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Graphene based ceria nanocomposite synthesized by hydrothermal method for enhanced supercapacitor performance
Journal Pre-proof Graphene based ceria nanocomposite synthesized by hydrothermal method for enhanced supercapacitor performance
Sheeba Britto, Velavan Ramasamy, Priya Bernaurdshaw Neppolian, Thangavel Kavinkumar
Murugesan,
PII:
S0925-9635(19)30988-4
DOI:
https://doi.org/10.1016/j.diamond.2020.107808
Reference:
DIAMAT 107808
To appear in:
Diamond & Related Materials
Received date:
23 December 2019
Revised date:
11 February 2020
Accepted date:
16 March 2020
Please cite this article as: S. Britto, V. Ramasamy, P. Murugesan, et al., Graphene based ceria nanocomposite synthesized by hydrothermal method for enhanced supercapacitor performance, Diamond & Related Materials (2018), https://doi.org/10.1016/ j.diamond.2020.107808
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© 2018 Published by Elsevier.
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Graphene Based Ceria Nanocomposite Synthesized by Hydrothermal Method for Enhanced Supercapacitor Performance
Sheeba Britto1,2, Velavan Ramasamy2, Priya Murugesan1,3,, Bernaurdshaw Neppolian4 and Thangavel Kavinkumar4 R&D centre, Bharathiyar University, Coimbatore, India
2
Department of Physics, Bharath Institute of Higher Education and Research, Chennai, India
3
Department of Physics, Saveetha college of Engineering, Chennai, India
4
Research Department, Chemical Sciences, SRM Institute of Science and Technology,
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Kattankulathur, Chennai, India
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*Corresponding author: Tel: 91-7871770315
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Email: [email protected] (Priya Murugesan)
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Abstract: Reduced graphene oxide – cerium oxide (rGO-CeO2) nanocomposite is prepared by simple hydrothermal method for enhanced charge storage supercapacitor performance. The prepared nanocomposite is characterized by various techniques, such as X-ray diffraction (XRD), Fourier Transform – Raman Spectra (FT-Raman), Field Emission - Scanning Electron Microscope (FE-SEM), High Resolution –Transmission Electron Microscope (HR-TEM), Thermo-gravimetric and Differential Thermal Analysis (TG-DTA), Cyclic Voltametric (CV),
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Galvanostatic Charge/Discharge (GCD) and Electrochemical Impedance Spectroscopy (EIS) studies to understand its morphology, composition, thermal stability and charge storage
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efficiency as electrode material. The nanocomposite formation is confirmed with FE-SEM and HR-TEM images where the ceria is anchored on the surface of the graphene sheets and
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the interplanar distances are observed as fringes. The Energy Dispersive Analysis of X-Ray
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(EDAX) has provided substantial evidence for the nanocomposite formation with the elemental composition. The maximum specific capacitance is measured as 280 F/g using
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Teller (BET) analysis.
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GCD studies. The surface area of the composite is determined using the Brunauer-Emmett-
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Keywords:Graphene oxide, reduced graphene oxide, cerium oxide, electrochemical studies, cyclic voltametry, galvanostatic charge/discharge, supercapacitors
Journal Pre-proof Introduction: Considering the perspective of overcoming environmental and energy crisis, the renewed interest in finding sustainable energy sources and innovating energy storage system are of vital importance, especially in the field of mobile electronics, electrical vehicles, power sources, portable electronics and other devices [1,2]. Many alternate energy sources are explored to overcome the energy crisis. Electrochemical supercapacitor is one among the alternate energy storage / conversion device which possesses higher power density and longer life cycles than batteries. Similarly, supercapacitors hold high energy density than conventional dielectric capacitors. Hence, supercapacitor is recognized as promising and
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favourable device for charge storage [3].
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Supercapacitors are separated in to two major classifications, i) Electrical Double
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Layer Capacitors (EDLC) and ii) Pseudocapacitors. EDLC utilizes carbon-based materials as electrodes for charge storage with high specific power, whereas the pseudocapacitors make
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advantage of using transition metal oxides or polymers like polyaniline as electrodes for charge storage with high specific energy. Hybrid supercapacitor is a combination of both
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EDLC and pseudocapacitor to produce high power density and energy density [4]. Amid the carbon-based materials, graphene is identified as the cheaper and efficient candidate of
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choice for supercapacitor electrodes.
Graphene is a single layered graphitic planar structure, formed by Sp2 hybridised
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carbon atoms which possess high thermal and electrical surface area, high electrical conductivity, good electrochemical property, high mechanical strength and very high charge transfer properties. Graphene oxide (GO) is a derivative of graphene with functional groups such as epoxy, hydroxyl and carbonyl hydroxyl or carboxylic acid groups attached to it [5, 6]. The degree of oxidation can be evaluated by the presence of carbon / oxygen ratio. The attachment of these functional groups leads to the buckling of sp3 hybridised graphene oxide, resulting the interplanar distance ranging 0.7 – 0.8 nm. GO is reduced to produce reduced graphene oxide (rGO) via chemical, thermal or electro chemical methods[7]. rGO tends to align towards pristine graphene structure which is used for charge storage application. Carbonaceous nanomaterials have been demonstrated as the potential candidate for high performance charge storage supercapacitors. Several transition metal oxides have been widely accepted as a potential pseudocapacitor due to their high surface area and porosity. Ruthenium oxide (RuO2) is the
Journal Pre-proof dominant metal oxide with high specific capacitance, which is investigated in detail under various conditions. Moreover, MnO2[8], NiO[9], Fe3O4[10], CoO2, Co3O4[11], V2O5[12], NiCo2O4[13] and mixed metal oxides have been studied for the electrochemical charge storage. The capacitance retention of these metal oxides deteriorates after several cycles while used as electrodes in concentrated acidic electrolytes. rGO –metal oxide is the nanocomposite material of current research interest with applications in batteries[14], supercapacitors[15], tracing of heavy and toxic elements in aqueous media [16], photocatalytic dye degradation [17], biosensor [18] and electrocatalysts
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[15,19] in oxide fuel cells. The nanocomposite of graphene with metal oxide likeRuO2 (20), SnO2[21], MnO2 [22], Co3O4[23], Nb2O5[24], V2O5[25], Fe2O3[26], MoO2[27], WO3[28] and
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ZnO [29] have proven track record of enhanced specific capacitance, capacitance retention after several cycles, high power density and energy density. Among these nanocomposites,
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the graphene-ruthenium oxide is observed to be an outstanding performer of supercapacitor
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studied so far. The main demerit of this nanomaterial is very expensive leading to high cost supercapacitor fabrication. The present scientific community is in search of cheaper charge
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storage materials, alternate to graphene - ruthenium oxide nanomaterials.The ceria is claimed to be a cheaper material and found abundant in nature with highly active redox property.
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Hence, graphene-based ceria nanocomposite is focussed for the present study. This article deals with the preparation of rGO-CeO2 using hydrothermal method and the charge storage
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capacity is quantified with CV and GCD studies. 2.Experimental Section 2.1 Materials
Graphite, sodium nitrate, potassium permanganate, hydrogen peroxide, cerium chloride, and hydrazine hydrate of analytical grade purchased from Thermo Fischer Scientific are used for the preparation of nanomaterials. 2.2.Synthesis of GO and CeO2 The graphene oxide is prepared as reported in our earlier work [30] using modified Hummer’s method. For pure ceria preparation,0.1M cerium chloride and 0.3M sodium hydroxide solution are chosen as the primary concentration of the starting materials. Initially, sodium hydroxide solution is gradually added drop wise to the cerium chloride solution under constant magnetic
Journal Pre-proof stirring. As a result, the solution observed as yellowish white later solution changes to pale yellow colloidal suspension. Subsequently, the suspension is transferred to a 100 ml Teflon – lined stainless steel autoclave and sealed tightly. Further, autoclave is placed in a muffle furnace and heated at 120 °C for 15 hours. On cooling to room temperature, the yellow precipitate is collected and washed three times using deionised water and ethanol. The gathered powder is dried at 100 °C for 12 hours[31] and grinded to a fine powder using mortar. 2.4. Synthesis of rGO- CeO2
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The rGO - CeO2 nanocomposite is synthesised by hydrothermal method. 50 mg of GO is dissolved in 20 ml of deionised water and sonicated for 1 hour. Then, 0.1 M of cerium
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chloride is added to the dispersed GO solution under constant stirring. 0.3 M sodium hydroxide solution and 2 ml of hydrazine hydrate are added to the above mixture. After 30
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min of stirring, the mixture is transferred to the Teflon – lined stainless steel auto-clave and
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maintained at 160 °C for 24 hours. After cooling to room temperature, the black precipitate collected by centrifugation is washed with deionised water / ethanol for several times and
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2.5. Characterization
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dried at 60°C for 24 hours in a muffle furnace.
To shed more light on rGO-CeO2 structure and their efficacy of charge storage the
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prepared sample is subjected to various characterizations. The structural studies are performed by BRUKERD8 ADVANCE POWDER X- ray diffractometer using Cu-Kα radiation (λ = 1.540 Å). Fourier Transform - Raman (FT-Raman) analyses of the samples are accomplished by BRUKER RFS 27:Stand-alone FT-Raman spectrometer using KBr pellet method in the range 4000 – 50 cm-1. The wavelength of the Raman laser beam is 1064 nm. The morphology of the CeO2 and CeO2-rGO nanocomposites are examined using High resolution – Scanning Electron Microscope (HR-SEM) and Field Emission -Scanning Electron Microscope (FE-SEM), respectively, using FEI Quanta FEG 200 microscope at voltage of 20 kV. The High Resolution – Transmission Electron Microscope (HR-TEM) images of the rGO-CeO2 composite is recorded using BrukerXFlash 6T130 EDX detector. In prior to HR-TEM analysis, the sample is dispersed in isopropyl alcohol and few drops are coated on copper grids and allowed to dry in air atmosphere. The Selected Area Electron Diffraction (SAED) is recorded to validate the sample as crystalline or amorphous in nature. The elemental composition of the nanocomposites is analysed with the help of EDAX
Journal Pre-proof spectroscopy using EDX detector. The thermal stability of the sample is explored using Thermo Gravimetric – Differential Thermal Analyser (TG-DTA), NETZSCH-STA2500 STA2500A-0061-N. The Brunauer-Emmett-Teller (BET) analyser is employed to obtain the surface area and pore size of the prepared sample using Quantachrome Instruments – Autosorb Series. Electrochemical
studies
such
as
Cyclic
Voltametric
(CV),
Galvanostatic
Charge/Discharge (GCD) and Electrochemical Impedance Spectroscopy (EIS) of single electrode have been conducted using Bio-Logic SP-150 electrochemical workstation in 2 M
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KOH aqueous solution using a three-electrode configuration. In a three-electrode system, the working electrode is fabricated by prepared material dispersed in dimethylformamide (DMF)
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using ultrasonication in order to form homogeneous viscous paste. A known weight (1.5 mg) of the paste is cast onto a Ni foam sheet (1 cm ×1 cm) and dried at 80 C overnight. Here, the
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KCl saturated Ag/AgCl electrode and a platinum wire are used as reference and counter
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electrodes, respectively, whereas the prepared material is used as the working electrode. CV is performed for different sweep rates (3, 5, 10, 20, 50, 100, 150 and 200 mV/s) within a
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potential range of 0 to 0.5 V for pure CeO2, and potential range of 0 to 0.6 V for rGO-CeO2 composite. GCD measurements are tested to study the charge/discharge time of the electrodes
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for various current densities (1, 2, 3, 4, 5 and 10 A/g) in an operating potential of 0 to 0.45 V. Electrochemical Impedance spectroscopy (EIS) technique is executed in the frequency range
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of 0.01 Hz – 100 kHz. 3. Results and Discussions
3.1.X-ray diffraction analysis
The X-ray diffraction patterns of GO, rGO, pure CeO2 and rGO / CeO2 nanocomposite are stacked one over the other as shown in Fig. 1a.
The characteristic
intensity peak observed at 10.83° confirms the formation of GO and the intensity peak is emanating from the (001) plane with d spacing of 0.82nm. On reduction of GO (rGO), the diffracted intensity peaks are observed at 24.68 ° and 43 °, which corresponds to (002) plane and (100) plane, respectively. The inter-planar distance is observed to be 0.361 nm for rGO, resembling the graphite like structure. Regarding CeO2, seven distinctive diffraction peaks are noticed, and these peaks are indexed as (111), (200), (220), (311), (222), (400), (331) planes, with reference to JCPDS file No 65-5923. Thus, the diffraction image of CeO2 substantiates the formation of face centred cubic structure. On comparing the simulated and
Journal Pre-proof experimental XRD patterns of CeO2 (Fig. 1b), most of the intensity peaks agree well with each other with minor ambiguity. It is obvious that no significant diffraction peak of GO is found in the rGO-CeO2 nanocomposite, which accounts for the existence of productive exfoliation of stacked graphene layers [32]. The average crystallite size of the nanocomposite
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Fig. 1 (a)- Stacked X-ray diffraction patterns of GO, rGO, CeO2 and rGO-CeO2. (b) Simulated and experimental XRD patterns of pure ceria overlapped for comparison
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3.2.FT-Raman Spectroscopic Studies
Figure 2 shows the FT-Raman spectra of rGO, pure CeO2, and rGO–CeO2 nanocomposite. For pure CeO2, highly intense peak is recorded at 460 cm-1which attributes to the triply degenerate F2g mode of the cubic fluorite crystal structure. In addition, a low intensity second order Raman peak is also observed at 1063 cm-1, which represents the 2ωR(X) phonon mode [33]. The Raman spectrum of rGO clearly shows the characteristic D band and G band at 1290 cm-1 and 1593 cm-1, respectively, whereas in rGO-CeO2 nanocomposite, the same bands are observed at 1288 cm-1 and 1594 cm-1, respectively. Further, the band at 456 cm-1 is registered for the nanocomposite spectrum, which corresponds to ceria peak. Thus, the D band, G band and ceria Raman peak confirms the formation of nanocomposite. The defects in the graphitic structure can be identified by the variation in the intensity contribution of D band and G band. In specific, D band reveals the structural defects related to geometrical parameters such as bond lengths, bond angles and
Journal Pre-proof hybridisation. The ID/IG ratio is used as an indicator to validate the structural purity of the graphitic structure. In the present study, the ID/IG ratio is observed to be same (0.81) for both rGO and rGO-CeO2 nanocomposite, which illustrates the attachment of CeO2 on rGO doesn’t
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affect the crystalline sp2 region as reported in the earlier studies [34, 35].
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Fig. 2 - FT-Raman Spectra of rGO, rGO – CeO2, CeO2
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3.3. SEM TEM and EDX
The HR-SEM image of pure CeO2 is shown in Fig. 3a. The HR-SEM micrograph of
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pure CeO2 determines the surface morphology as spherical in shape which concurs with the reported literature [36]. For better illustration of graphene sheet and bound ceria in hybrid nanomaterial, the obtained Field Emission Scanning Electron microscope (FE-SEM) micrographs are shown in Fig. 3b and 3c. Both images support the fine attachments of ceria on top of the folded graphene sheet which are clearly visible.
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Fig.3 - Scanning Electron Microscope images (a) HR-SEM image of pure CeO2 (b) & (c) FE-SEM images of rGO-CeO2.
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The HR-TEM images of rGO-CeO2 are shown in Figures 4a, 4b,4c and 4d. The images certify the formation of nanocomposite, where CeO2 is decorated on the graphene sheets (Fig.
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4c) and the embedded ceria is spherical in shape with slight disorder. Moreover, the
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morphology of nanoparticle varies based on precursor concentration, temperature and several other factors[31]. Here, the nanocomposite heated at 160 C for 24 hours, results in variation
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of the diameter of the spherical ceria particle between 7.99 nm and 15.6 nm and are anchored on the well-defined graphene sheets. Fig. 4d demonstrates the lattice fringes observed for
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ceria nano particle and graphene nanosheet with the interplanar distances 0.283 nm and 0.356 nm, respectively. The d spacing of 0.283 nm corresponds to 200 plane of ceria nanoparticle
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and 0.356 nm corresponds to the 002 plane of graphene nanosheet.
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The Selected Area Electron Diffraction pattern (Fig.4e) exhibits the polycrystalline nature of the nanocomposite with bigger grain size and finer grain size. The diffraction pattern agrees well with the indexed XRD pattern of composite. The compositional analysis of the composite is revealed using EDX spectrum (Fig.4f). Here, the major elements of the sample are carbon with wt % 72.81 and cerium with wt % 17.10 along with the minor composition of oxygen with wt % 2.53. The low content of oxygen may be due to the incomplete reduction of GO. Furthermore, it is evident that the detected copper signals originated from the copper grid with wt % 7.56.
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(c)
(d)
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cps/eV
C
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(f)
Element CK OK CeL CuK wt % 72.81 2.53 17.10 7.56
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10
8
6
4
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Element CK OK CeL CuK
Ce
Ce Cu
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wt % 72.81 2.53 17.10 7.56 at % 93.82 2.45 1.89 1.84
at % 93.82 2.45 1.89 1.84
Cu
0 0
1
2
3
4
keV
5
6
7
8
9
(g)
Fig.4(a) (b) & (c) HR-TEM images of rGO-CeO2 nanocomposite (d) HR-TEM image of the composite with the highlighted ‘d’ spacing of ceria nanoparticle and graphene sheet (e) SAED pattern for the nanocomposite (f) EDX spectrum of rGO-CeO2 nanocomposite with the normalized and atomic weight % of the elements in the inset
Journal Pre-proof 3.4. TG-DTA Studies The TG-DTA curves for CeO2 and rGO–CeO2 are shown in Fig.5a & b. The weight loss occurs in two distinct stages (Fig.5a) for pure form of CeO2. The first stage falls in the temperature range of 50 C – 200 C where maximal weight loss (7.25 %) takes place and this may be due to the vaporization of moisture and adsorbed solvent molecules. Beyond 200 C, a minimal weight loss occurs due to the removal of combustible organic residuals. Above 600 C, a slight increase in weight % is noticed, which may be connected to pure phase formation of CeO2 [37]. Finally, CeO2 suffers 13% weight loss at 799 C. Upon
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binding of ceria on graphene nanosheet, the thermal stability is enhanced for the composite (Fig. 5a) than the ceria alone. In the present study, the prepared composite (hydrothermal
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method) is thermally more stable than the earlier prepared rGO-CeO2 xerogel composite [38].
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The total mass loss of the composite is insignificant with 6.36 % at 801 C. The DTA thermogram of CeO2 explicitly shows the endothermic peak observed at
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102.2 C, followed by broad exothermic peak stretched across temperature 200 – 500 C (Fig 5b). The endothermic peak represents the elimination of surface adsorbed water molecules at
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102.2 C, whereas the extended exothermic peak may be due to the decomposition of theorganic components (weight loss ~ 3 %) in CeO2[39]. The DTA thermogram of the
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nanocomposite.
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composite reflects the TGA curve and gives supportive evidence for the stability of the
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Fig. 5(a) TGA and DTA curve for pure CeO2 (b) nanocomposite rGO-CeO2.
Journal Pre-proof 3.5. BET Studies A few experimental techniques are available in determination of surface area of porous materials but BET study is the widely accepted efficient method to estimate the surface area[40]. The N2 adsorption-desorption curve for pure CeO2 and rGO-CeO2 composite are delineated in Fig. 6a using multi-point BET surface area analyser. The adsorption-desorption curve increase gradually at the low relative pressure ranging between 0.0- 0.75 and sharp increase is observed above 0.75 relative pressure. Both the curves take perfect hysteresis loop, belonging to the IUPAC type IV category [41]. The surface area of the nanomaterials CeO2 and rGO-CeO2 are 85.5 m2/g and 87.1 m2/g, respectively. The
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Barret–Joyner—Halenda (BJH) pore size distribution plots (Fig. 6b) enunciate the pore size
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distribution of the CeO2 and rGO-CeO2, in which the major distribution of pore diameter is 3.4 nm for the both the nanoparticles. According to the IUPAC nomenclature, the obtained
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pore size testifies the formation of mesoporous structure [2 – 50 nm], and the result is in close
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proximity to the earlier reported literature [42].
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Fig.6 - (a) Nitrogen adsorption-desorption isotherm curve and (b) pore size distribution plotof pure CeO2 and rGO-CeO2
Journal Pre-proof 3.6. Electrochemical Studies For the better understanding of the electrochemical properties of ceria and rGO-ceria, a detailed investigation on the CV and GCD studies have been carried out for pure ceria and the composite rGO-ceria nanomaterials. Fig. 7a shows the CV curves for rGO-CeO2 working electrode at different scan rates 3, 5,10, 20, 50, 100, 150 and 200 mV/s and their potential window ranging from 0 to 0.6V. Here, the redox peaks are prominent and takes nonrectangular shape for all CV curves of different scan rates which represents the pseudocapacitance behaviour of the nanocomposite. The observation of pseudocapacitance
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feature for rGO-CeO2 matches well with the earlier electrochemical studies using KOH as the electrolyte [15, 43, 44]. In contrast, Dezfuli et al., showed that the rGO-CeO2 redox peaks
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are in rectangular shape for different scan rates using Na2SO4 as electrolyte [35]. Obviously, Fig. 7a demonstrates the voltammetric current of each CV curve, which increases on
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increasing the scan rate. As the intensity peak current is directly proportional to the square
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root of scan rate, the waxing of voltammetric current occurs. Nevertheless, the specific capacitance decreases drastically on increasing the scan rate. The rationale behind waning of
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specific capacitance is as follows: at low scan rate, complete electrolyte ions diffusion takes place into the material with maximum active sites accessible for maximum charge storage. In
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case of high scan rate, the electrolyte ion movement is restricted due to lack of time, resulting less charge storage [45]. Furthermore, the increment of scan rate leads to slight shift in the
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anodic peak towards higher end potential and the cathodic peaks shift towards lower end potential [44,46] and this may be due to the presence of internal resistance of the electrode. In Fig.7b shows the overlapped CV curves for the pure ceria and rGO-ceria composites at the specific scan rate 50 mV/s. The area swept by the composite CV curve is larger than the pure CeO2, resulting high specific capacitance. The major contribution of the pseudocapacitance behaviour of the composite is mainly facilitated by the CeO2 redox reaction. The galvanostatic charging and discharging curves for rGO-CeO2at different current densities are shown in Fig.7c. The specific capacitance (Cs) is obtained from the GCD curve using the formula It Cs = ---------mV
Journal Pre-proof where I, t, m and V corresponds to oxidation or reduction current, time differential, mass of the active electrode material and potential scan window, respectively. The calculated specific capacitance values of ceria are 91 F/g, 85 F/g, 78 F/g, 70 F/g and 65 F/g for current densities 1 A/g, 2 A/g 3 A/g 4 A/g and 5 A/g, respectively. Similarly, the calculated specific capacitance values of composite rGO-ceria are 280 F/g, 234 F/g, 227 F/g, 222 F/g, and 214 F/g for 1 A/g, 2 A/g 3 A/g 4 A/g and 5 A/g, respectively. The maximum specific capacitance obtained for the present study is 280 F/g for rGO-CeO2 composite, which is higher than the reported values, 265 F/g [44] and 208 F/g [15]. Here the calculated specific capacitance values for the rGO-CeO2 are larger than the pure ceria-based electrode, which explicitly
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reveals the transformation of ceria from poor electrical conductivity to better electrical
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conductivity. The electrons percolate at faster rate throughout the electrode, due to good electrical conductivity of graphene nanosheet, leading to higher capacitance attainment [47].
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In the charge-discharge curves (Fig. 7c), the discharge occurs in two phases. The potential drops as a slope in phase I, 0.5 - 0.3 Volts, which represents the pseudocapacitance behaviour
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of the electrode due to the redox reaction at the interface of the electrode / electrolyte. In
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phase II, the potential declines abruptly in the range of 0 – 0.3 Volts, which denotes the electrical double layer capacitance nature of the electrode as reported earlier [15]. The GCD curves of pure ceria and rGO-ceria composite are superimposed (registered at1 A/g) in Fig.
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7d to distinguish the area enclosed and their significant contribution towards specific capacitance of pure ceria and composite. The specific capacitance is inversely proportional to
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both the scan rate and the current density. The progression of scan rate and current density resembles same as a function of specific capacitance where the diffusion of electrolyte ions into the electrode material are restricted, yielding low specific capacitance. The variation of specific capacitance with respect to current density for pure ceria and composite rGO-CeO2 is illustrated precisely in Fig. 7e. The specific capacitance for the composite is higher than the pure ceria at different current densities and this may be due to the morphology of rGO/CeO2 composite compared to pure CeO2. It is evident from the SEM and TEM images, the CeO2 particles are uniformly embedded on the graphene sheets, which attributes to the synergistic effect of combination of rGO and CeO2, resulting in elevated electrolyte ion movement at the interface of the electrode/electrolyte with enhanced specific capacitance. The cyclic stability is an important parameter in electrochemical studies to decipher the long-term supercapacitor performance of the electrode material. In the present study, it is monitored up to 5000 cycles for the composite rGO-CeO2 using GCD and found to be more
Journal Pre-proof stable all along 5000 cycles. After 5000 cycles, a remarkable 98 % capacitance retention is accomplished (Fig. 7f), which is better than the already reported value i.e. 94.1% capacitance retention after 4000 cycles [43], 86% capacitance retention after 1000 cycles [48]. Thus, capacitance retention after 5000 cycles is a good sign of long-term stability of supercapacitor with enhanced performance and more likely to be a potential electrode material for
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Fig. 7(a) & (c) CV curves and GCD curves of the composite rGO-CeO2 plotted at different scan rates and current densities, respectively. (b)&(d) Comparison of CV curves and GCD curves for pure CeO2 and composite rGO-CeO2 at scan rate 50 mV/s and current density 1 A/g, respectively. (e) Obtained specific capacitance values of pure CeO2 and composite rGOCeO2 plotted as a function of current densities. (f) Specific capacitance retention (%) as a
Journal Pre-proof function of cycle number plotted for pure ceria and composite rGO-CeO2 electrode in KOH electrolyte In order to establish the supercapacitance behaviour and the electrical conductivity of the pure ceria and the composite rGO-CeO2 electrode materials, EIS technique is employed in the frequency range of 0.01 Hz – 100 kHz using alternate current potential. Fig. 8 displays the Nyquist plot of pure ceria and rGO – ceria composite, a graphical representation of real part of impedance (Z’) vs imaginary part of impedance (Z”). An apparent semi-circle is observed at high frequency, succeeded by linear line at low frequency for both the samples.
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The Warburg linear line region of low frequency explains the prolonged ion diffusion path lengths and elevated barrier against ion movement, which represents the ideal capacitor
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behaviour. The internal resistance Rs is evaluated from the intercept of high frequency at real axis whereas the charge transfer resistance (Rct) is measured from the diameter of the semi-
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circle [49]. The diameter of the semi-circleis smaller for composite material compared to pure
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ceria, resulting higher charge transfer rate of the electrode material with better performance
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Fig.8 - Nyquist plot for pure ceria and rGO-ceria electrodes in KOH electrolyte
4. Conclusion The nanomaterial of our interest rGO-CeO2 is prepared by hydrothermal method. The experimental investigations like XRD, FT-RAMAN, SEM, TEM and EDAX confirmed the formation of rGO-CeO2 composite. The composite is thermally more stable than pure ceria with limited weight loss of 6.34 % at 801 C. The pseudocapacitance behaviour is dominant for the composite material as the redox peaks of CV curves promote nonrectangular shape in KOH electrolyte. The GCD studies unravel the enhanced supercapacitor performance of
Journal Pre-proof composite with specific capacitance 280 F/g. The significant long-term supercapacitor performance of the composite electrode material is achieved to be 98 % capacitance retention than the original, starting value after 5000 cycles. Eventually, these results endorse the composite as economical and potential electrode material for charge storage application. Conflicts of interest
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Electrodes. Adv Func Mater 2014; 24(22): 3405-12. [4] Muzaffar A, Ahamed MB, Desmukh K, Thirumala J.A review on recent advances in
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hybrid supercapacitors: Design, fabrication and applications. Renew Sust Energy Rev
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2019; 101 (C): 123-45.
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[31] Kavitha P, Ramesh R, Rajan MR, Stella C. Synthesis and Characterization of Cerium Oxide Nanoparticles by Using Rapid Precipitation Method. Ind J Res 2015; 4(12): 91– 3. [32] Raccichini R, Varzi A, Passerini S, Scrosati B. The role of graphene for electrochemical energy. Storage. Nat Mater 2014; 14(3): 271-79. [33] Kourouklis GA, Jayaraman A, Espinosa GF. High-pressure Raman study of CeO2 to 35 Gpa and pressure-induced phase transformation from the fluorite structure. PhysRev B, 1988; 37(3): 4250-3. [34] Joung D, Singh V, Park S, Schulte A, Seal S, Khondaker SI. Anchoring ceria nanoparticles on reduced graphene oxide and their electronic transport properties. J. Phys. Chem. C 2011; 115: 24494–500. [35] Dezfuli AS, Ganjali MR, Naderi HR, Norouzi P. A HighPerformance Supercapacitor Based on Ceria/Graphene Nanocomposite Synthesized by a Facile Sonochemical Method. RSC Adv 2015; 5 (57): 46050- 58.
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Journal Pre-proof Highlights The experimental investigations of the prepared rGO-CeO2 like XRD, FT-RAMAN, SEM, TEM and EDAX confirmed the formation of rGO-CeO2 composite. The composite is thermally more stable than pure ceria with limited weight loss of 6.34 % at 801 C. The pseudocapacitance behaviour is dominant for the composite material as the redox peaks of CV curves promote nonrectangular shape in KOH electrolyte. The GCD studies proves the enhanced supercapacitor performance of composite electrode
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material achieved to be 98 % than the original value after 5000 cycles
Sheeba Britto, Velavan Ramasamy, Priya Bernaurdshaw Neppolian, Thangavel Kavinkumar
Murugesan,
PII:
S0925-9635(19)30988-4
DOI:
https://doi.org/10.1016/j.diamond.2020.107808
Reference:
DIAMAT 107808
To appear in:
Diamond & Related Materials
Received date:
23 December 2019
Revised date:
11 February 2020
Accepted date:
16 March 2020
Please cite this article as: S. Britto, V. Ramasamy, P. Murugesan, et al., Graphene based ceria nanocomposite synthesized by hydrothermal method for enhanced supercapacitor performance, Diamond & Related Materials (2018), https://doi.org/10.1016/ j.diamond.2020.107808
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© 2018 Published by Elsevier.
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Graphene Based Ceria Nanocomposite Synthesized by Hydrothermal Method for Enhanced Supercapacitor Performance
Sheeba Britto1,2, Velavan Ramasamy2, Priya Murugesan1,3,, Bernaurdshaw Neppolian4 and Thangavel Kavinkumar4 R&D centre, Bharathiyar University, Coimbatore, India
2
Department of Physics, Bharath Institute of Higher Education and Research, Chennai, India
3
Department of Physics, Saveetha college of Engineering, Chennai, India
4
Research Department, Chemical Sciences, SRM Institute of Science and Technology,
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Kattankulathur, Chennai, India
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*Corresponding author: Tel: 91-7871770315
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Email: [email protected] (Priya Murugesan)
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Abstract: Reduced graphene oxide – cerium oxide (rGO-CeO2) nanocomposite is prepared by simple hydrothermal method for enhanced charge storage supercapacitor performance. The prepared nanocomposite is characterized by various techniques, such as X-ray diffraction (XRD), Fourier Transform – Raman Spectra (FT-Raman), Field Emission - Scanning Electron Microscope (FE-SEM), High Resolution –Transmission Electron Microscope (HR-TEM), Thermo-gravimetric and Differential Thermal Analysis (TG-DTA), Cyclic Voltametric (CV),
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Galvanostatic Charge/Discharge (GCD) and Electrochemical Impedance Spectroscopy (EIS) studies to understand its morphology, composition, thermal stability and charge storage
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efficiency as electrode material. The nanocomposite formation is confirmed with FE-SEM and HR-TEM images where the ceria is anchored on the surface of the graphene sheets and
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the interplanar distances are observed as fringes. The Energy Dispersive Analysis of X-Ray
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(EDAX) has provided substantial evidence for the nanocomposite formation with the elemental composition. The maximum specific capacitance is measured as 280 F/g using
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Teller (BET) analysis.
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GCD studies. The surface area of the composite is determined using the Brunauer-Emmett-
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Keywords:Graphene oxide, reduced graphene oxide, cerium oxide, electrochemical studies, cyclic voltametry, galvanostatic charge/discharge, supercapacitors
Journal Pre-proof Introduction: Considering the perspective of overcoming environmental and energy crisis, the renewed interest in finding sustainable energy sources and innovating energy storage system are of vital importance, especially in the field of mobile electronics, electrical vehicles, power sources, portable electronics and other devices [1,2]. Many alternate energy sources are explored to overcome the energy crisis. Electrochemical supercapacitor is one among the alternate energy storage / conversion device which possesses higher power density and longer life cycles than batteries. Similarly, supercapacitors hold high energy density than conventional dielectric capacitors. Hence, supercapacitor is recognized as promising and
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favourable device for charge storage [3].
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Supercapacitors are separated in to two major classifications, i) Electrical Double
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Layer Capacitors (EDLC) and ii) Pseudocapacitors. EDLC utilizes carbon-based materials as electrodes for charge storage with high specific power, whereas the pseudocapacitors make
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advantage of using transition metal oxides or polymers like polyaniline as electrodes for charge storage with high specific energy. Hybrid supercapacitor is a combination of both
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EDLC and pseudocapacitor to produce high power density and energy density [4]. Amid the carbon-based materials, graphene is identified as the cheaper and efficient candidate of
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choice for supercapacitor electrodes.
Graphene is a single layered graphitic planar structure, formed by Sp2 hybridised
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carbon atoms which possess high thermal and electrical surface area, high electrical conductivity, good electrochemical property, high mechanical strength and very high charge transfer properties. Graphene oxide (GO) is a derivative of graphene with functional groups such as epoxy, hydroxyl and carbonyl hydroxyl or carboxylic acid groups attached to it [5, 6]. The degree of oxidation can be evaluated by the presence of carbon / oxygen ratio. The attachment of these functional groups leads to the buckling of sp3 hybridised graphene oxide, resulting the interplanar distance ranging 0.7 – 0.8 nm. GO is reduced to produce reduced graphene oxide (rGO) via chemical, thermal or electro chemical methods[7]. rGO tends to align towards pristine graphene structure which is used for charge storage application. Carbonaceous nanomaterials have been demonstrated as the potential candidate for high performance charge storage supercapacitors. Several transition metal oxides have been widely accepted as a potential pseudocapacitor due to their high surface area and porosity. Ruthenium oxide (RuO2) is the
Journal Pre-proof dominant metal oxide with high specific capacitance, which is investigated in detail under various conditions. Moreover, MnO2[8], NiO[9], Fe3O4[10], CoO2, Co3O4[11], V2O5[12], NiCo2O4[13] and mixed metal oxides have been studied for the electrochemical charge storage. The capacitance retention of these metal oxides deteriorates after several cycles while used as electrodes in concentrated acidic electrolytes. rGO –metal oxide is the nanocomposite material of current research interest with applications in batteries[14], supercapacitors[15], tracing of heavy and toxic elements in aqueous media [16], photocatalytic dye degradation [17], biosensor [18] and electrocatalysts
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[15,19] in oxide fuel cells. The nanocomposite of graphene with metal oxide likeRuO2 (20), SnO2[21], MnO2 [22], Co3O4[23], Nb2O5[24], V2O5[25], Fe2O3[26], MoO2[27], WO3[28] and
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ZnO [29] have proven track record of enhanced specific capacitance, capacitance retention after several cycles, high power density and energy density. Among these nanocomposites,
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the graphene-ruthenium oxide is observed to be an outstanding performer of supercapacitor
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studied so far. The main demerit of this nanomaterial is very expensive leading to high cost supercapacitor fabrication. The present scientific community is in search of cheaper charge
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storage materials, alternate to graphene - ruthenium oxide nanomaterials.The ceria is claimed to be a cheaper material and found abundant in nature with highly active redox property.
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Hence, graphene-based ceria nanocomposite is focussed for the present study. This article deals with the preparation of rGO-CeO2 using hydrothermal method and the charge storage
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capacity is quantified with CV and GCD studies. 2.Experimental Section 2.1 Materials
Graphite, sodium nitrate, potassium permanganate, hydrogen peroxide, cerium chloride, and hydrazine hydrate of analytical grade purchased from Thermo Fischer Scientific are used for the preparation of nanomaterials. 2.2.Synthesis of GO and CeO2 The graphene oxide is prepared as reported in our earlier work [30] using modified Hummer’s method. For pure ceria preparation,0.1M cerium chloride and 0.3M sodium hydroxide solution are chosen as the primary concentration of the starting materials. Initially, sodium hydroxide solution is gradually added drop wise to the cerium chloride solution under constant magnetic
Journal Pre-proof stirring. As a result, the solution observed as yellowish white later solution changes to pale yellow colloidal suspension. Subsequently, the suspension is transferred to a 100 ml Teflon – lined stainless steel autoclave and sealed tightly. Further, autoclave is placed in a muffle furnace and heated at 120 °C for 15 hours. On cooling to room temperature, the yellow precipitate is collected and washed three times using deionised water and ethanol. The gathered powder is dried at 100 °C for 12 hours[31] and grinded to a fine powder using mortar. 2.4. Synthesis of rGO- CeO2
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The rGO - CeO2 nanocomposite is synthesised by hydrothermal method. 50 mg of GO is dissolved in 20 ml of deionised water and sonicated for 1 hour. Then, 0.1 M of cerium
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chloride is added to the dispersed GO solution under constant stirring. 0.3 M sodium hydroxide solution and 2 ml of hydrazine hydrate are added to the above mixture. After 30
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min of stirring, the mixture is transferred to the Teflon – lined stainless steel auto-clave and
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maintained at 160 °C for 24 hours. After cooling to room temperature, the black precipitate collected by centrifugation is washed with deionised water / ethanol for several times and
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2.5. Characterization
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dried at 60°C for 24 hours in a muffle furnace.
To shed more light on rGO-CeO2 structure and their efficacy of charge storage the
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prepared sample is subjected to various characterizations. The structural studies are performed by BRUKERD8 ADVANCE POWDER X- ray diffractometer using Cu-Kα radiation (λ = 1.540 Å). Fourier Transform - Raman (FT-Raman) analyses of the samples are accomplished by BRUKER RFS 27:Stand-alone FT-Raman spectrometer using KBr pellet method in the range 4000 – 50 cm-1. The wavelength of the Raman laser beam is 1064 nm. The morphology of the CeO2 and CeO2-rGO nanocomposites are examined using High resolution – Scanning Electron Microscope (HR-SEM) and Field Emission -Scanning Electron Microscope (FE-SEM), respectively, using FEI Quanta FEG 200 microscope at voltage of 20 kV. The High Resolution – Transmission Electron Microscope (HR-TEM) images of the rGO-CeO2 composite is recorded using BrukerXFlash 6T130 EDX detector. In prior to HR-TEM analysis, the sample is dispersed in isopropyl alcohol and few drops are coated on copper grids and allowed to dry in air atmosphere. The Selected Area Electron Diffraction (SAED) is recorded to validate the sample as crystalline or amorphous in nature. The elemental composition of the nanocomposites is analysed with the help of EDAX
Journal Pre-proof spectroscopy using EDX detector. The thermal stability of the sample is explored using Thermo Gravimetric – Differential Thermal Analyser (TG-DTA), NETZSCH-STA2500 STA2500A-0061-N. The Brunauer-Emmett-Teller (BET) analyser is employed to obtain the surface area and pore size of the prepared sample using Quantachrome Instruments – Autosorb Series. Electrochemical
studies
such
as
Cyclic
Voltametric
(CV),
Galvanostatic
Charge/Discharge (GCD) and Electrochemical Impedance Spectroscopy (EIS) of single electrode have been conducted using Bio-Logic SP-150 electrochemical workstation in 2 M
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KOH aqueous solution using a three-electrode configuration. In a three-electrode system, the working electrode is fabricated by prepared material dispersed in dimethylformamide (DMF)
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using ultrasonication in order to form homogeneous viscous paste. A known weight (1.5 mg) of the paste is cast onto a Ni foam sheet (1 cm ×1 cm) and dried at 80 C overnight. Here, the
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KCl saturated Ag/AgCl electrode and a platinum wire are used as reference and counter
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electrodes, respectively, whereas the prepared material is used as the working electrode. CV is performed for different sweep rates (3, 5, 10, 20, 50, 100, 150 and 200 mV/s) within a
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potential range of 0 to 0.5 V for pure CeO2, and potential range of 0 to 0.6 V for rGO-CeO2 composite. GCD measurements are tested to study the charge/discharge time of the electrodes
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for various current densities (1, 2, 3, 4, 5 and 10 A/g) in an operating potential of 0 to 0.45 V. Electrochemical Impedance spectroscopy (EIS) technique is executed in the frequency range
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of 0.01 Hz – 100 kHz. 3. Results and Discussions
3.1.X-ray diffraction analysis
The X-ray diffraction patterns of GO, rGO, pure CeO2 and rGO / CeO2 nanocomposite are stacked one over the other as shown in Fig. 1a.
The characteristic
intensity peak observed at 10.83° confirms the formation of GO and the intensity peak is emanating from the (001) plane with d spacing of 0.82nm. On reduction of GO (rGO), the diffracted intensity peaks are observed at 24.68 ° and 43 °, which corresponds to (002) plane and (100) plane, respectively. The inter-planar distance is observed to be 0.361 nm for rGO, resembling the graphite like structure. Regarding CeO2, seven distinctive diffraction peaks are noticed, and these peaks are indexed as (111), (200), (220), (311), (222), (400), (331) planes, with reference to JCPDS file No 65-5923. Thus, the diffraction image of CeO2 substantiates the formation of face centred cubic structure. On comparing the simulated and
Journal Pre-proof experimental XRD patterns of CeO2 (Fig. 1b), most of the intensity peaks agree well with each other with minor ambiguity. It is obvious that no significant diffraction peak of GO is found in the rGO-CeO2 nanocomposite, which accounts for the existence of productive exfoliation of stacked graphene layers [32]. The average crystallite size of the nanocomposite
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Fig. 1 (a)- Stacked X-ray diffraction patterns of GO, rGO, CeO2 and rGO-CeO2. (b) Simulated and experimental XRD patterns of pure ceria overlapped for comparison
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3.2.FT-Raman Spectroscopic Studies
Figure 2 shows the FT-Raman spectra of rGO, pure CeO2, and rGO–CeO2 nanocomposite. For pure CeO2, highly intense peak is recorded at 460 cm-1which attributes to the triply degenerate F2g mode of the cubic fluorite crystal structure. In addition, a low intensity second order Raman peak is also observed at 1063 cm-1, which represents the 2ωR(X) phonon mode [33]. The Raman spectrum of rGO clearly shows the characteristic D band and G band at 1290 cm-1 and 1593 cm-1, respectively, whereas in rGO-CeO2 nanocomposite, the same bands are observed at 1288 cm-1 and 1594 cm-1, respectively. Further, the band at 456 cm-1 is registered for the nanocomposite spectrum, which corresponds to ceria peak. Thus, the D band, G band and ceria Raman peak confirms the formation of nanocomposite. The defects in the graphitic structure can be identified by the variation in the intensity contribution of D band and G band. In specific, D band reveals the structural defects related to geometrical parameters such as bond lengths, bond angles and
Journal Pre-proof hybridisation. The ID/IG ratio is used as an indicator to validate the structural purity of the graphitic structure. In the present study, the ID/IG ratio is observed to be same (0.81) for both rGO and rGO-CeO2 nanocomposite, which illustrates the attachment of CeO2 on rGO doesn’t
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Fig. 2 - FT-Raman Spectra of rGO, rGO – CeO2, CeO2
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3.3. SEM TEM and EDX
The HR-SEM image of pure CeO2 is shown in Fig. 3a. The HR-SEM micrograph of
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pure CeO2 determines the surface morphology as spherical in shape which concurs with the reported literature [36]. For better illustration of graphene sheet and bound ceria in hybrid nanomaterial, the obtained Field Emission Scanning Electron microscope (FE-SEM) micrographs are shown in Fig. 3b and 3c. Both images support the fine attachments of ceria on top of the folded graphene sheet which are clearly visible.
(b)
(a)
300 nm
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Fig.3 - Scanning Electron Microscope images (a) HR-SEM image of pure CeO2 (b) & (c) FE-SEM images of rGO-CeO2.
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The HR-TEM images of rGO-CeO2 are shown in Figures 4a, 4b,4c and 4d. The images certify the formation of nanocomposite, where CeO2 is decorated on the graphene sheets (Fig.
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4c) and the embedded ceria is spherical in shape with slight disorder. Moreover, the
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morphology of nanoparticle varies based on precursor concentration, temperature and several other factors[31]. Here, the nanocomposite heated at 160 C for 24 hours, results in variation
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of the diameter of the spherical ceria particle between 7.99 nm and 15.6 nm and are anchored on the well-defined graphene sheets. Fig. 4d demonstrates the lattice fringes observed for
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ceria nano particle and graphene nanosheet with the interplanar distances 0.283 nm and 0.356 nm, respectively. The d spacing of 0.283 nm corresponds to 200 plane of ceria nanoparticle
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and 0.356 nm corresponds to the 002 plane of graphene nanosheet.
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The Selected Area Electron Diffraction pattern (Fig.4e) exhibits the polycrystalline nature of the nanocomposite with bigger grain size and finer grain size. The diffraction pattern agrees well with the indexed XRD pattern of composite. The compositional analysis of the composite is revealed using EDX spectrum (Fig.4f). Here, the major elements of the sample are carbon with wt % 72.81 and cerium with wt % 17.10 along with the minor composition of oxygen with wt % 2.53. The low content of oxygen may be due to the incomplete reduction of GO. Furthermore, it is evident that the detected copper signals originated from the copper grid with wt % 7.56.
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Element CK OK CeL CuK wt % 72.81 2.53 17.10 7.56
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Element CK OK CeL CuK
Ce
Ce Cu
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wt % 72.81 2.53 17.10 7.56 at % 93.82 2.45 1.89 1.84
at % 93.82 2.45 1.89 1.84
Cu
0 0
1
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Fig.4(a) (b) & (c) HR-TEM images of rGO-CeO2 nanocomposite (d) HR-TEM image of the composite with the highlighted ‘d’ spacing of ceria nanoparticle and graphene sheet (e) SAED pattern for the nanocomposite (f) EDX spectrum of rGO-CeO2 nanocomposite with the normalized and atomic weight % of the elements in the inset
Journal Pre-proof 3.4. TG-DTA Studies The TG-DTA curves for CeO2 and rGO–CeO2 are shown in Fig.5a & b. The weight loss occurs in two distinct stages (Fig.5a) for pure form of CeO2. The first stage falls in the temperature range of 50 C – 200 C where maximal weight loss (7.25 %) takes place and this may be due to the vaporization of moisture and adsorbed solvent molecules. Beyond 200 C, a minimal weight loss occurs due to the removal of combustible organic residuals. Above 600 C, a slight increase in weight % is noticed, which may be connected to pure phase formation of CeO2 [37]. Finally, CeO2 suffers 13% weight loss at 799 C. Upon
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binding of ceria on graphene nanosheet, the thermal stability is enhanced for the composite (Fig. 5a) than the ceria alone. In the present study, the prepared composite (hydrothermal
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method) is thermally more stable than the earlier prepared rGO-CeO2 xerogel composite [38].
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The total mass loss of the composite is insignificant with 6.36 % at 801 C. The DTA thermogram of CeO2 explicitly shows the endothermic peak observed at
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102.2 C, followed by broad exothermic peak stretched across temperature 200 – 500 C (Fig 5b). The endothermic peak represents the elimination of surface adsorbed water molecules at
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102.2 C, whereas the extended exothermic peak may be due to the decomposition of theorganic components (weight loss ~ 3 %) in CeO2[39]. The DTA thermogram of the
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nanocomposite.
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composite reflects the TGA curve and gives supportive evidence for the stability of the
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Fig. 5(a) TGA and DTA curve for pure CeO2 (b) nanocomposite rGO-CeO2.
Journal Pre-proof 3.5. BET Studies A few experimental techniques are available in determination of surface area of porous materials but BET study is the widely accepted efficient method to estimate the surface area[40]. The N2 adsorption-desorption curve for pure CeO2 and rGO-CeO2 composite are delineated in Fig. 6a using multi-point BET surface area analyser. The adsorption-desorption curve increase gradually at the low relative pressure ranging between 0.0- 0.75 and sharp increase is observed above 0.75 relative pressure. Both the curves take perfect hysteresis loop, belonging to the IUPAC type IV category [41]. The surface area of the nanomaterials CeO2 and rGO-CeO2 are 85.5 m2/g and 87.1 m2/g, respectively. The
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Barret–Joyner—Halenda (BJH) pore size distribution plots (Fig. 6b) enunciate the pore size
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distribution of the CeO2 and rGO-CeO2, in which the major distribution of pore diameter is 3.4 nm for the both the nanoparticles. According to the IUPAC nomenclature, the obtained
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pore size testifies the formation of mesoporous structure [2 – 50 nm], and the result is in close
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proximity to the earlier reported literature [42].
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Fig.6 - (a) Nitrogen adsorption-desorption isotherm curve and (b) pore size distribution plotof pure CeO2 and rGO-CeO2
Journal Pre-proof 3.6. Electrochemical Studies For the better understanding of the electrochemical properties of ceria and rGO-ceria, a detailed investigation on the CV and GCD studies have been carried out for pure ceria and the composite rGO-ceria nanomaterials. Fig. 7a shows the CV curves for rGO-CeO2 working electrode at different scan rates 3, 5,10, 20, 50, 100, 150 and 200 mV/s and their potential window ranging from 0 to 0.6V. Here, the redox peaks are prominent and takes nonrectangular shape for all CV curves of different scan rates which represents the pseudocapacitance behaviour of the nanocomposite. The observation of pseudocapacitance
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feature for rGO-CeO2 matches well with the earlier electrochemical studies using KOH as the electrolyte [15, 43, 44]. In contrast, Dezfuli et al., showed that the rGO-CeO2 redox peaks
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are in rectangular shape for different scan rates using Na2SO4 as electrolyte [35]. Obviously, Fig. 7a demonstrates the voltammetric current of each CV curve, which increases on
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increasing the scan rate. As the intensity peak current is directly proportional to the square
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root of scan rate, the waxing of voltammetric current occurs. Nevertheless, the specific capacitance decreases drastically on increasing the scan rate. The rationale behind waning of
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specific capacitance is as follows: at low scan rate, complete electrolyte ions diffusion takes place into the material with maximum active sites accessible for maximum charge storage. In
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case of high scan rate, the electrolyte ion movement is restricted due to lack of time, resulting less charge storage [45]. Furthermore, the increment of scan rate leads to slight shift in the
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anodic peak towards higher end potential and the cathodic peaks shift towards lower end potential [44,46] and this may be due to the presence of internal resistance of the electrode. In Fig.7b shows the overlapped CV curves for the pure ceria and rGO-ceria composites at the specific scan rate 50 mV/s. The area swept by the composite CV curve is larger than the pure CeO2, resulting high specific capacitance. The major contribution of the pseudocapacitance behaviour of the composite is mainly facilitated by the CeO2 redox reaction. The galvanostatic charging and discharging curves for rGO-CeO2at different current densities are shown in Fig.7c. The specific capacitance (Cs) is obtained from the GCD curve using the formula It Cs = ---------mV
Journal Pre-proof where I, t, m and V corresponds to oxidation or reduction current, time differential, mass of the active electrode material and potential scan window, respectively. The calculated specific capacitance values of ceria are 91 F/g, 85 F/g, 78 F/g, 70 F/g and 65 F/g for current densities 1 A/g, 2 A/g 3 A/g 4 A/g and 5 A/g, respectively. Similarly, the calculated specific capacitance values of composite rGO-ceria are 280 F/g, 234 F/g, 227 F/g, 222 F/g, and 214 F/g for 1 A/g, 2 A/g 3 A/g 4 A/g and 5 A/g, respectively. The maximum specific capacitance obtained for the present study is 280 F/g for rGO-CeO2 composite, which is higher than the reported values, 265 F/g [44] and 208 F/g [15]. Here the calculated specific capacitance values for the rGO-CeO2 are larger than the pure ceria-based electrode, which explicitly
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reveals the transformation of ceria from poor electrical conductivity to better electrical
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conductivity. The electrons percolate at faster rate throughout the electrode, due to good electrical conductivity of graphene nanosheet, leading to higher capacitance attainment [47].
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In the charge-discharge curves (Fig. 7c), the discharge occurs in two phases. The potential drops as a slope in phase I, 0.5 - 0.3 Volts, which represents the pseudocapacitance behaviour
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of the electrode due to the redox reaction at the interface of the electrode / electrolyte. In
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phase II, the potential declines abruptly in the range of 0 – 0.3 Volts, which denotes the electrical double layer capacitance nature of the electrode as reported earlier [15]. The GCD curves of pure ceria and rGO-ceria composite are superimposed (registered at1 A/g) in Fig.
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7d to distinguish the area enclosed and their significant contribution towards specific capacitance of pure ceria and composite. The specific capacitance is inversely proportional to
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both the scan rate and the current density. The progression of scan rate and current density resembles same as a function of specific capacitance where the diffusion of electrolyte ions into the electrode material are restricted, yielding low specific capacitance. The variation of specific capacitance with respect to current density for pure ceria and composite rGO-CeO2 is illustrated precisely in Fig. 7e. The specific capacitance for the composite is higher than the pure ceria at different current densities and this may be due to the morphology of rGO/CeO2 composite compared to pure CeO2. It is evident from the SEM and TEM images, the CeO2 particles are uniformly embedded on the graphene sheets, which attributes to the synergistic effect of combination of rGO and CeO2, resulting in elevated electrolyte ion movement at the interface of the electrode/electrolyte with enhanced specific capacitance. The cyclic stability is an important parameter in electrochemical studies to decipher the long-term supercapacitor performance of the electrode material. In the present study, it is monitored up to 5000 cycles for the composite rGO-CeO2 using GCD and found to be more
Journal Pre-proof stable all along 5000 cycles. After 5000 cycles, a remarkable 98 % capacitance retention is accomplished (Fig. 7f), which is better than the already reported value i.e. 94.1% capacitance retention after 4000 cycles [43], 86% capacitance retention after 1000 cycles [48]. Thus, capacitance retention after 5000 cycles is a good sign of long-term stability of supercapacitor with enhanced performance and more likely to be a potential electrode material for
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Fig. 7(a) & (c) CV curves and GCD curves of the composite rGO-CeO2 plotted at different scan rates and current densities, respectively. (b)&(d) Comparison of CV curves and GCD curves for pure CeO2 and composite rGO-CeO2 at scan rate 50 mV/s and current density 1 A/g, respectively. (e) Obtained specific capacitance values of pure CeO2 and composite rGOCeO2 plotted as a function of current densities. (f) Specific capacitance retention (%) as a
Journal Pre-proof function of cycle number plotted for pure ceria and composite rGO-CeO2 electrode in KOH electrolyte In order to establish the supercapacitance behaviour and the electrical conductivity of the pure ceria and the composite rGO-CeO2 electrode materials, EIS technique is employed in the frequency range of 0.01 Hz – 100 kHz using alternate current potential. Fig. 8 displays the Nyquist plot of pure ceria and rGO – ceria composite, a graphical representation of real part of impedance (Z’) vs imaginary part of impedance (Z”). An apparent semi-circle is observed at high frequency, succeeded by linear line at low frequency for both the samples.
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The Warburg linear line region of low frequency explains the prolonged ion diffusion path lengths and elevated barrier against ion movement, which represents the ideal capacitor
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behaviour. The internal resistance Rs is evaluated from the intercept of high frequency at real axis whereas the charge transfer resistance (Rct) is measured from the diameter of the semi-
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circle [49]. The diameter of the semi-circleis smaller for composite material compared to pure
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ceria, resulting higher charge transfer rate of the electrode material with better performance
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Fig.8 - Nyquist plot for pure ceria and rGO-ceria electrodes in KOH electrolyte
4. Conclusion The nanomaterial of our interest rGO-CeO2 is prepared by hydrothermal method. The experimental investigations like XRD, FT-RAMAN, SEM, TEM and EDAX confirmed the formation of rGO-CeO2 composite. The composite is thermally more stable than pure ceria with limited weight loss of 6.34 % at 801 C. The pseudocapacitance behaviour is dominant for the composite material as the redox peaks of CV curves promote nonrectangular shape in KOH electrolyte. The GCD studies unravel the enhanced supercapacitor performance of
Journal Pre-proof composite with specific capacitance 280 F/g. The significant long-term supercapacitor performance of the composite electrode material is achieved to be 98 % capacitance retention than the original, starting value after 5000 cycles. Eventually, these results endorse the composite as economical and potential electrode material for charge storage application. Conflicts of interest
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There are no conflicts to declare.
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Graphical abstract
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Journal Pre-proof Highlights The experimental investigations of the prepared rGO-CeO2 like XRD, FT-RAMAN, SEM, TEM and EDAX confirmed the formation of rGO-CeO2 composite. The composite is thermally more stable than pure ceria with limited weight loss of 6.34 % at 801 C. The pseudocapacitance behaviour is dominant for the composite material as the redox peaks of CV curves promote nonrectangular shape in KOH electrolyte. The GCD studies proves the enhanced supercapacitor performance of composite electrode
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material achieved to be 98 % than the original value after 5000 cycles