanosheets in the nanocomposites. Cyclic voltammetry and galvanostatic charge– discharge experiments demonstrated a high specific capacitance of 131 F/g in 1 M Na2SO4 electrolyte at the current density of 0.5 A/g for RGM-0.5 nanocomposite. A 99% of capacitance retention was observed for 500 cycles charge-discharge at a current density of 5 A/g conformed the excellent stability of the RGM electrodes. The prepared Mn3O4/RGO nanocomposites are promising for electrochemical energy storage.

Keywords: Mn3O4, supercapacitors, nanocomposite, graphene, manganese hexacyanoferrate

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Physical Chemistry Chemical Physics Accepted Manuscript

DOI: 10.1039/C3CP54788D

Physical Chemistry Chemical Physics

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DOI: 10.1039/C3CP54788D

Introduction:

owing to their unique properties such as high power density, long cycle life, easy maintenance


and low cost. However, its poor energy density limits their wide applications in high energy applications such as electric vehicles (EVs).1 Supercapacitors are broadly classified in to two types namely (i) electrochemical double layer capacitors (EDLC) and (ii) pseudocapacitors, based on their charge storage mechanism. In EDLC, the opposite charges are separated by ion adsorption/desorption at the electrode/electrolyte interface, no chemical reaction occurs in the electrode materials that results excellent stability to the electrodes.2 But, the specific capacitance of commonly used EDLC electrodes is very poor and it mainly depends on the electrode active surface area. Generally carbon based materials are used extensively in EDLC due to their high surface area, good conductivity, light weight etc,.3,4 On the other hand, surface or near surface reactions will occur in pseudocapacitors, where the electrode materials will undergo a reversible oxidation/reduction reactions during charging and discharging process. Thereby, the specific capacitance is increased tremendously with sacrificing the cycle life compared to EDLC. Metal oxides such as RuO2, MnO2, Mn3O4, NiO, Co3O4, Fe3O4 and conductive polymers such as polyaniline, polypyrrole etc have been reported.5–9 Attempts have been made to integrate the EDLC and pseudocapacitve materials to attain supreme specific capacitance with optimum cycle life. The recent developments in materials science offer a variety of synthetic strategies to design and synthesis of desired electrode materials and nanocomposites with desired morphology and size.10–13 Graphene, a single or few atomic layer carbon atoms linked together in a hexagonal network by strong and flexible sp2-bonds. It exhibits very high theoretical specific surface area (2630 m2 g-1),14 high mobility of charge carriers (200000 cm2 v-1 s-1),15 low mass density, high 2


Supercapacitors have gathered much attention in electrochemical energy storage devices

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thermal conductivity (5000 W m-1 k-1),16 high young’s modulus (1100 GPa)17 and high optical

properties show its potential application in various fields. The proposed high surface area and


conductivity assure that graphene as a promising candidate for electrochemical energy storage applications particularly for supercapacitors. However, it is difficult to attain the proposed specific surface area of graphene as the monolayers of graphene sheets are more vulnerable to restacking owing to strong van der Walls’ force of attraction between the graphene layers. To reduce the re-stacking rate, attempts have been made to functionalize the graphene sheets with surfactants, polymers, metal oxides etc. Decorating pseudocapacitve metal oxides on the graphene surface reduces the re-stacking of graphene sheets and increasing the specific capacitance. In addition, the conductivity of the metal oxides improved by decorating on the graphene surface and the specific capacitance also increased due to pseudocapacitance contribution from the decorated metal oxides and improved EDLC due to the additional active graphene surface. Moreover, the decoration of metal oxides on graphene surface creates porosity to the nanocomposites that enables the electrolyte to easily penetrate within the electrodes and access all possible active electrode surfaces.

Among the various metal oxides, RuO2 is well known for pseudocapacitance applications and have been studied widely due to its conductive nature and different oxidation states available within the potential window of 1.2 V.19 The high cost and the small potential window in aqueous electrolyte limits its extensive applications in supercapacitors. RuO2 has been replaced with other inexpensive transition-metal oxides such as nickel hydroxide, nickel oxide, cobalt hydroxide, cobalt oxide, and manganese oxide. However, the potential window of many transition metal

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transmittance (∼97.7%).18 These superior physical, chemical, mechanical, electrical and thermal



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oxides is very small that leads to a poor energy density, as the energy density is proportional to

and Mn3O4 have attracted much attention due to their variable oxidation states as like RuO2.


Attempts have been made to exploit Mn3O4 as electrode material for supercapacitors because of its low cost, eco-friendly, better stability, high abundance and high capacitance.20–23 However, the poor electronic conductivity limits its wide utilization in supercapacitors. In general, the poor electronic conductivity of metal oxides is surmounted by making composites with electronically conductive carbon nanostructures. Various carbon-Mn3O4 nanocomposites such as CNT-Mn3O4, MWCNTs-Mn3O4 and mesoporous carbon-Mn3O4 have been reported.24–28 Among the available carbon nanomaterials, graphene nanosheets are ideal choice for supercapacitor electrodes due to its high conductivity and vast surface area for uniform decoration of Mn3O4 nanoparticles. Recently, various attempts have been made to prepare graphene-Mn3O4 nanocomposite electrodes for energy storage applications by adopting different synthetic strategies.29–33 L. Li et al reported a microwave hydrothermal technique for the rapid synthesis of Mn3O4-graphene nanocomposite and studied their Li-ion storage properties.29 H. Wang et al report the two-step solution based method to grow Mn3O4 nanoparticles on graphene surface for Li-ion battery applications.30 X. Zhang et al report a solvothermal method and D. Wang et al report a hydrothermal methods for the preparation of Mn3O4-graphene nanocomposites with a specific capacitance 147 F/g in 1 M Na2SO4 and 236 F/g in 2 M KOH solution, respectively.31,32 B. Wang et al report the Mn3O4 nanoparticles embedded into graphene nanosheets by subsequent ultrasonication processing and heat treatment and it shows a specific capacitance of 175 F/g in 1 M Na2SO4 electrolyte and 256 F/g in 6 M KOH electrolyte, respectively.33 It is clear from the reported literature that the method of preparation, size, morphology of the Mn3O4, and the

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the square of the cell voltage. Manganese oxide based electrode materials such as MnO2, Mn2O3

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electrolyte plays a vital role on the supercapacitance performance. Also, the uniform dispersion

performance. It is known that the particle size, morphology and porosity of the materials largely


depend on the method of preparation and the heat treatment. In our present study, MnHCF complex was directly grown on graphene surface by reaction with manganese ions and potassium ferricyanide on graphene surface. Subsequently, the MnHCF on graphene surface was decomposed to Mn3O4 nanocubes by 1 M NaOH solution. The decomposition process is taking place instantaneously on graphene surface without any heat treatment. This process enables the direct growth of crystalline Hausmannite Mn3O4 nanocubes on graphene at room temperature. The ratio of Mn3O4 nanocubes and RGO in the nanocomposite has been varied by keeping the GO weight as constant and adjusting the MnHCFs weight. The electrochemical charge storage behavior of above prepared Mn3O4/RGO nanocomposite is studied in aqueous medium. Experimental section Materials Graphite powder (≤20µm, 99%), Polytetrafluoroethylene (PTFE) (-CF2-CF2-)n were purchased from Sigma-Aldrich, India. Sodium Nitrate (NaNO3, 99 wt%), potassium permanganate (KMnO4, 99.5 wt%), hydrogen peroxide (H2O2,30 wt%), hydrazine hydrate (H6N2O, 99 wt%) were purchased from E-merck Co. Ltd. Sulfuric acid (H2SO4, 98 wt%), potassium ferricyanide (K3Fe(CN)6, 99 wt%), manganese (II) chloride (MnCl2, 95 wt%) were purchased from Ranbaxy laboratories., Ltd. Sodium hydroxide pellets (NaOH, 98 wt%) and Ethanol (CH2CH2OH, 99 wt%) were purchased from SRL Pvt. Ltd, India. All purchased chemicals and reagents were in analytical grade and used as received without any further purification. Deionized (DI) water was obtained through MILLIPORE water system.

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of Mn3O4 nanoparticles on graphene surface is essential to achieve the better electrode



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Preparation of graphene oxide (GO)

In a typical synthesis, 0.5 g of graphite and 0.5 g of NaNO3 was mixed with 25 ml of


concentrated H2SO4 in a 500 ml round – bottom flask with ice – bath and continues magnetic stirring for 30 min. Then, KMnO4 (3 g) was gradually added with constant stirring and the reaction temperature was kept at 5 °C. Then, the ice water bath was removed and 40 ml DI water was slowly added with vigorous stirring and the temperature was raised to 90 °C. After 30 min stirring, the solution was further diluted by addition of 100 ml DI water followed by 3 ml of H2O2 (30%), and the resulting mixture was stirred for 1 h. Then, the solution was filtered and washed with hot DI water for several times until the pH of the solution becomes ~6. The filter cake was then dispersed in water by mechanical agitation and low-speed centrifugation was done at 1000 rpm for 5 min, and the large particles were removed from the precipitates. The final sediment was re-dispersed in water with mechanical agitation and mild sonication, giving a solution of exfoliated GO. Synthesis of Mn3O4/RGO nanocomposites Mn3O4/RGO nanocomposites were synthesized through the decomposition of MnHCF complex directly on graphene surface (Scheme-1). In the first step MnHCF complex was formed on GO nanosheets surface using manganese chloride and K3Fe(CN)6. Then the above complex was decomposed to Mn3O4 nanocubes by 1M NaOH solution. Finally, the GO was reduced to RGO by strong reducing agent. In a typical synthesis, MnCl2.4H2O dissolved in 30 ml of DI water was mixed with 15 ml of GO solution and the resulting solution was stirred for 30 min followed by 15 min ultra-sonication. Then, K3Fe(CN)6 dissolved in 30 ml of DI water (MnCl2 : K3Fe(CN)6 = 1:1) was added to the above mixture and stirred for 3 h. The resulting solution was

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GO was synthesized from synthetic graphite flakes by modified Hummer’s method.34,35

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washed with DI water until the solution becomes neutral. The obtained manganese

Mn3O4 nanocubes, the above MnHCF-GO powder was re-dispersed in DI water and stirred for


30 min and 1 M NaOH solution was added slowly with constant stirring and the mixture was stirred for 3 h. The resulting precipitate was filtered and washed with DI water and dried at 90 °C for 5 h. the obtained Mn3O4-GO powder was re-dispersed in DI water and stirred for 30 min at 90 °C and 0.5 ml of hydrazine hydrate was added and stirred for another 2 h. This reduced sample was filtrated and washed with hot water until the filtrate becomes neutral. The resulting fine Mn3O4-RGO nanocomposite powder was dried at 90 °C for 5 h. The weight ratio of Mn3O4 and RGO in the nanocomposite has been varied by keeping the GO weight as constant and adjusting the MnHCFs weight appropriately and the resulting nanocomposites have been named as RGM–X, where the x indicates the Mn3O4/RGO weight ratio in the nanocomposite. For comparison purpose, pure Mn3O4 nanocubes were prepared using the same experimental procedure without GO and represented as pure Mn3O4, similarly pure RGO (without Mn3O4 nanocubes) was prepared by reducing the GO with hydrazine hydrate solution.

Materials characterization The crystalline nature of the nanocomposite was examined by powder X-ray diffraction (XRD) measurements using a PAN Analytical X’ Per PRO Model X-ray Diffractometer with Cu K radiation (α=1.5418Å) from 10-80° at 0.02° step and a count time of 0.2 s. Fourier Transform α

Infrared (FT-IR) spectra were recorded in TENSOR 27 spectrometer (Bruker) using KBr pellet technique from 400 to 4000 cm-1. Thermogravimetric analyses (TGA) of the RGM-X nanocomposites was carried out using TGA/DTA instruments (Model SDT Q 600) from room 7


hexacyanoferrate complex-GO (MnHCF-GO) was dried at 60 °C in hot air oven. To obtain



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temperature to 800 °C with a heating rate of 10 °C/min in air. The chemical nature of the

Raman microscope) equipped with a semiconducting laser with a wavelength of 633 nm. The


morphology and surface nature of the Mn3O4/RGO nanocomposite was characterized using fieldemission scanning electron microscopy (FE-SEM) using Carl Zeiss AG (Supra 55VP) with an acceleration voltage of 5-30kV. The particle size and dispersion of Mn3O4 nanocubes on graphene in the Mn3O4/RGO nanocomposite was examined using transmission electron microscope (TEM, TecnaiTM G2 20) working at an accelerating voltage of 200 kV. Electrochemical characterization The electrochemical properties of Mn3O4/RGO nanocomposites were studied using cyclic voltammetry (CV) with three electrode cell configuration. The working electrode was fabricated by mixing the nanocomposite materials with conductive carbon (acetylene black) and binder (PTFE) at the weight ratio of 75:20:5. The electrode paste (3~5 mg) was pressed on stainless steel mesh current collector and used as working electrode. A slice of Pt foil and Ag/AgCl are used as counter electrode and reference electrode, respectively. Cyclic voltammetry and galvanostatic charge–discharge measurements were carried out using potentiostat-galvanostat (PG30, AUTOLAB) instruments in 1M Na2SO4 electrolyte at different scan and current densities, respectively. Results and discussion: The formation of Mn3O4 nanocubes involves two steps chemical reaction such as formation of MnHCF complex followed by its decomposition in alkaline medium. The chemical reactions involved in the preparation are shown in equation 1 to 5 as follows.36,37

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prepared nanocomposites was characterized using a laser Raman system (RENISHAW Ivia laser

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!


!"#$% !"

+ !"#$! . 4!! ! → !"#$% !" !

+ 2!"#$ → !" !"

!

!

+ 2!"#

+ ! ! + 2!"! + !" (!")!! !

….. (1) ….. (2)

Mn(OH)! → MnO + H! O

….. (3)

2MnO + O! → 2MnO!

….. (4)

2MnO + MnO! → Mn! O!

….. (5)

For Mn3O4/RGO nanocomposite preparation (Scheme-1), the MnHCF complex was directly grown on GO surface. The functional group presents on the graphene surface enables the homogeneous formation of MnHCF complex on its surface. Decomposition of MnHCF complex in the further step results the uniform dispersion of Mn3O4 nanocubes. These Mn3O4 nanocubes grown on GO surface prevents the restacking of graphene sheets during the reduction of GO to RGO by hydrazine hydrate solution. It is worthy to note here that there was no high temperature heat treatment involved during the nanocomposite preparation thus the agglomeration of Mn3O4 nanocubes on the graphene is completely prevented.

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!! !" !"



Scheme 1: Preparation of Mn3O4/RGO nanocomposites.

Fig. 1a shows the XRD pattern of RGO nanosheets, Mn3O4 and RGM-1 nanocomposite. It can be clearly seen that the RGO shows a broad peak at 24° and 43.1° corresponding to (002) and (100) plane, respectively.38 Similarly, as-prepared Mn3O4 nanocubes shows diffraction lines corresponding to Hausmannite tetragonal crystalline system (JCPDS No. 024-0734). The RGM-1 nanocomposite shows the presence of Hausmannite Mn3O4 and RGO, and the diffraction lines are slightly broad than the Mn3O4 nanocubes prepared without GO. This clearly indicates that the direct growth of Mn3O4 on RGO surface reduces the agglomeration of Mn3O4 nanocubes. The particle size calculated using Debye–Scherrer equation39 showed an average size of ~14 nm

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Mn3O4 nanocubes on graphene surface. Fig. 1b shows the XRD pattern of Mn3O4/RGO

all the compositions, whereas the presence of RGO could be seen when the weight ratio of


Mn3O4/RGO is 1 (RGM-1) and below. At high Mn3O4/RGO ratio, the intensity of (002) plane corresponding to RGO is very small due to strong Mn3O4 lines.

Fig. 1 XRD pattern of (a) (i) RGO, (ii) Mn3O4 and (iii) RGM-1, and (b) (i) RGM-0.5, (ii) RGM1, (iii) RGM-5, (iv) RGM-10 and (v) RGM-15 nanocomposites. To examine the weight ratio of Mn3O4 and RGO in the RGM nanocomposites, thermogravimetric analysis (TGA) were carried out from 35 to 800 °C at a heating rate of 10 °C min-1 in air (Fig. 2). The TGA profile of RGM-1 and RGM-0.5 nanocomposites showed a small weight loss due to the physically and chemically adsorbed water molecules at below 200 °C. Further increase in temperature results the decomposition of RGO present in the nanocomposites from 250 ~ 420 °C. Generally, graphene and graphene oxide are completely burned to CO2 in air between 250 to 600 °C.40 At above 600 °C the weight loss is almost zero up to 800 °C indicating the presence of highly stable Mn3O4 nanocubes alone in the RGM nanocomposites. From the 11


nanocomposites prepared with different weight ratios. The presence of Mn3O4 could be seen at



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weight loss profile, the amount of graphene decomposed and remaining amount of Mn3O4 in

RGM-0.5 and RGM-1 that when the RGO amount is more in the nanocomposite the


decomposition of RGO occurs at low temperature. This may be ascribed to the exothermic reaction occurs during the decomposition of carbon materials in the nanocomposites. The observed Mn3O4/RGO weight ratio of 49/40 and 25/54 for the RGM-1 and RGM-0.5 are in good agreement with the theoretically calculated weight ratio during the synthesis of RGM nanocomposites, respectively.

Fig. 2 TGA profile of (i) RGM-1 and (ii) RGM-0.5 nanocomposites. FT-IR spectrum of GO showed the presence of various functional groups such as hydroxyl, epoxy, carboxyl, alkoxy and carbonyl groups on graphene oxide (Fig. 3a). The broad – OH peak observed at 3406 cm-1 indicates the presence of hydroxyl groups in GO nanosheets. The C-H stretching, carbonyl/carboxyl C=O, aromatic C=C, epoxy C-O, alkoxy C-O and carboxyl C-O can be seen at 2915, 1712, 1582, 1411, 1202 and 1046 cm-1, respectively.41 FT-IR spectrum of RGO showed that peak corresponding to various oxygen containing functional 12


RGM nanocomposites was calculated. It could be clearly seen from the decomposition profile of

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groups are disappeared or the intensity reduced drastically (Fig. 3a). Similarly, the RGM-5

but two new peaks at 622 cm-1 and 508 cm-1 are corresponding to the vibration of the Mn-O


stretching modes, associated with Mn in tetrahedral and octahedral sites, respectively.42 Also, the –OH peak appears at 3406 cm-1 due to the surface hydroxyl groups on Mn3O4 nanocubes in RGM-X nanocomposites. FT-IR spectra of pure Mn3O4 nanocubes and RGM nanocomposite with different mass ratio are shown in Fig. 3b. It can be clearly seen that when the Mn3O4 weight ratio increased, the intensity of Mn-O stretching mode is also increased and it becomes close to the bare Mn3O4 nanocubes from RGM-15.

Fig. 3 FT-IR spectra of (a) (i) GO, (ii) RGO and (iii) RGM-5; and (b) (i) pure Mn3O4 and RGM nanocomposites (ii) RGM-15, (iii) RGM-10, (iv) RGM-5 and (v) RGM-1.

Raman spectroscopy is a fundamental analytical tool for the characterization of carbon based materials particularly for graphene based materials to understand its chemical bonding. Certainly, it can preciously distinguish sp2 and sp3 hybridized bonding of carbon atoms in graphene and extensively used to estimate the number of graphene layers. The G band is assigned to the E2g phonon of sp2 carbon atoms and D band is assigned the extent of defects in 13


sample showed (Fig. 3a) the absence of oxygen containing function groups corresponding to GO,



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the graphene sheets.43–45 The ratio of D band and G band intensities (ID/IG) substantiates the

RGM-1 and RGM -10 nanocomposites are shown in Fig. 4. The G band at 1585 cm-1 is sharp


and intense, and the D band intensity in graphite is almost negligible compared to RGO and its nanocomposites. Whereas, the RGO and the nanocomposites (RGM-1 and RGM-10) shows a broad G and D bands at 1592 cm-1 and 1334 cm-1, respectively. The intensity of D band (ID) is generally low for highly ordered graphene materials and it will be high for the defective graphene.46 RGO shows a high ID/IG ratio (ID/IG = 1.11) compared to the graphite (ID/IG = 0.13) indicating the decrease in the average size of sp2 domains in RGO due to strong oxidation of graphite during the GO preparation and exfoliation process. Mn3O4 nanocubes show a characteristic Raman lines at 648 cm-1 due to the A1g active mode.47,48 The nanocomposites RGM-1 and RGM-10 also show a similar line that reveals the presence of Mn3O4 nanocubes in the nanocomposites. However, G and D bands corresponding to RGO could be seen for RGM-1 sample, the absence of these bands in RGM-10 is due to very low concentration, this is in good agreement with the results observed in XRD studies.

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extent of its defects in the graphene nanosheets. Raman spectra of raw graphite, RGO, Mn3O4,

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Fig. 4 Raman spectra of (i) graphite, (ii) RGO, (iii) pure Mn3O4, (iv) RGM-10 and (v) RGM-1 nanocomposite.

The surface morphology of the nanocomposites RGM-0.5, RGM-1, RGM-10 and RGM15 was investigated using FE-SEM images (Fig. 5 a-d). It could be clearly seen from the images that formation of Mn3O4 nanocubes by the decomposition of MnHCF complex. The Mn3O4 nanocubes dispersion is high for lower Mn3O4/RGO mass ratio (Fig 5 a&b). When the amount of Mn3O4 increased, the RGO gets completely covered and the presence of RGO sheets could not be seen in the FE-SEM images (Fig. 5 c&d).

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Fig. 5 FE-SEM images of (a) RGM-0.5, (b) RGM-1, (c) RGM-10 and (d) RGM-15; TEM images of (e) GO and (f) RGO nanosheets.

Fig. 5 e&f represent the TEM images of GO and RGO prepared by modified Hummers method and its reduction by hydrazine hydrate solution, respectively. Formation of few layered GO and RGO nanosheets can be clearly seen from the trasparent nature of the observed 16



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nanosheete. The disperssion of Mn3O4 nanocubes on the RGO nanosheets was studied using

and RGM-5 reveal that when Mn3O4 content in the nanocompstie increase the RGO surface was


completely covered with Mn3O4 nanocubes and excess particles could be seen throughout the nanocomposites (Fig. 6 a-d). TEM image of RGM-15 at high magnification (Fig. 6b) clealry shows the Mn3O4 nanocube morphology with an average size of 30 nm. Similarly, TEM image of RGM-5 at high manification (Fig. 6e) revealed the Mn3O4 nanocubes decoration on RGO nanosheets. The average crystaline size of the Mn3O4 nanocubes in the nanocomposites is 25 nm, it is slighly higher than the particle size calculated from the XRD data. The SAED pattern of RGM-15 (inset of Fig. 6a) showed presence of graphitic carbon plane (002) and polycrystalline Hausmanite Mn3O4 nanocrystals in the nanocomposites.49 Fig. 6f shows the lattice fringes of Mn3O4 nanocubes decorated on the RGO surface in RGM-5 nanocomposites. The calculated ‘d’ values of 0.32 nm and 0.27 nm from the lattice fringes image are in good correspondance with the ‘d’ values calculated from XRD pattern for (112) and (103) planes of Hausmannite Mn3O4 nanocubes, repectively.

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TEM images at different magnificaions as shown in Fig. 6. TEM images of RGM-15, RGM-10



Fig. (6): TEM images of (a) RGM-15, (b) RGM-10, (c) RGM-5, (d) RGM-0.5, and (e&f) RGM5 nanocomposites at high magnifications (inset of (a) shows SAED pattern of RGM-15).

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Electrochemical properties of RGM-X nanocomposites, RGO and Mn3O4 nanocubes were

electrolyte from -800 to 600 mV and -900 to 0 mV (vs. Ag/AgCl). CV profile of RGO, Mn3O4


and RGM-0.5 nanocomposites at 20 mVs-1 scan rate are shown in Fig 7a. Among the three samples, the RGM-0.5 showed a better performance, it shows better performance in anode region compared to cathode region due to better contribution of RGO in the nanocomposite. It is known that RGO is widely used as anodes and Mn3O4 is widely used as cathodes in supercapaciors. CV profile of RGM nanocomposite with different weight ratios of Mn3O4 and RGO at 20 mV s-1 scan rate is shown in Fig 7b. It clearly indicates that when the weight ratio of Mn3O4/RGO increase from 0.2 to 0.5, the current density increase and further increase Mn3O4weight in the nanocomposite decreases current density. Besides, it is observed that there is no significant change in the current at the cathodic region compared to anode region. The CV profile of RGM0.5 at different scan rates (Fig. 7c) clearly demonstrates that the anodic region undergoes severe change compared to cathodic region when the scan rates were altered. The galvanostatic discharge profile at 0.5A/g shows a specific capacitance of 65, 118 and 130 F/g for Mn3O4, RGO and RGM-0.5 nanocomposites, respectively (Fig. 7d). It is clear from the above observations that the Mn3O4 nanocubes on RGO surface enhance the specific capacitance of RGO by hindering the re-stacking of graphene sheets. And, addition of RGO to Mn3O4 nanocubes does not help much to improve the specific capacitance of Mn3O4 nanocubes. Thus, the potential window is changed to -0.9 to 0 V vs Ag/Agcl for further studies to understand the change in the specific capacitance of RGO alone in RGM nanocomposites.

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investigated using CV and galvanostatic charge-discharge experiments in 1 M Na2SO4 aqueous



Fig. 7 Cyclic voltammetry profile of (A) (a) RGM-0.5, (b) RGO and (c) Mn3O4 electrodes at 20 mV s−1, (B) RGM-X nanocomposites with different mass ratios at 20 mV s-1 (C) RGM-0.5 electrode at various scan rates, and (D) galvanostatic discharge curves of (a) RGO, (b) Mn3O4 and (c) RGM-0.5 nanocomposites at 0.5A/g.

In Fig. 8a, the CV profile of RGO, Mn3O4 and RGM-0.5 nanocomposites at 20 mV s-1 scan rates are shown. Among the three samples, the RGM-0.5 show better performance in the studied potential range. Fig. 8b reveals the CV profile of RGM nanocomposite with different weight ratios of Mn3O4 and RGO at 20 mV s-1 scan rate. It clearly indicates that when the weight ratio of Mn3O4/RGO increase to more than 1, the current density decrease with increasing the Mn3O4 in the nanocomposite. Whereas, when the ratio is less than one, the current density 20



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increase up to 0.5 (RGM-0.5) further decreases in Mn3O4 weight will decrease the current

shown in Fig. 8c. In all the studied scan rates the CV profile is in ideal rectangular shape and


exhibit mirror-image characteristics as like EDLC. It is clear from the above observations that the Mn3O4 nanocubes on RGO surface should be optimum to obtain better charge storage.

Galvanostatic charge–discharge experiments revealed that the specific capacitance of Mn3O4, RGO and RGM-0.5 nanocomposites are 2 F/g, 125 F/g and 131 F/g at 0.5A/g current density, respectively. Though, Mn3O4 nanocubes shows a poor specific capacitance (2 F/g) in 0.9 to 0 V potential window compared 65 F/g in -0.6 to 0.6 V, the specific capacitance of RGM0.5 nanocomposite is similar in both the potential windows. This clearly indicates that the Mn3O4 is acting as spacer molecule to hinder the re-stacking of RGO nanosheeets. Fig. 8d represents the discharge profile of RGM-0.5 nanocomposite at 0.5, 1, 2 and 5A/g current densities and the corresponding specific capacitance are 131, 118, 107 and 86 F/g, respectively. Fig. 8e shows the specific capacitance of RGO and RGM-0.5 at different current densities, it could be clearly seen that though the specific capacitance of RGO is comparable to RGM-0.5 at low current density (0.5A/g). However, the specific capacitance of RGO decreases drastically when the discharge current density is increased. The RGM-0.5 nanocomposite shows ~65.64 % capacitance retention whereas RGO shows only ~24 % capacitance retention when the current density is increased from 0.5A/g to 5A/g. Fig. 8f shows the charge-discharge profile of RGM-0.5 nanocomposite at a current density of 5 A/g for 500 cycles. There is no significant decrease in the specific capacitance up to 500 cycles, the capacitance retention is 99% compared to the first cycle specific capacitance. This clearly indicates that the electrode materials are stable enough in the

21


density of the nanocomposite (RGM-0.2). The CV profile of RGM-0.5 at different scan rate is



DOI: 10.1039/C3CP54788D

studied potential window. The inset of Fig. 8f shows the charge-discharge profile of RGM-0.5

Fig. 8 Cyclic voltammetry profile of (a) (i) RGM-0.5, (ii) Mn3O4 and (iii) RGO electrodes at 20 mV s−1, (b) RGM-X nanocomposites with different mass ratio at 20 mV s-1 (c) RGM-0.5 electrode at various scan rates, (d) galvanostatic discharge curves of RGM-0.5 nanocomposite electrode at different current densities, (e) specific capacitance of (i) RGM-0.5 nanocomposite and (ii) RGO electrodes at various current densities and (f) cycling performance of RGM-0.5 nanocomposite electrode for 500 cycles and charge-discharge profile (inset) at 5 Ag−1. 22



nanocomposite during the cycling study.

Page 23 of 27


DOI: 10.1039/C3CP54788D

Conclusions:

been prepared directly on graphene surface by chemical decomposition of MnHCF complex in 1


M NaOH solution. The electron microscopic images revealed the cubic morphology of the Mn3O4 nanocubes with an average size of ~25 nm decorated on the graphene surface. At low Mn3O4/RGO ratio the nanocubes decoration on graphene surface is uniform and increasing the Mn3O4 amount in the nanocomposite results the excess nanocubes throughout the nanocomposites. Raman and FT-IR spectra confirmed the chemical nature of the Mn3O4 and RGO in the nanocomposites. Cyclic Voltammetry and galvanostatic charge–discharge experiments showed that the specific capacitance of RGO and RGM-0.5 nanocomposites are very close at low current rate whereas when the current rate is increased the RGM-0.5 nanocomposite performance is superior to RGO. RGM-X nanocomposites exhibited excellent rate capability and good electrochemical stability at 5 A/g for 500 cycles. Acknowledgments K. Subramani is thankful to DST, India for financial support through DST - INSPIRE fellowship and D. Jeyakumar and M. Sathish thank CSIR for financial support through MULTIFUN (CSC0101) project. Reference: 1

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DOI: 10.1039/C3CP54788D

Manganese hexacyanoferrate derived Mn3O4 nanocubes/ reduced graphene oxide nanocomposites and their charge storage characteristics in supercapacitors


K. Subramani, D. Jeyakumar and M. Sathish*

Mn3O4 nanocubes/RGO nanocomposites were prepared using manganese-hexacyanoferrate decomposition on graphene and a specific capacitance of 131 F/g was achieved at 0.5 A/g in 1 M Na2SO4.

1


Graphical abstract:-

Hierarchical nanocomposites of polyaniline nanowire arrays on reduced graphene oxide sheets for supercapacitors.

Fabrication of graphene-carbon nanotube papers decorated with manganese oxide nanoneedles on the graphene sheets for supercapacitors.

Significantly enhanced photocatalytic activities and charge separation mechanism of Pd-decorated ZnO-graphene oxide nanocomposites.

Nanoarchitectured graphene-based supercapacitors for next-generation energy-storage applications.

carbon nanotubes, graphene and graphene oxide as water-oxidizing composites in artificial photosynthesis.

Effect of pristine graphene incorporation on charge storage mechanism of three-dimensional graphene oxide: superior energy and power density retention.

Electrospun manganese-cobalt oxide hollow nanofibres synthesized via combustion reactions and their lithium storage performance.

Nitrogen-doped reduced graphene oxide electrodes for electrochemical supercapacitors.

Bismuth oxide nanotubes-graphene fiber-based flexible supercapacitors.

reduced graphene oxide nanocomposites.

Preparation and mechanical properties of graphene oxide: cement nanocomposites.

Geometry-controllable graphene layers and their application for supercapacitors.

Facile synthesis of Mesoporouscobalt Hexacyanoferrate Nanocubes for High-Performance Supercapacitors.

NMR Study of Ion Dynamics and Charge Storage in Ionic Liquid Supercapacitors.

graphene oxide nanocomposites for energy applications.

Thermally reduced kaolin-graphene oxide nanocomposites for gas sensing.

Fabrication of ultralong hybrid microfibers from nanosheets of reduced graphene oxide and transition-metal dichalcogenides and their application as supercapacitors.

Facile synthesis of graphene-CeO2 nanocomposites with enhanced electrochemical properties for supercapacitors.

Electrostatic Spray Deposition-Based Manganese Oxide Films-From Pseudocapacitive Charge Storage Materials to Three-Dimensional Microelectrode Integrands.

In situ NMR spectroscopy of supercapacitors: insight into the charge storage mechanism.

High-performance solid-state supercapacitors based on graphene-ZnO hybrid nanocomposites.

X-ray Absorption Study of Graphene Oxide and Transition Metal Oxide Nanocomposites.

Shape-controlled ceria-reduced graphene oxide nanocomposites toward high-sensitive in situ detection of nitric oxide.

Co@Carbon and Co 3 O4@Carbon nanocomposites derived from a single MOF for supercapacitors.