CdS-CoFe2O4@reduced graphene oxide Nano hybrid: An excellent

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CdS-CoFe2O4@reduced graphene oxide Nano hybrid: An excellent electrode material for Supercapacitor Applications Amrita De Adhikari, Ramesh Oraon, Santosh Kumar Tiwari, Pupulata Saren, Joong-Hee Lee, Nam Hoon Kim, and Ganesh Chandra Nayak Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04885 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 9, 2018

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Industrial & Engineering Chemistry Research

CdS-CoFe2O4@reduced graphene oxide Nano hybrid: An excellent electrode material for Supercapacitor Applications. Amrita De Adhikari a, Ramesh Oraon a, Santosh Kumar Tiwaria, Pupulata Sarena, Joong Hee Leeb,c, Nam Hoon Kim b,c and Ganesh Chandra Nayak*a a. Department of Applied Chemistry, IIT (ISM) Dhanbad,826004, Jharkhand, India b. Department of BIN Fusion Technology, Chonbuk National University,Jeonju, Jeonbuk 571-756, Republic of Korea c. Dept of Polymer & Nano Science and Technology, Chonbuk National University,Jeonju, Jeonbuk 571-756, Republic of Korea E-mail address: [email protected]

Abstract: CoFe2O4 nanospheres ornamented CdS nanorods were successfully assembled over the RGO nanosheets. Such hierarchical morphology established by FESEM and TEM studies, with high surface area offered a high specific capacitance of 1487Fg-1 at a current density of 5A/g owing to fast diffusion of ions, facile transportation of electrons and great synergism between the components which led to reversible redox reactions. Furthermore, the electrode material has specific capacitance retention of 78% up to 5000 cycles, thus demonstrating its good reversibility and cyclic stability. The resulting CdS-CoFe2O4@reduced graphene oxide nano hybrid can deliver excellent electrochemical performance and can be a potential candidate for supercapacitors application. Keywords: CdS nanorods, CoFe2O4 nanospheres, supercapacitor, specific capacitance.

1. Introduction: The ever-increasing consumption of fossil fuel reserves and the escalating global environmental problems necessitates the development of the energy storage devices to meet the increasing demand of daily life.1,2 Such drastic conditions have compelled the scientific communities to indulge in sustainable energy storage technologies.2 Supercapacitors (SCs) are one of the ideal candidate for the green energy storage owing to their higher power density, excellent reversibility, fast recharge ability and longer cycle life than conventional electrostatic and electrolytic capacitors and the secondary batteries.3 Although the SCs have smaller energy densities than the conventional batteries, it can offer higher power density due to its faster ion flow.4 The advances of the portable and wearable energy technologies, has a high demand in the development of the environment friendly SCs.2 Depending on the charge storage mechanism, the SCs can be classified as electrochemical double layer capacitors (EDLCs) and the pseudocapacitors, which stores charge via ion adsorption and surface faradic reactions

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respectively2,4 Commonly three types of materials have been chosen for the SCs electrode material viz, carbon materials (such as carbon nanotubes, carbon black, onion-like carbons, activated carbon, graphene),5,6,7 conducting polymers (Polyaniline, polpyrrole, polyindole etc )811 and metal oxides or hydroxides (MnO2, V2O5,Co3O4, Ni(OH)2). 12-14 The carbonaceous materials have been widely used in the EDLCs owing to their high surface area, good electrical conductivity and thermal stability. The interior surface of the micropores can increase the surface area of the carbon materials; however the diffusion of the electrolyte onto the total surface area is difficult which leads to poor rate capability at higher current densities.1 Therefore increasing the rate capacity and improved energy capacity of carbon based nanomaterials have become an ongoing challenge of research. The energy densities of the pseudocapacitors (i.e. conducting polymers and metal or metal oxides) are many times higher than the EDLCs, thus an effective strategy to utilize the nanostructured carbon materials with such pseudocapacitive materials, can optimize the electrode properties with high capacitance having increased surface area and short ion transport pathways.15,4Numerous efforts have been made to search for the low cost transition metal oxides such as MnO2,16,17,18 NiO, Fe3O4,19 Co3O4,20V2O5,13 CdS etc.21 The capacitive performance of the metal sulfides are better as compared to the metal(hydro-) oxides, having good redox reversibility, superior rate capability and high specific capacitance.1 Among the array of the widely studied metal sulfides, CdS has been studied for the energy storage applications due to its unique properties.It is a n-type semiconductor with a narrow band gap of 2.4eV.It has been widely used in cadmium nickel batteries displaying its long cycle life, high discharge rates, good environmental stability, relatively low toxicity and high energy density.1 Various research groups have reported the utilization of CdS in various energy storage devices. Xu et al have reported porous cadmium sulphide on nickel foam as an electrode material which showed a specific capacitance of 909 Fg-1 at 2mA cm-2.22 Again, Wang et al reported Ni2S3@CdS core shell nanostructure exhibited a specific capacitance of 2100Fg-1 at 2mA cm-2.1 Zhang et al have reported RGO/CdS hydrogel for energy storage application with a specific capacitance of 300 Fg-1 at 5 mV sec-1.23 Huang et al have reported the electrochemical capacitive performance of Cd(OH)2 nanowires on Ni foam which showed a specific capacitance of 1164.8 F g-1 at 1 Ag-1.16 Thus, to study the performance of CdS as supercapacitor electrode material is very desirable to explore. Again, CdS has a high theoretical specific capacitance value of 1675 Fg-1, which also predicts its favorable potential to be exploited in the energy storage applications.22 Nowadays, mixed metal oxide systems like Mn/Fe oxides24, Mn/Ni/Co oxide 25, Ni/Mn oxides 26, have attributed and shown high improvements in the electrochemical performance. Spinel ferrites are such materials which has various remarkable properties like electrical, optical, exhibits different redox states, electrochemical stability and so on. 27, 28 Among various spinels, CoFe2O4 has been explored as a pseudocapacitor material for the SCs application. Deng et al have demonstrated the fabrication of CoFe2O4 nanostructures and their comparison with the electrochemical properties.29 However, it has been observed that pure CoFe2O4 cannot exhibit appreciable conducting performance which led to the research of synthesizing their composites with other metal and conducting materials.30,31 CoFe2O4 is a p-type


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semiconductor with a band gap of 0.8eV. CdS being an n-type semiconductor and CoFe2O4 as ptype semiconductor, p-n heterojunctions can be obtained and such heterojunctions can engender materials with superior electrical performance owing to the inherent charge transfer driving force. 1 Further, by incorporation of carbonaceous material in such binary composites, can offer increased surface area and short ion transport pathways which can be beneficial for the SCs application. Recently, graphene, a sp2-bonded single one atom thick 2D carbon layer has attracted immense concern as electrode materials for the SCs application. 32 It has certain intriguing properties like light weight, high electrical and thermal conductivity, strong thermal and mechanical stability (~1TPa) and high tunable surface area (upto 2675 m2g-1). 32 Graphene-based nanocomposites with the metal oxides can offer a conductive network through various redox reactions. The graphitic layers facilitate the dispersion of metal oxides/hydroxides which acts as a highly conductive matrix for increasing the electrical conductivity. In this work, CdS-CoFe2O4 @ reduced graphene oxide (RGO) nano hybrid has been synthesized hydrothermally for the SCs application. Here, the binary CdS-CoFe2O4 nanocomposite acts as a p-n junction diode, where the electrons in the valence band (VB) of CoFe2O4 moves to the conduction band (CB) and further the electrons from the CB of CoFe2O4 moves to the CB of CdS, where RGO acts as an electronic acceptor and transferor. The ternary electrode material offered high specific capacitance of 1487 Fg-1.

2. Experimental Section: 2.1. Materials: All the chemicals were of analytical grade and used without further purification. Graphite powder was obtained from S.D Fine Chemicals Limited, Mumbai (India) (99.9% particle size and 100 micron). Nitric acid and potassium permanganate was procured from RFCL limited, New Delhi (India). Sulphuric acid (98% pure), ortho-phosphoric acid 88%, methanol, sodium hydroxide and hydrogen peroxide were obtained from Merck Specialist Pvt. Limited, Mumbai. Cadmium nitrate [Cd (NO3)3] and Cobalt nitrate [Co (NO3)2.6H2O] were obtained from Loba Chemie Pvt. Limited, Mumbai. Fe (NO3)3.9H2O, Thiourea, ethylene diamine and ethylene gycol was obtained from Alfa Aesar Pvt. Limited. 2.2. Synthesis of Reduced graphene oxide: Graphene oxide (GO) can be prepared from graphite flakes via improved Hummer’s method. In brief, 9:1 mixture of concentrated H2SO4/H3PO4 was added to graphite ﬂakes (1.0 g, 1 wt equiv) .The reaction was kept under continuous stirring in an ice bath followed by addition of KMnO4 (8.0 g, 8 wt equiv). The colour of the reaction mixture turned slowly from dark black to greenish brown. About 100ml of DI water was added to the above reaction medium and the temperature was increased to 900C and maintained for 30 mins. Again, about 200ml DI water



was added followed by the slow addition of 30% H2O2 (3 mL) under dark condition and kept under stirring condition for about 6 hrs. The reaction mixture was then centrifuged (10000 rpm) and washed with methanol and water for several times. The solid obtained, was vacuum-dried overnight at room temperature to obtain GO. Further, RGO was obtained by treating GO with hydrazine hydrate.GO was ultrasonicated for about 30 mins. and then a few drops of hydrazine hydrate were added and the entire mixture was stirred at 900 C for 1 hr. The brownish GO solution gradually turned blackish. The reaction mixture was cooled down at room temperature, washed several times with water/ethanol mixture and dried under vacuum at 640C to obtain RGO. 2.3. Preparation of CdS: 1.85g Cd (NO3)3 and 3.65 g of thiourea was mixed in 50ml of ethylene diamine. After 30 mins of stirring, the mixture was transferred into 50ml autoclave. Hydrothermal reaction was carried out at 2000C for 10 hrs. The product obtained was yellow in colour which was washed and dried at 800C to obtain CdS. 31 2.4. Preparation of CdS-CoFe: 1g of CdS and 5mg/ml of citric acid solution was dispersed in 30ml solution of Co (NO3)2.6H2O (0.436g) and Fe (NO3)3.9H2O (1.212g). This was followed by the addition of 5 ml, 5(M) NaOH solution. The mixture was stirred for 3 hrs. at 1000C to obtain the binary composite. 28

2.5. Preparation of CdS-CoFe-RGO: 10 mg of CdS-CoFe was dispersed in 10ml ethylene gycol. This was then added to 0.3mg/ml of RGO suspension in DI water. To this 4ml of ammonia solution was added maintaining the pH-10 and stirred for half an hour. Then 2 ml of hydrazine was added and heated in oil bath for 6 hrs. The resulting paste obtained was washed with DI water and heated at 600C for 2 hrs. 31 All the synthetic schemes are summarized in scheme 1.

3. Characterization: 3.1.

Morphology and structural characterization:

The X-ray diffraction data were recorded at a scan rate of 10 min-1 for 2θ angles between 200 and 800. The FTIR spectra of the as-synthesized nanocomposite were recorded within the wavelength range of 400 to 4000 cm-1 with a NEXUS 870 FTIR (Thermo Nicolet) to investigate the bonding properties in the resultant nanocomposites. Raman analysis was carried out with a Nanofinder 30 (Tokyo Instruments Co., Japan). XPS characterization was carried out with a Theta Probe ARXPS system from Thermo Fisher Scientific (UK) with monochromated AlKα as X-ray source (hv=1486.6 eV, X-ray energy=15 kV), 150 W, and a 400 mm spot size. The surface morphology of the as-prepared nanocomposites was characterized by means of field-emission


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scanning electron microscopy with a Supra 55 (Carl Zeiss, Germany) instrument. The BET surface-area measurement of the as-synthesized nanocomposite was characterized with a Quantachrome Nova Win at a degassing temperature of 1200C. 3.2.

Electrochemical measurements:

The electrochemical properties of CdS based composites were characterized by using a threeelectrode system and 1(M) NEt4BF4–acetonitrile as electrolyte at room temperature with Biologic SP-300. The CdS based nanocomposites were used as the working electrode, and Ag/AgCl and platinum wire were used as the reference and counter electrode, respectively. The working electrodes were prepared by adhering the respective electrode materials on the tip of the graphite rods. The electrochemical impedance spectrum (EIS) was analyzed at 5 mV of amplitude and in the frequency range of 1 to 100000 Hz. Cyclic voltammetry was performed in the potential range of 0 to 1.6 V at scan rate of 5mV/sec. The specific capacitance was calculated from the charge/discharge plots using the formula: C = I∆t/mV (F g-1), 33 Where I is the charge–discharge current, ∆t is the discharge time, V is the potential window and m is the mass of the active material. The specific energy density and the power density were calculated as: Energy density (E) =CV2/2 (W h kg-1) 33 Power density = E/∆t (W kg-1). 33

4. Results and Discussion: The XRD patterns were used to analyze the crystallinity of the CdS based ternary nanocomposite as shown in figure 1(a). The diffraction pattern of pure CdS was well indexed with the hexagonal structure of CdS. 33In case of CdS-CoFe, it has been found that all the peak corresponding to CdS and as well as CoFe2O4 were observed, which suggested the occurrence of both cubic phase of CoFe2O4 and the hexagonal phase of CdS.34Again, the XRD planes in the nanocomposite CdS-CoFe and CdS-CoFe-G are shifted to lower 2θ value, which suggests some interaction of CdS with CoFe and RGO. However, the diffraction peaks in case of CdS-CoFe were widened because of the small size effect.33 The diffraction peak corresponding to the ternary nanocomposite clearly demonstrated the presence of CdS and CoFe2O4 component which signifies that the crystal structure of the components remains intact after their decoration on the RGO nanosheets.The peak shifting in the ternary nanocomposite is much more pronounced which suggested that the d-spacing increases, thus justifying the fact that the graphitic layers have reduced restacking after the incorporation of CdS-CoFe moeity.



Scheme 1: Synthetic route of the CdS based nanocomposites

FTIR spectra in figure 1 (b) shows a band at around 3200 cm-1 which corresponded to the strong H-bonding.35 The other peaks below 1000 cm-1 at 511, 653 and 827 cm-1 is related to the structure vibration of Fe-O, Co-O and Cd-S bonds respectively.36 The peak at around 1116 cm-1 corresponds to the stretching bands of the sulfide compounds.35 It has been clearly observed that the ternary nanocomposite constitutes all the peaks thus confirming the successful incorporation of CdS and CoFe2O4 within the RGO nanosheets, which has been further confirmed by the Raman and FESEM analysis.

Figure 1: (a) XRD patterns and (b) FTIR spectra of the as synthesized composites


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Raman spectroscopy was carried out to identify the different crystal structures and their decoration on the RGO sheets. Figure 2 (a) depicts the Raman spectra of the nanocomposite CdS-CoFe-G. Two bands at around 1364.9 cm-1 and 1584.6 cm-1 i.e. the D band and G band in figure 2(a) corresponds to the breathing modes of the aromatic rings and the first order scattering of the E2g phonon of the sp2 C atoms respectively, thus suggesting the presence of RGO nanosheets. 36 However, the D and G bands are shifted to somewhat lower wavenumber as compared to pure RGO nanosheets which suggests some interactions between CdS-CoFe moeity with RGO sheets. Further, the magnified view of the dotted area in figure 2(a) is shown in figure 2(b) which corresponds to the different Raman peaks of CdS and CoFe2O4.Two prominent peaks at around 294.8 cm-1 and 592 cm-1 occurs due to the longitudinal optical photon mode (1LO) and overtone (2LO) of CdS nanorods. Compared to pure CdS, there is a slight blue shift in the peaks which is caused due to the small size effects in the composite (CdS-CoFe-G).37 Other peaks designated as T2g(177 cm-1), Eg (327 cm-1), T2g(479 cm-1),T2g(573 cm-1), A1g(641 cm-1) and A1g(705 cm-1) corresponds to the CoFe2O4 spinel crystal which confirmed the presence of cubic spinel ferrite in the nanocomposite.38 Thus, the Raman spectra well documented the incorporation of CdS-CoFe heterostructures into the RGO nanosheets in the nanocomposite CdS-CoFe-G.

Figure 2: (a) Raman Spectra of CdS-CoFe-G and (b) Magnified view of spectra as in (a) The FESEM images vividly explain the surface morphology of the as synthesized materials. In figure 3(a) the CdS nanorods are observed to be stacked over one another but in case of figure 3(b,c) it has been observed that the CdS nanorods gets somewhat separated from each other because of incorporation and uniform distribution of CoFe2O4 nanospheres over the CdS nanorods and is further confirmed by the elemental mapping study which revealed that Cd is present in the inner core which has been covered or surrounded by Co and Fe from CoFe2O4.In figure 3 (c), i.e. the magnified view of the image 3(b) clearly showed the deposition of nanospheres (as marked with red arrow) over the CdS nanorods. Again on fabricating the CdS-



CoFe moieties with RGO it has been observed that there occurs decoration of the CdS-CoFe moieties over the RGO sheets as shown in figure 3 (d). In figure 3 (d), the RGO sheet (marked by yellow arrow) has been decorated by the CdS-CoFe moieties (shown by red rectangular CdS nanorods which has CoFe2O4 nanosphere over them as shown by yellow arrow in figure 3(c)).Assimilation of RGO nanosheets along with CdS and CoFe2O4 decreases the stacking of the CdS nanorods and also increases the surface area for the ion interactions as required for the SCs electrode material which is further accompanied by the BET surface area studies.

Figure 3: FESEM images of (a) CdS (b) CdS-CoFe (c) Magnified view of CdS-CoFe and (d) CdS-CoFe-G nanocomposites The FESEM study of the composites successfully described the surface morphology which was further supplemented by the HRTEM images and the elemental mapping. Figure 4 (a) depicts the dark field image of CdS-CoFe and the corresponding elemental mapping of this particular area describes the presence of Fe, Co and Cd as shown in figure 4 (b), (c) and (d) respectively. The elemental mapping justifies that in case of CdS-CoFe, Fe and Co is present on


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the outer surface as observed by the prominent dark colored Fe and Co whereas Cd is present in the inner core. Similarly the HRTEM image of CdS-CoFe-G, also shows the presence of RGO sheet (yellow encircled in figure 4 (f)) which covers the CdS-CoFe decorated nanostructures. The elemental mapping of the area in figure 4(e) also confirmed the presence of RGO sheet as observed from the C mapping in figure 4 (g). The RGO nanosheets are fabricated by the CdSCoFe moieties which are depicted by their distribution through the elemental mapping. Thus, the TEM study revealed the presence of various metals and their distribution in the nanostructured RGO sheets.

Figure 4: (a) Dark field image of CdS-CoFe and area under elemental mapping showing the mapping of Fe(b),Co(c),Cd(d); (e)Dark field image of the nanocomposite CdS-CoFe-G (f) Bright field image of the nanocomposite CdS-CoFe-G and area under elemental mapping showing the mapping of C(g),Fe(h),Co(i), Cd(j)and (k)EDX spectra showing the presence of different metals. XPS analysis was used to study the composition and the electronic structure of the materials which provides information about the chemical environment of the elements in the



composite as depicted in the survey plot as in figure 5(a). The high resolution Cd 3d spectra in figure 5(b) contains two peaks centered at 404.9 eV and 411.7eV which indicated the typical divalent source of Cd from CdS. 39 The XPS spectra of Co2 p and Fe2p as in figure 5 (c) and 5(d) respectively confirmed the incorporation of the Cobalt ferrite particles in the ternary nanocomposite, CdS-CoFe-G. The Fe2p spectra exhibited two peaks at 711.29 eV and 724 eV along with the shakeup satellite peaks at 718.9 eV and 733 eV. These peaks are identified as the important surface peaks of α-Fe2O3 with the presence of Fe3+ inside Co(II)Fe(III)3O4. The Co2p spectrum exhibited two main peaks at around 780.6 eV and 796.7 eV and two satellite peaks at around 803 eV and 787.6 eV which proved the Co2+ valence states.40 The XPS spectra of S2p shown in figure 5(e) contains two peaks corresponding to 2P3/2 and 2P1/2 with a slight shift in peak from 165eV to 161.5 eV i.e. to lower binding energy when compared with the standard reported values of CdS.37 Similar results has also been found in case of the binary composite CdS-CoFe as shown in supporting information, figure S3(b). Both the nanocomposites, CdSCoFe and CdS-CoFe-G shows a very prominent peak at about 168.2 eV which can be attributed to the presence of oxidized S atoms and the oxygen atoms chemically bonded to the sulfur atoms displays a peak at 533.2 eV in O1s spectra as shown in figure 5(g).41 Such oxidation of the S atoms, promoted the formation of more ionic chemical bond with Cd atoms as observed from the Cd3d spectra. Figure S3 (a) in the supporting information clearly depicts the difference between the Cd3d spectra of both the composites. It has been observed the Cd3d spectra corresponding to the nanocomposite CdS-CoFe-G has much narrower peaks as compared to CdS-CoFe with a chemical shift to higher binding energy, suggesting higher oxidation degree of the Cd atoms.41 Furthermore, the energy level structure of the CdS and RGO reports that, conduction band (CB) and the valence band (VB) position of CdS are about -4 eV and -6.5 eV respectively. The work function of RGO has been calculated to be -4.42eV, i.e., below the CB of CdS and thus the electrons will be transferred from CdS to RGO when they are in contact, resulting in shift in XPS peak in Cd3d spectra.42 The C1s peak in figure 5 (f) shows four peaks corresponding to the CC/C=C (sp2 bonded,284.7 eV), C-OH (hydroxyls, 286.7 eV), C-O (carbonyls,287.9 eV) and OC=O (carboxyl,288.6 eV) respectively.37 The O1s peak in figure 5 (g) can be resolved into 4 peaks at 529.5, 530.8, 531.5 and 533.2 eV. The peak at 530.8eV is associated with the metal oxygen bond i.e. O-Co in CoFe2O4 interfacial bonding structure. 43The other peaks at 531.5 and 533.2 eV corresponds to the hydroxyl groups adsorbed on the surface (metal-OH) and the oxygen in surface-adsorbed carbonate anions or the oxygen atoms chemically bonded to the S atoms respectively.41,43Again the peak at around 529 eV, can be ascribed to the metal-oxygencarbon bond, suggesting some covalent bonding with RGO sheets.44


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Figure 5: (a) XPS survey plot; XPS spectra of (b) Cd3d, (c) Co2p, (d) Fe2p, (e) S2p, (f) C1s, (g) O1s Figure 6 (a) shows the liquid nitrogen adsorption-desorption curves and figure 6(b) shows their corresponding pore size distribution. All the plots are of type II according to the IUPAC nomenclature and such samples have either no or very little micropores and are mainly composed of mesoporous or nonporous particles.45 The surface morphology study (FESEM) was



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further corroborated by the BET surface area analysis which confirmed the increased surface area of about 230 m2/g in case of the nanocomposite CdS-CoFe-G. Such efficient conformation can provide large surface area and desired electrical connection for fast redox kinetics, which potentially increases the efficiency of energy storage.46The result, is in good agreement with the pore-size distribution calculated through non-local density functional theory (NLDFT) method which showed that maximum number of pores lay in the mesoporous region. The higher surface area in case of CdS-CoFe-G is due to the decreased interlayer stacking of RGO nanosheets on introducing CdS-CoFe moieties, offering better charge transfer phenomenon thus resulting in good electrochemical property. The BET surface area results are summarized in table 1. Table 1: Surface area and the pore size distribution of the nanocomposites Sample

BET surface area (m2g-1)

Pore width(nm)

Pore volume(cc/g)

CdS nanorods

30.05

3.169

0.367

CdS-CoFe

91.13

3.09

0.275

CdS-CoFe-G

230

2.89

0.082

Figure 6: (a) BET surface area and (b) Pore size distribution of the nanomaterials In order to investigate the reversibility of the nanocomposites, cyclic voltammetry was performed. The cyclic voltammograms (CV) as in figure 7 (a), were performed at a lower scan rate i.e. 5 mV/sec, which allowed longer duration for the ions to access the bulk of the electrode material. A pair of anodic and cathodic peak has been clearly visible and this is different from the quasi-rectangular behavior which indicated the reversible and continuous faradic reactions of CdS and CdS-CoFe.22 Especially in case of CdS-CoFe-G there is a remarkable increase in current density and redox peaks, and as we know the electron-transfer reactions in the redox


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systems are dependent on the surface chemistry, microstructure and the density of the electronic states near the fermi potential, the above result explains that on introduction of RGO nanosheets, there occurred enhanced electron transfer between the samples and electrode. 47 Figure S2 (b) also confirms such redox reactions taking place on the electrode surface. It is clearly visible that there occur various redox reactions that have been presented by their corresponding redox peaks which indicate that the electrode material is differing from only double layer capacitance which has rectangular shaped CV cures. Such redox peaks occurs due to the surface faradic reactions. One pair of redox peak at around 1 V is observed due to the presence of Co ions.48 The peak around 1.4-2V can be ascribed to the oxidation of Fe to Fe3+ and Co to Co2+.49Again the cathodic and the anodic peak at around -0.55V and -0.66 V respectively occurs due to the Cd2+/Cd couple.50 Thus, it is evident from the above CV plot that the system CdS-CoFe-G offers pseudocapacitive behavior as depicted by the distinct redox peaks and such mechanism has also been confirmed by the Trasatti method of analysis (see supporting information), which showed that the binary composite offered a high pseudocapacitive behavior of 99.22% with a very little EDLC contribution of 0.66%. But on incorporation of RGO, the pseudocapacitive contribution was found to be 96.62%, with an increase in EDLC behavior of 3.37% compared to that of CdSCoFe. Galvanostatic charging-discharging (GCD) was carried out in order to evaluate the SCs performance and was performed at a current density of 5 A/g as shown in figure 7(b). Discharge time is the dominant parameter for the electrochemical performance of the electrode material and a greater discharge time corresponds to the higher specific capacitance of that electrode material. Very high discharge time of the nanocomposite CdS-CoFe-G occurs due to the synergistic effect between RGO and CdS-CoFe moeity along with various surface redox reactions as confirmed from the CV analysis and has a good capacitive behavior with a specific capacitance of 1487 F/g as shown in the bar plot figure 7(d). Such high capacitive behavior exhibited by the nanocomposite, CdS-CoFe-G occurs owing to the interfacial covalent metal-O-C interaction as proved from XPS. 44 Further, the restacking property of the RGO nanosheets has also been reduced on fabrication of CdS-CoFe moieties, which enhanced the effective surface area (as observed in BET surface area plots) required for better ion exchange. Table 2 summarizes the specific capacitance values of the corresponding nanocomposites in three electrode configuration. GCD of the nanocomposite CdS-CoFe-G, was also carried out at different current densities i.e. at 5,10,15,20 A/g in three electrode system, which showed a gradual decrease in discharge time as in figure 7 (c) because of increased voltage drop and insufficient active electrode material involved in redox reactions at higher current densities. Table 3 summarizes the specific capacitance at different current densities and the rate capability of the CdS-CoFe-G nanocomposite was 49.5%.The nanocomposite, CdS-CoFe-G has also been subjected for CV analysis in a two electrode configuration and the relevant plots are provided in the supporting information (figure S4(,b) and table S4).



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Table 2: The specific capacitance of various nanocomposites Sample

Specific Capacitance (F/g)

Energy Density (Wh/Kg)

Power Density (W/Kg)

CdS

771.875

274.44

3000

CdSCoFe

990.625

352.22

3500

CdSCoFe-G

1487

528.8

4000

Table 3: Specific capacitance of CdS-CoFe-G at different current densities Current Density (A/g) 5 10 15 20

Specific Capacitance(F/g) 1487 1043.75 843.75 750


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Figure 7: (a) Cyclic volatammograms at 5mV/sec and (b) Galvanostatic chargingdischarging plots of the nanocomposites; (c) Galvanostatic Charging-discharging plots at different current densities for the nanocomposite CdS-CoFe-G; (d) Bar plots depicting the specific capacitance, energy density and power density of all the synthesized composites The different charge storage mechanisms of CdS and its synthesized composites are evident from the EIS (Electrochemical impedance spectroscopy) study as in figure 8. The radius of the arc on the EIS Nyquist plot, reflects the reaction occurring at the electrode surface.The plot constitutes two distinct part with a semicircular arc at the higher frequency region.The semicircular arc in the higher frequency region corresponds to the electron transfer resistance which controls the kinetics at the electrode interface.23,51 The radii of the arcs decreases significantly when pure CdS is coupled with CoFe2O4 and RGO nanosheets. The smallest radius is obtained in case of the nanocomposite CdS-CoFe-G which justifies the fact that effective separation of electron-hole pair occurs in case of CdS-CoFe and again an enhanced charge transfer occurs on incorporation of RGO nanosheets i.e. in CdS-CoFe-G. Again, the high frequency region shows a straight line which is associated with adsorption process.46 The nanocomposite CdS-CoFe-G has a very low electrolyte resistance (Rs) and charge transfer resistance (Rct) (see supporting information) which proposes more efficient charge transfer in the hybrid material thus, suggesting its good capacitive behavior.



Figure 8: Nyquist plot showing the impedance study of the composites (inset: magnified view of the plots) Cyclic stability of the nanocomposite CdS-CoFe-G has also been carried out for 5000 consecutive cycles as shown in figure 9 with 78% capacitance retention of the initial value, thus demonstrating the good reversibility and cyclic stability of the electrode material. Further, a blue LED was successfully lightened by an asymmetric device using CdS-CoFe-G as positive electrode and carbon black as negative electrode (Figure S9, supporting video 1). Additionally a mini fan was also successfully ran by the device (supporting video 2).

Figure 9: Cyclic stability of the nanocomposites upto 5000 cycles


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5. Conclusion: In summary, a cost effective CdS nanorods based nanocomposites were synthesized via facile and inexpensive chemical process. The composite CdS-CoFe-G exhibited a high specific capacitance of 1487 F/g with retention of 78% upto 5000 cycles, thus highlighting its excellent cyclic stability. Furthermore, the electrode material can be cycled reversibly within a wide potential window of 0-1.6V having a high energy density of 528.8 Wh/Kg and power density 4000 W/kg. The CdS-CoFe moieties reduce the restacking of the RGO nanosheets resulting in a hierarchical architecture. Again, CdS-CoFe can offer more active sites for the electrochemical reactions, shortens the electrons or ions transport pathways which can lead to high specific capacitance and rate capability. The RGO nanosheets offer high surface area leading to high electrical conductivity and improved mechanical and cyclic stability of the composite. Therefore the ultrahigh specific capacitance occurs due to the effective utilization of active material CdS-CoFe on the conductive RGO sheets. Thus, all the components assemble into a mesoporous structure with good synergism and can be a promising electrode for SCs application.

Associated Content: Supporting information Fitted Nyquist plot, Equivalent circuit, and fitted value of Equivalent circuit element obtained by the simulation of impedance spectra; CV of the nanocomposite CdS-CoFe-G in 3 electrode system at different scan rates (0 – 1.6 V); CV of CdS-CoFe-G within wide potential window of -1 to 2V; XPS of Cd 3d in case of CdS-CoFe(bottom panel) and CdS-CoFe-G (Upper panel) and S2P of CdS-CoFe; CV and GCD of CdS-CoFe-G in two electrode system at various scan rates and different current densities; Specific Capacitance table of CdS-CoFeG at different current densities; Trasatti method; Trasatti plots of CdS-CoFe and CdS-CoFeG; Percentage of capacitance contribution evaluated for the electrodes based on trasatti analysis; HRTEM image of CdS-CoFe-G; TGA plot of the nanocomposites; Cell assembly of the ternary nanocomposite (CdS-CoFe-G) (PDF) Supporting video 1(.avi) Supporting Video 2(.avi)

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Table of content Synopsis: CdS-CoFe2O4@reduced graphene oxide Nano hybrid for the supercapacitor application




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CdS-CoFe2O4@reduced graphene oxide Nano hybrid: An excellent

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