Journal of Science: Advanced Materials and Devices (2019) 515e523 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article A facile synthesis of graphene/Co3V2O8 nanocomposites and their enhanced charge storage performance in electrochemical capacitors Wei Hau Low a, d, Chiu Wee Siong b, Chin Hua Chia c, Siew Shee Lim a, Poi Sim Khiew d, * a Department of Chemical and Environmental Engineering, Faculty of Engineering, The University of Nottingham Malaysia Campus, Jalan Broga, 43500, Semenyih, Selangor, Malaysia b Low Dimensional Materials Research Center, Department of Physics, Faculty of Science, University of Malaya, 50603, Kuala Lumpur, Malaysia c Faculty Science and Technology, School of Applied Physics, University Kebangsaan Malaysia, 43600, Bangi, Selangor, Malaysia d Center of Nanotechnology and Advanced Materials, Faculty of Engineering, University of Nottingham Malaysia Campus, Jalan Broga, 43500, Semenyih, Selangor, Malaysia a r t i c l e i n f o a b s t r a c t Article history: Received July 2019 Received in revised form 26 September 2019 Accepted October 2019 Available online 14 October 2019 The electrochemical capability for the charge and energy storage of supercapacitors can be augmented by fabricating the hybrid and binder-free electrodes In this work, the novel graphene/Co3V2O8 micropencils nanohybrids were successfully developed via the one-pot solvothermal method The interactive effect between graphene and Co3V2O8 was investigated by varying their mass ratios Benefiting from the peculiar morphology of Co3V2O8 micro-pencils and the homogenous distribution of Co3V2O8 on the graphene sheets, G-4CVO manifested a relatively promising specific capacitance of 528.17 F$gÀ1 at 0.5 A$gÀ1 while 80% of the charge storage capability was still retained even after 5000 continuous cycles G-4CVO also delivered a remarkable energy density of 73 Wh/kg and these advanced electrochemical performances could be explicated by the integration of graphene sheets which shortens the ions and electrons transportation pathway and at the same time acts as a scaffold to alleviate the volume variation during the inter/de-intercalation process © 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: One-pot solvothermal Graphene Graphene/Co3V2O8 Supercapacitor Introduction Energy alteration and storage have played a critical role in the past decades of human civilisation [1] Nowadays, countless research efforts have been devoted to miniaturising the electronic equipment into portable, flexible and wearable devices [2] These growing concerns of the world-wide energy security accelerate the design and development of efficient, durable and reproducible energy storage systems [3,4] Notably, supercapacitor is regarded as one of the most sought-after charge storage sources by virtue of its fast charge-discharge ability, stable and enduring shelf life, excellent specific power, high efficiency and environmental benignness [4] These peculiar properties enable it to be utilised in hybrid cars, portable electronic devices and military appliances [3] Nonetheless, relatively poor energy density of supercapacitors has hindered their commercial applications The electrode materials and * Corresponding author E-mail address: poisim.khiew@nottingham.edu.my (P.S Khiew) Peer review under responsibility of Vietnam National University, Hanoi morphologies are widely acknowledged as the decisive variables affecting the capacitive activity of supercapacitors [1,5] In consideration of the mechanism of charge storing, supercapacitors generally can be categorised into two main groups, namely the electrical double layer capacitors (EDLC, carbon based materials) and pseudocapacitors (metal oxide based materials) The former normally will intercalate charges at the electrode/electrolyte interface through the electrostatic interactions while the latter store charges through a series of oxidation/reduction reactions during the charge-discharge process [6] On the basis of charge storage capacity, the pseudocapacitor has outperformed the EDLC Till date, myriad single component transition metal oxides (TMO) such as ZnO, MnO2, NiO, CuO and V2O5 have been identified as the effective materials for supercapacitor electrode fabrication [3,7] However, these materials encounter undesirable volume expansion during the persistent charge-discharge (inter-deintercalation) process, which eventually contribute to their low specific capacitance, inferior electrical conductivity and poor capacitance retention [3,7] Other than TMO, mixed transition metal oxides (MTMO) like metal molybdate, metal cobaltite, metal tungstate, metal vanadate etc have been developed as the faradiac electrode since their https://doi.org/10.1016/j.jsamd.2019.10.001 2468-2179/© 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) 516 W.H Low et al / Journal of Science: Advanced Materials and Devices (2019) 515e523 diverse oxidation states allow reversible redox reaction Cobalt vanadate has been recognised as one of the potential electroactive materials due to its low toxicity, cost-effectiveness and remarkable charge storage activity as a result of the integration of Co2O3 (significant contribution to specific capacitance) and multiple polymorphism of V2O5 [7,8] So far, limited studies have been attempted to configure it into an active electrode material for supercapacitors As an example, Liu and his co-workers [9] synthesised the Co3V2O8 nanoparticles via the hydrothermal approach which delivered a value of 505 F$gÀ1 specific capacitance at 0.625 A$gÀ1 In addition, Co3V2O8 3D porous nanoroses were fabricated by Zhang et al [10] through the solvothermal method The 3D porous nanoroses displayed a specific capacitance of 371.3 F$gÀ1 when the electrode is analysed under a current density of 0.5 A$gÀ1 However, Co3V2O8 suffers from low specific capacitance, short shelf life and inferior rate capability due to the serious volume variation during cycling analysis, which hinders its practical application [1,11] As a result, integration of Co3V2O8 with carbonaceous material appears as one of the promising solutions to rectify this conundrum in which the electrical conductivity of the nanohybrid could be significantly improved Particularly, graphene is a potential matrix for the deposition of MTMO owing to its inimitable properties which comprise good electrochemical stability, huge specific surface area, high structural tenacity, excellent electrical conductivity and superior mechanical properties [1,12,13] Nevertheless, the restacking or aggregation of the pure graphene sheet unavoidably enhances the ion diffusion resistances which contributes to the poor capacitive performance In this context, integration of graphene with MTMO is perceived as a promising solution to this problem, for instance, the MTMO serves as a spacer which can inhibit the aggregation of the graphene sheet Based on the above considerations, this paper presents the graphene/Co3V2O8 micro-pencils nanocomposites produced by using the solvothermal method and then utilised as the main active electrode material for a symmetric supercapacitor To the extent of our knowledge, this is the first work reporting the utilisation of graphene/Co3V2O8 nanohybrid as an electrode material for advanced supercapacitor system The modified Hummers' method usually involves the use of harsh acids and oxidisers, whereas the graphene in this work was synthesised via the top down liquid phase exfoliation of HOPG by adopting ethanol and water as the exfoliating medium in an optimum ratio [5,14] Furthermore, the inference of mass loading of Co3V2O8 on the electrochemical performance of the nanocomposite was investigated by preparing different mass ratios of graphene/Co3V2O8 nanomaterials Experimental 2.1 Materials and chemicals Highly pyrolytic graphite flakes (HOPG, Bay Carbon USA) were used to synthesise graphene Cobalt chloride hexahydrate (CoCl2.6H2O, R&M Chemicals), ammonium metavanadate (NH4VO3, Acros Organics) and lithium hydroxide (LiOH, R&M Chemicals) were used for the preparation of pure Co3V2O8 and its nanocomposites Carbon black (Alfa Aesar), polyvinylidene fluoride (PVDF, SigmaeAldrich), potassium hydroxide (KOH, SigmaeAldrich) and N-methyl-2-pyrrolidinone (NMP, SigmaeAldrich) were used to fabricate the electrode 2.2 Fabrication of graphene and graphene/Co3V2O8 micro-pencils nanocomposites Graphene sheets were fabricated by exfoliating the HOPG through liquid-phase exfoliation [15] Typically, 50 mg of HOPG was added into a 100 mL aqueous ethanol-water solution (2:3) and sonicated for h under ambient condition The graphene solution was then washed with the ethanol-water solution through centrifugation and dried at 80 C overnight As for graphene/Co3V2O8 nanocomposites, the solvothermal technique was applied and the entire procedure is delineated in Fig S1 Firstly, NH4VO3 (0.02 mol) was dissolved into a 170 mL deionised water at 80 C under vigorous stirring to obtain a transparent light green colour solution Then, a reddish brown solution was obtained after mixing LiOH (0.02 mol) and CoCl2$6H2O (0.004 mol) with the solution In the next step, the graphene suspension was slowly added to the aforementioned mixed solution and kept agitating for half an hour to reach homogeneity The resulting mixture was then transferred into a 200 ml Teflon-lined stainless steel autoclave and heated at 200 C for 16 h After that, the as-synthesised precipitates were collected, cleaned thoroughly with ethanol-water solution, dried at 80 C in the hot air oven for h and annealed at 500 C for h to obtain the graphene/Co3V2O8 micro-pencils nanomaterial Pure Co3V2O8 was fabricated in the same experimental pathway without graphene For comparison, variation on quantity of graphene in the nanocomposites was conducted to investigate the impact of the mass ratio on the structure and the energy storage capability of the nanocomposites The detailed graphene/Co3V2O8 nanocomposite formulation is manifested in Table S1 2.3 Instrumentation and sample characterisation The crystal structures of the nanocomposites were assessed by the powder X-ray diffraction (XRD, PANanalytical X'pert-Pro) The molecular fingerprints of the nanocomposites were analysed using Raman spectroscopy and recorded on the Renishaw inVia spectrometer The morphology of the nanocomposites was characterised by field emission scanning electron microscopy (FESEM, Quanta 400F USA) and transmission electron microscopy (TEM, JEOL-2100F Japan) The elemental composition and surface chemical states of the nanocomposites were assessed by energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCAlab 250Xi), respectively 2.4 Electrochemical measurements The electrochemical performances of the active electrode materials were characterised via cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) analysis using a Metrohm Autolab Potentiostat (PGSTAT302F)) The galvanostatic charge-discharge (GCD) is conducted using the Arbin multichannel galvanostat (BT-2000) A symmetric two electrodes configuration with mol/L aqueous KOH as electrolyte was fabricated The working electrodes were assembled by mixing 70% of the active materials with 20% of carbon black and 10% of PVDF Finally, the active electrode was obtained by coating the as-prepared paste on an aluminium foil and drying the coated aluminium foil at 80 C for h Results and discussion 3.1 Phase and morphological study In order to provide an insight on the crystallographic data and phase composition of the as-prepared samples, XRD analysis was performed Fig 1a demonstrates the XRD spectra of graphene, Co3V2O8 micro-pencils and graphene/Co3V2O8 nanohybrids The graphene nanosheet exhibited two broad peaks at 26.8 and 54.9 , which correspond to the (002) and (004) crystal planes of graphene and this is in accordance with our previous reports [5,16] W.H Low et al / Journal of Science: Advanced Materials and Devices (2019) 515e523 517 Fig (a) XRD patterns of graphene, Co3V2O8 micro-pencils and graphene/Co3V2O8 nanocomposites, (b) Raman spectra of the graphene and G-4CVO nanocomposite From the diffraction spectrum of the graphene/Co3V2O8 nanocomposites, the distinctive peaks (except the two typical peaks originating from graphene) of the cubic crystal structure of Co3V2O8 (JCPDS No 16-0675) are located at 30.0 , 35.6 , 43.7, 57.8 and 63.2 , which could be assigned to the (220), (311), (400), (511) and (440) reflection planes, respectively [17,18] In addition, the samples are highly crystallised as indicated by the sharp diffraction peaks, reflecting their stable crystalline structures The highly stable Fig FESEM micrographs of (a) graphene, (b) pure Co3V2O8 micro-pencils, (c) G-CVO, (d) G-2CVO, (e) G-4CVO and (f) G-6CVO 518 W.H Low et al / Journal of Science: Advanced Materials and Devices (2019) 515e523 structures might enhance the electrochemical stability by alleviating the volume variation during the continuous intercalationdeintercalation process [19] These results reveal that the graphene/Co3V2O8 hybrid nanomaterials comprising of cubic Co3V2O8 micro-pencils and graphene nanosheets were successfully synthesised Raman spectroscopy has also been applied to determine the degree of graphitisation and the bonding modalities between the elements of the pristine graphene and the G-4CVO nanomaterial (Fig 1b) From the Raman analysis of G-4CVO, the prominent bands of graphene, namely the D, G and 2D bands are located at 1360, 1582 and 2726 cmÀ1, respectively The G band corresponds to the in-plane bond stretching motion of C sp2 atoms, while the D and 2D bands represent the disorder or defective graphitic structure [19,20] Besides, two peaks at around 335 and 810 cmÀ1 are observed, which can be correlated to the Raman spectra of Co3V2O8 [21] The bands at 335 cmÀ1 and 810 cmÀ1 are correlated to the asymmetric stretching vibration of the VeOeCo bonds and the symmetric vibration of the VeO bonds, respectively [5,18,19] These results further affirm the successful synthesis of the graphene/ Co3V2O8 nanocomposites In addition, a redshift of the G band can be noticed in the Raman spectra of G-4CVO, implying the intimate interaction between graphene and Co3V2O8 [22] In order to determine the surface morphological structure of the graphene, sole Co3V2O8 and graphene/Co3V2O8 nanocompo sites, FESEM was conducted and the outcomes are portrayed in Fig From Fig 2b, it can be noticed that Co3V2O8 possess an exterior geometry of short pencil-like structure with hexagonal prisms and these Co3V2O8 micro-pencils have a consistent microscale size distribution of approximately mm in length and mm in height [17,18] Besides, both the graphene sheet (Fig 2a) and the 3D skeletal configuration of the Co3V2O8 micro-pencils (Fig 2b) are discernible in Fig 2(cef), implying the successful decoration of the Co3V2O8 micro-pencils onto the surface of graphene to form the hybrid nanocomposites Such novel nanoarchitectures could effectively sustain the volume variation within the lattice of active materials during the inter/de-intercalation cycles due to their improved structural strength and tenacity [23] In addition to enlarging the effective surface area of the nanostructure, the graphene nanomaterial also serves as a conductive scaffold to facilitate the ions and charges migration [24,25] The morphological disparities of the graphene/Co3V2O8 nanocomposites with a variation of mass loadings are indicated in Fig 2cef In contrast to G-CVO and G-2CVO, G-4CVO manifested an evenly distribution of the Co3V2O8 micro-pencils on the surface of graphene as a result of a sufficient quantity of Co3V2O8 micro-pencils available to perfectly cover the whole surface of graphene and this configuration is believed to contribute significantly towards the enhancement of the electrochemical activity However, excess mass loading of precursors in G6CVO (Fig 2f) resulted in structural agglomeration, which inevitably reduces the homogeneity and the active sites of the nanocomposites, leading to inferior capacitive performance Furthermore, TEM was conducted to further determine the morphology of the G-4CVO (Fig 3) It is noteworthy that the graphene sheets not stack together and the high density of the Co3V2O8 micro-pencils can be observed on the graphene surface, which is congruent with the SEM results (Fig 2e) In addition, the EDS analysis of the as-prepared G-4CVO nanocomposite has been conducted and the results are depicted in Fig S2a The peaks in the EDS spectrum can be ascribed to the C, Co, V and O elements, validating their contribution in the assynthesised nanocomposite (G-4CVO) No other peaks were observed in the EDS analysis, that further verifying the purity of the nanomaterial Meanwhile, the EDS mapping analysis of G-4CVO (Fig S2b) shows the even scattering of the Co, V, O and C elements, which further advocates the homogenous dispersion of Co3V2O8 on the graphene surface The atomic ratio of Co:V in Table S2 was approximately 1.54:1, which coincides with the stoichiometric ratio of Co3V2O8 The XPS spectrum of G-4CVO is delineated in Fig to give an insight on the chemical composition and the oxidation states of the graphene/Co3V2O8 sample Four distinctive peaks assigned to the C 1s, O 1s, Co 2p and V 2p are evident from the wide-scan XPS survey spectrum of G-4CVO (Fig 4a), testifying the presence of these elements in the G-4CVO nanocomposites From Fig 4b, a doublet centered at 780 and 795 eV can be assigned to the spin orbit coupling levels of Co 2p3/2 and Co 2p1/2, respectively and they are accompanied by two prominent satellite peaks at 786.5 and 802.5 eV Fig 4b further confirms the presence of the two oxidation states of the cobalt element: Co2ỵ(779.4 and 795.8 eV) and Co3ỵ(782.8 and 799.5 eV) [18,26e28] As for V element (Fig 4c), the peaks at ca 515.6 and 523 eV can be assigned to the V 2p3/2 and V 2p1/2 of V5ỵ states [10,18] Fig 4d depicts the O1s orbital spectrum, where the two resolved peaks at 529 and 531.7 eV can be described by the metaleoxygen bonds (CoeO and VeO bonding) and the adsorbed oxygen, respectively [14,27,29,30] In addition, the high resolution C 1s spectrum (Fig 4e) can be de-convoluted into fittings peaks: 284.8, 285 (C¼C/CeC carbon species) and 282.8 eV (carbidic CoeC bonds) [27,31,32] 3.2 Electrochemical measurement The charge storage performances of the nanocomposites were assessed by performing the cyclic voltammetry analysis (CV), the galvanostatic charge-discharge (GCD) and the electrochemical impedance spectroscopy (EIS) tests The CV curves of the pristine Co3V2O8 and the graphene/Co3V2O8 nanocomposites in the potential ranging from À1 to V with the scanning rate of mVsÀ1 are delineated in Fig 5a It is worth noting that two pairs of didymous anodic and cathodic peaks are visible in each voltammogram, implying the electrode materials are strongly pseudocapacitive in nature [10,33] Notably, the redox reactions of Co2ỵ/Co3ỵ with the OHÀ ions is Fig TEM image of the G-4CVO nanocomposite W.H Low et al / Journal of Science: Advanced Materials and Devices (2019) 515e523 reversible and hence contributing to these didymous redox peaks The corresponding faradiac electrochemical reactions are described in Equations (1)e(4), [33,34]: Co2ỵ Co3ỵ ỵ e (1) Co2ỵ ỵ 2OH /CoOHị2 (2) CoOHị2 þ OHÀ CoOOH þ H2 O þ eÀ (3) CoOOH ỵ OH 4CoO2 ỵ H2 O ỵ e (4) Besides, the CV curves of the anode and cathode are almost 519 symmetrical, reflecting their superior reversibility and close-to-ideal capacitive behaviour The enhanced charge storage ability of the graphene/Co3V2O8 nanocomposites is reflected by their larger integrated area of the CV loop as compared to that of the pure Co3V2O8 electrode This can be elucidated by the positive interaction between the graphene and the Co3V2O8 micro-pencils which resulted in rapid transportation of ions and electrons [26] Moreover, the presence of graphene provides abundant exposed surface area for the ion adsorption, leading to the effective faradiac redox reaction [35,36] From Fig 5a, the integrated area under of the CV curves of the nanocomposites was ranked in the following order: G-4CVO > G2CVO > G-6CVO > G-CVO, suggesting that the amount of Co3V2O8 anchoring on the surface of the graphene nanomaterial has a Fig XPS spectra of G-4CVO: (a) Full survey spectra, (b) Co 2p, (c) V 2p, (d) O 1s and (e) C 1s 520 W.H Low et al / Journal of Science: Advanced Materials and Devices (2019) 515e523 substantial effect on the capacitive performance of the electrode material [14] Fig 5b manifests the CV curves of G-4CVO obtained at various scan rates (25, 50 and 100 mVsÀ1) The outstanding rate capability and rapid electrochemical response of G-4CVO was indicated by the unaltered CV curves at a wide range of the potential sweep rates [37,38] Additionally, the increased peak current at the higher scan rate indicates a steerable ion transportation process with the rapid interfacial kinetics [26,33] It is noticeable from Fig 5b that the increment in the scan rate resulted in the shifting of the redox peaks For instance, the anodic peak is shifted to a more positive direction while the cathodic peak moved to the more negative potential This phenomenon can be ascribed to the limited ion diffusion to attain the electronic neutralisation during the reversible reaction at high scan rates [39] In addition, the charge storage abilities of the as-prepared electrode materials were assessed by the conducting galvanostatic charge-discharge analysis (GCD) Fig 5c illustrates the galvanostatic charge-discharge profiles of the nanocomposites obtained at the following conditions: working potential is ranging from to V at 0.5 A$gÀ1 From Fig 5c, the good capacitive activity and the excellent reversibility of the electrode materials can be advocated by the nearly symmetrical charge-discharge curves [26] The non-linearity of these curves could be attributed to the pseudocapacitive nature of the Co3V2O8 micro-pencils and the occurrence of the faradiac redox reaction, which tally with the CV Fig CV analysis of (a) pure Co3V2O8, G-CVO, G-2CVO, G-4CVO and G-6CVO nanocomposites at a scan rate of 50 mVsÀ1 (b) G-4CVO at scan rates of 25, 50 and 100 mVsÀ1 and GCD profiles of (c) pristine Co3V2O8, G-CVO, G-2CVO, G-4CVO and G-6CVO nanocomposites at 0.5 A gÀ1, (d) G-4CVO at 0.5, 0.8, 1, 1.5 and A gÀ1 Fig (a) Specific capacitances of G-4CVO at current densities of 0.5, 0.8, 1, 1.5 and A$gÀ1, (b) Cycling analysis (Purple) and Faraday efficiency (Cyan) of G-4CVO electrode obtained after 5000 charge-discharge cycles W.H Low et al / Journal of Science: Advanced Materials and Devices (2019) 515e523 profiles [5] It is noteworthy that G-4CVO possessed the longest discharge time as compared to the pure Co3V2O8 and other nanocomposites revealing its excellent charge storage performance As for all the electrode materials, their specific capacitances were computed by the Equation (5): Cs ¼ I Â Dt ðDV Â mÞ (5) where Cs is the specific capacitance (in F$gÀ1), DV is the potential window (in V), Dt is the discharge time (in s), I is the discharge current (in A), m is the mass of the electrode materials (in g) and is the constant multiplier to convert two electrodes into a single one The computed specific capacitance of G-4CVO was about 528.17 F$gÀ1 at 0.5 A$gÀ1, which outperformed the pure Co3V2O8 (267.85 F$gÀ1), G-CVO (368.59 F$gÀ1), G-2CVO (460.53 F$gÀ1) and G-6CVO (406.25 F$gÀ1) This can be attributed to the inimitable nanoarchitecture of the nanocomposite and the effective hybridisation of the ample Co3V2O8 micro-pencils onto the surface of the graphene sheets Nonetheless, overloading of the Co3V2O8 micropencils in G-6CVO results in the deterioration on the specific capacitance This can be ascribed to the agglomeration of the nanocomposite, leading to the loss of exposure active sites [14,36] Fig 5d demonstrates the charge-discharge profiles of G-4CVO at different current densities of 0.5, 0.8, 1, 1.5 and A$gÀ1 From Fig 5d, the shortened discharge time at the higher current densities can be elucidated based on the ion transfer resistance and the ion diffusion time At the high current density, the larger ion transfer resistance and the limited diffusion time hinder the diffusion process of the electrolyte ions from the surface of the electrode material to its interior [39] Besides, the specific capacitances of G4CVO were calculated at different current densities and the outcomes are shown in Fig 6a The calculated specific capacitances of G-4CVO at various current densities were as follows: 528.17 F$gÀ1 (0.5 A$gÀ1), 507.04 F$gÀ1 (0.8 A$gÀ1), 492.96 F$gÀ1 (1 A$gÀ1), 485.92 F$gÀ1 (1.5 A$gÀ1) and 464.79 F$gÀ1 (2 A$gÀ1) In addition, a capacitance retention of 80% was achieved even when a high current density of A$gÀ1 was used, indicating its superior rate capability The outstanding rate capability of G-4CVO is due to the positive interaction between the graphene and the Co3V2O8 micro-pencils, the large specific surface area and the well preserved nanostructures [26,37] Furthermore, the cycling stability of the G-4CVO electrode was determined over 5000 cycles at 0.5 A$gÀ1 and the result is portrayed in Fig 6b After 5000 cycles of a continuous GCD process, the specific capacitance of the G-4CVO nanocomposite declined from 528.17 to 422.54 F$gÀ1 (i.e 80% retention of its initial capacitance), implying its eminent electrochemical cycling stability This exceptional cycle-ability is believed due to the strong interaction between the graphene sheet and the Co3V2O8 micro-pencils and that the presence of the Co3V2O8 micro-pencils impede the graphene from restacking during the cycling analysis [37] In addition, the graphene nanosheets (buffering scaffold) in the G-4CVO nanocomposite alleviated the volume changes of the active material (expansion and contraction) during the continuous cyclic analysis, leading to the eminent cycling stability [38] Besides, the coulombic efficiency (h) of the optimised electrode material (G-4CVO) over the charge-discharge cycles was computed by Equation (6) and the corresponding results are delineated in Fig 6b: h¼ tD Â 100% tC 521 The coulombic efficiency of G-4CVO was initially about 41% and it increases continuously to nearly 86% after 5000 cycles, further confirming its good electrochemical reversibility [38] The EIS test was conducted to further evaluate the electrochemical properties of the as-prepared electrode materials and the corresponding Nyquist plots of the pure Co3V2O8 and the graphene/ Co3V2O8 nanocomposites are plotted in Fig As seen from Fig 7, all the plots present a quasi-semicircle (denoted as charge transfer resistance, Rct) and the oblique line (Warburg impedance) in the high and low frequency regions, respectively In the high frequency region, the interception with the real x-axis is regarded as the solution resistance (Rs), which is a combination of the internal resistances of the electrode material, namely the electrolyte resistance as well as the contact resistance at the interfaces of electrode/electrolyte [26,40] Here, the Rs values ranging from 0.8 to 1.1 U were delivered by all the electrode materials, suggesting their low internal resistances and high conductivity of the electrolyte [11,41] Besides, the Rct values of the pure Co3V2O8 and the graphene/Co3V2O8 nanocomposites were obtained in the following order: G-4CVO (1.25 U) < G-2CVO (1.73 U) < G-6CVO (2.11 U) < G-CVO (2.38 U) < CVO (4.22 U) Other than improving the electrical conductivity of the electrode material, the integration of graphene with Co3V2O8 also shorten the charge transfer pathway, which in turn contributes to the lowest Rct value and enhanced the electrochemical performance of G-4CVO [42] The feasibility of the electrode materials for practical supercapacitor application can be evaluated based on two important criteria, namely the associated energy and power densities A Ragone plot illustrating these two important characteristics of the pure Co3V2O8 and the graphene/Co3V2O8 nanocomposites is depicted in Fig It can be observed from Fig that the G-4CVO electrode outperformed the pure Co3V2O8, other combinations of the graphene/ Co3V2O8 nanocomposites and the symmetric/asymmetric based supercapacitor reported in other studies [5,11,16,27,43,44], as indicated by its relatively high energy and power densities of 73 Wh/kg and 41 kW/kg, respectively This suggests the capability of the graphene/Co3V2O8 nanocomposite based supercapacitor as a credible energy storage device There are several factors contributing to the superior energy storage ability of the graphene/Co3V2O8 nanomaterials: (1) The intimate contact between the graphene sheet and the Co3V2O8 (6) where tC is the charge time (in s) and tD is the discharge time (in s) Fig Nyquist plots of pristine Co3V2O8 and graphene/Co3V2O8 nanocomposites in the symmetrical two-electrode system 522 W.H Low et al / Journal of Science: Advanced Materials and Devices (2019) 515e523 Appendix A Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jsamd.2019.10.001 References Fig Ragone plot nanocomposites of the pristine Co3V2O8 and the graphene/Co3V2O8 micro-pencils guarantees an excellent electrical conductivity and shorten the ion diffusion pathway substantially, which is beneficial for the transfer of ions and charges and the acceleration of the faradiac redox reaction [44,45] (2) The role of the graphene sheet as a buffering matrix in alleviating the volume variation during the intercalation-deintercalation process [45] (3) The structure of the liquid phase exfoliated graphene nanosheets can be well preserved, leading to the advanced supercapacitive performance [43] (4) The homogenous dispersion of the Co3V2O8 micro-pencils on the surface of the graphene sheets offers a large accessible specific surface area for the electrolyte ions, assuring the effective utilisation of the active materials [46] Conclusion In summary, the Co3V2O8 micro-pencils were successfully decorated on the graphene nanosheets via the solvothermal technique followed by an annealing process The FESEM and TEM analysis confirmed the exfoliation of the graphene sheet and it was uniformly coated with Co3V2O8 micro-pencils Benefiting from the unique microstructure of Co3V2O8 and the strong interactive effect between the Co3V2O8 and graphene, the optimised graphene/ Co3V2O8 nanocomposite (G-4CVO) holds great potential as an effective electrode material for an advanced supercapacitor system, as affirmed by its remarkable specific capacitance of 528.17 F$gÀ1 at a current density of 0.5 A$gÀ1 Furthermore, the electrode material maintained 80% of its charge storage capability after 5000 cycles, signifying its outstanding cycling stability Additionally, G-4CVO delivered impressive energy and power densities (73 Wh/kg at 41 kW/kg) These eminent electrochemical properties suggested that the graphene/Co3V2O8 nanocomposite (i.e the electrode material) is promising for future supercapacitor applications Declaration of Competing Interest There are no conflicts to declare Acknowledgments The financial support from the Ministry of Higher Education, Malaysia with the FRGS grant code of FRGS/1/2016/STG02/UNIM/ 02/1 and the technical support from The University of Nottingham Malaysia Campus are highly acknowledged [1] L Deng, J 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active electrode was obtained by coating the as-prepared paste on an aluminium foil and drying the coated aluminium... obtained at various scan rates (25, 50 and 100 mVsÀ1) The outstanding rate capability and rapid electrochemical response of G-4CVO was indicated by the unaltered CV curves at a wide range of. .. charge storage abilities of the as-prepared electrode materials were assessed by the conducting galvanostatic charge- discharge analysis (GCD) Fig 5c illustrates the galvanostatic charge- discharge