80% was achieved even when a high current density of 2 A $g 1 was used, indicating its superior rate capability. The outstanding rate capability of G-4CVO is due to the positive interac[r]
Trang 1Original Article
enhanced charge storage performance in electrochemical capacitors
a Department of Chemical and Environmental Engineering, Faculty of Engineering, The University of Nottingham Malaysia Campus, Jalan Broga, 43500,
Semenyih, Selangor, Malaysia
b Low Dimensional Materials Research Center, Department of Physics, Faculty of Science, University of Malaya, 50603, Kuala Lumpur, Malaysia
c Faculty Science and Technology, School of Applied Physics, University Kebangsaan Malaysia, 43600, Bangi, Selangor, Malaysia
d Center of Nanotechnology and Advanced Materials, Faculty of Engineering, University of Nottingham Malaysia Campus, Jalan Broga, 43500, Semenyih,
Selangor, Malaysia
a r t i c l e i n f o
Article history:
Received 7 July 2019
Received in revised form
26 September 2019
Accepted 6 October 2019
Available online 14 October 2019
Keywords:
One-pot solvothermal
Graphene
Graphene/Co 3 V 2 O 8
Supercapacitor
a b s t r a c t The electrochemical capability for the charge and energy storage of supercapacitors can be augmented by fabricating the hybrid and binder-free electrodes In this work, the novel graphene/Co3V2O8 micro-pencils nanohybrids were successfully developed via the one-pot solvothermal method The interac-tive effect between graphene and Co3V2O8was investigated by varying their mass ratios Benefiting from the peculiar morphology of Co3V2O8micro-pencils and the homogenous distribution of Co3V2O8on the graphene sheets, G-4CVO manifested a relatively promising specific capacitance of 528.17 F$g1 at 0.5 A$g1while 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/)
1 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,
excel-lent 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]
Nonethe-less, relatively poor energy density of supercapacitors has hindered
their commercial applications The electrode materials and
morphologies are widely acknowledged as the decisive variables affecting the capacitive activity of supercapacitors [1,5
In consideration of the mechanism of charge storing, super-capacitors 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 V2O5have 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 con-ductivity 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
* Corresponding author.
E-mail address: poisim.khiew@nottingham.edu.my (P.S Khiew).
Peer review under responsibility of Vietnam National University, Hanoi.
Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d
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
Journal of Science: Advanced Materials and Devices 4 (2019) 515e523
Trang 2diverse 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
(sig-nificant contribution to specific capacitance) and multiple
poly-morphism 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$g1specific capacitance at 0.625 A$g1 In addition,
Co3V2O83D porous nanoroses were fabricated by Zhang et al [10]
through the solvothermal method The 3D porous nanoroses
dis-played a specific capacitance of 371.3 F$g1when the electrode is
analysed under a current density of 0.5 A$g1 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 Co3V2O8with 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
gra-phene/Co3V2O8micro-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 thefirst 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 Co3V2O8on the electrochemical performance of the
nanocomposite was investigated by preparing different mass ratios
of graphene/Co3V2O8nanomaterials
2 Experimental
2.1 Materials and chemicals
Highly pyrolytic graphiteflakes (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
nano-composites Carbon black (Alfa Aesar), polyvinylidene fluoride
Sigma-eAldrich) and N-methyl-2-pyrrolidinone (NMP, SigmaSigma-eAldrich)
were used to fabricate the electrode
2.2 Fabrication of graphene and graphene/Co3V2O8micro-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 4 h under ambient condition The graphene solution was then washed with the ethanol-water solution through centri-fugation and dried at 80C overnight As for graphene/Co3V2O8
nanocomposites, the solvothermal technique was applied and the entire procedure is delineated inFig 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
200C for 16 h After that, the as-synthesised precipitates were collected, cleaned thoroughly with ethanol-water solution, dried at
80C in the hot air oven for 6 h and annealed at 500C for 4 h to obtain the graphene/Co3V2O8 micro-pencils nanomaterial Pure
Co3V2O8was 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 inTable 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 molecularfingerprints of the nanocomposites were analysed using Raman spectroscopy and recorded on the Renishaw inVia spec-trometer The morphology of the nanocomposites was charac-terised byfield emission scanning electron microscopy (FESEM, Quanta 400F USA) and transmission electron microscopy (TEM, JEOL-2100F Japan) The elemental composition and surface chem-ical states of the nanocomposites were assessed by energy
spectroscopy (XPS, Thermo Scientific ESCAlab 250Xi), respectively 2.4 Electrochemical measurements
The electrochemical performances of the active electrode ma-terials were characterised via cyclic voltammetry (CV) and elec-trochemical impedance spectroscopy (EIS) analysis using a Metrohm Autolab Potentiostat (PGSTAT302F)) The galvanostatic charge-discharge (GCD) is conducted using the Arbin multi-channel galvanostat (BT-2000) A symmetric two electrodes configuration with 2 mol/L aqueous KOH as electrolyte was fabri-cated 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 80C for 6 h
3 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,
Co3V2O8micro-pencils and graphene/Co3V2O8nanohybrids The graphene nanosheet exhibited two broad peaks at 26.8and 54.9, which correspond to the (002) and (004) crystal planes of graphene and this is in accordance with our previous reports [5,16]
Trang 3From the diffraction spectrum of the graphene/Co3V2O8
nano-composites, 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 1 (a) XRD patterns of graphene, Co 3 V 2 O 8 micro-pencils and graphene/Co 3 V 2 O 8 nanocomposites, (b) Raman spectra of the graphene and G-4CVO nanocomposite.
W.H Low et al / Journal of Science: Advanced Materials and Devices 4 (2019) 515e523 517
Trang 4structures might enhance the electrochemical stability by
allevi-ating the volume variation during the continuous
intercalation-deintercalation process [19] These results reveal that the
gra-phene/Co3V2O8hybrid nanomaterials comprising of cubic Co3V2O8
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 cm1, respectively The G band corresponds to the
in-plane bond stretching motion of C sp2atoms, while the D and 2D
bands represent the disorder or defective graphitic structure
[19,20] Besides, two peaks at around 335 and 810 cm1 are
observed, which can be correlated to the Raman spectra of Co3V2O8
[21] The bands at 335 cm1and 810 cm1are 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/
Co3V2O8nanocomposites 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 2
FromFig 2b, it can be noticed that Co3V2O8possess an exterior
geometry of short pencil-like structure with hexagonal prisms and
these Co3V2O8 micro-pencils have a consistent microscale size
distribution of approximately 3mm in length and 5mm 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 inFig 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
gra-phene nanomaterial also serves as a conductive scaffold to facilitate
the ions and charges migration [24,25] The morphological
dis-parities of the graphene/Co3V2O8nanocomposites with a variation
of mass loadings are indicated inFig 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 Co3V2O8micro-pencils available to perfectly cover the
whole surface of graphene and this configuration is believed to
contribute significantly towards the enhancement of the
electro-chemical activity However, excess mass loading of precursors in
G-6CVO (Fig 2f) resulted in structural agglomeration, which
inevi-tably reduces the homogeneity and the active sites of the
nano-composites, 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 do not stack together
and the high density of the Co3V2O8micro-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
as-synthesised 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 Co3V2O8on the graphene surface The atomic ratio of Co:V inTable S2was approximately 1.54:1, which coincides with the stoichiometric ratio
of Co3V2O8 The XPS spectrum of G-4CVO is delineated inFig 4to give an insight on the chemical composition and the oxidation states of the graphene/Co3V2O8sample
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: Co2þ(779.4 and 795.8 eV) and Co3þ(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 2p3/2and V 2p1/2of V5þ 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 3fittings 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 po-tential ranging from1 to 1 V with the scanning rate of 5 mVs1are
delineated inFig 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 Co2þ/Co3þ with the OH ions is
Trang 5reversible and hence contributing to these didymous redox peaks.
The corresponding faradiac electrochemical reactions are described
in Equations(1)e(4), [33,34]:
CoðOHÞ2þ OH4 CoOOH þ H2Oþ e (3)
CoOOHþ OH4CoO2þ H2Oþ e (4)
Besides, the CV curves of the anode and cathode are almost
symmetrical, reflecting their superior reversibility and close-to-ideal capacitive behaviour The enhanced charge storage ability of the graphene/Co3V2O8nanocomposites is reflected by their larger inte-grated 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 Co3V2O8micro-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> G-2CVO> G-6CVO > G-CVO, suggesting that the amount of Co3V2O8
anchoring on the surface of the graphene nanomaterial has a
W.H Low et al / Journal of Science: Advanced Materials and Devices 4 (2019) 515e523 519
Trang 6substantial 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 mVs1) 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 revers-ible reaction at high scan rates [39]
In addition, the charge storage abilities of the as-prepared electrode materials were assessed by the conducting galvano-static charge-discharge analysis (GCD).Fig 5c illustrates the gal-vanostatic charge-discharge profiles of the nanocomposites obtained at the following conditions: working potential is ranging from 0 to 1 V at 0.5 A$g1 FromFig 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 pseu-docapacitive nature of the Co3V2O8micro-pencils and the occur-rence of the faradiac redox reaction, which tally with the CV
Fig 5 CV analysis of (a) pure Co 3 V 2 O 8 , G-CVO, G-2CVO, G-4CVO and G-6CVO nanocomposites at a scan rate of 50 mVs1 (b) G-4CVO at scan rates of 25, 50 and 100 mVs1and GCD profiles of (c) pristine Co 3 V 2 O 8 , G-CVO, G-2CVO, G-4CVO and G-6CVO nanocomposites at 0.5 A g1, (d) G-4CVO at 0.5, 0.8, 1, 1.5 and 2 A g1.
Fig 6 (a) Specific capacitances of G-4CVO at current densities of 0.5, 0.8, 1, 1.5 and 2 A$g 1 , (b) Cycling analysis (Purple) and Faraday efficiency (Cyan) of G-4CVO electrode obtained
Trang 7profiles [5] It is noteworthy that G-4CVO possessed the longest
discharge time as compared to the pure Co3V2O8and other
nano-composites revealing its excellent charge storage performance As
for all the electrode materials, their specific capacitances were
computed by the Equation(5):
Cs¼ 4
I Dt
ðDV mÞ
(5)
where Csis the specific capacitance (in F$g1),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 4 is
the constant multiplier to convert two electrodes into a single one
528.17 F$g1at 0.5 A$g1, which outperformed the pure Co3V2O8
(267.85 F$g1), G-CVO (368.59 F$g1), G-2CVO (460.53 F$g1) and
G-6CVO (406.25 F$g1) This can be attributed to the inimitable
nanoarchitecture of the nanocomposite and the effective
hybrid-isation of the ample Co3V2O8micro-pencils onto the surface of the
graphene sheets Nonetheless, overloading of the Co3V2O8
micro-pencils 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 2 A$g1 From
Fig 5d, the shortened discharge time at the higher current densities
can be elucidated based on the ion transfer resistance and the ion
diffusion time At the high current density, the larger ion transfer
resistance and the limited diffusion time hinder the diffusion
pro-cess of the electrolyte ions from the surface of the electrode
ma-terial to its interior [39] Besides, the specific capacitances of
G-4CVO were calculated at different current densities and the
out-comes are shown inFig 6a
The calculated specific capacitances of G-4CVO at various
cur-rent densities were as follows: 528.17 F$g1 (0.5 A$g1), 507.04
F$g1(0.8 A$g1), 492.96 F$g1(1 A$g1), 485.92 F$g1(1.5 A$g1)
and 464.79 F$g1(2 A$g1) In addition, a capacitance retention of
80% was achieved even when a high current density of 2 A$g1was
used, indicating its superior rate capability The outstanding rate
capability of G-4CVO is due to the positive interaction between the
graphene and the Co3V2O8micro-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$g1and the result is portrayed inFig 6b After
5000 cycles of a continuous GCD process, the specific capacitance of
the G-4CVO nanocomposite declined from 528.17 to 422.54 F$g1
(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
Co3V2O8micro-pencils impede the graphene from restacking
dur-ing the cycldur-ing 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 inFig 6b:
h¼tD
where t is the charge time (in s) and t is the discharge time (in s)
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 electro-chemical properties of the as-prepared electrode materials and the corresponding Nyquist plots of the pure Co3V2O8and the graphene/
Co3V2O8nanocomposites are plotted inFig 7
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 Rsvalues ranging from 0.8 to 1.1Uwere delivered by all the electrode ma-terials, suggesting their low internal resistances and high conduc-tivity of the electrolyte [11,41] Besides, the Rctvalues of the pure
Co3V2O8 and the graphene/Co3V2O8 nanocomposites were ob-tained in the following order: G-4CVO (1.25 U) < G-2CVO (1.73U)< G-6CVO (2.11U)< G-CVO (2.38U)< CVO (4.22U) Other than improving the electrical conductivity of the electrode mate-rial, 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 super-capacitor 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 inFig 8
It can be observed fromFig 8that the G-4CVO electrode out-performed 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 indi-cated by its relatively high energy and power densities of 73 Wh/kg and 41 kW/kg, respectively This suggests the capability of the gra-phene/Co3V2O8nanocomposite 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
Fig 7 Nyquist plots of pristine Co 3 V 2 O 8 and graphene/Co 3 V 2 O 8 nanocomposites in the symmetrical two-electrode system.
W.H Low et al / Journal of Science: Advanced Materials and Devices 4 (2019) 515e523 521
Trang 8micro-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 Co3V2O8micro-pencils on the
sur-face of the graphene sheets offers a large accessible specific surface
area for the electrolyte ions, assuring the effective utilisation of the
active materials [46]
4 Conclusion
In summary, the Co3V2O8 micro-pencils were successfully
decorated on the graphene nanosheets via the solvothermal
tech-nique followed by an annealing process The FESEM and TEM
analysis confirmed the exfoliation of the graphene sheet and it was
uniformly coated with Co3V2O8micro-pencils Benefiting from the
unique microstructure of Co3V2O8 and the strong interactive
effect between the Co3V2O8and 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$g1at
a current density of 0.5 A$g1 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/Co3V2O8nanocomposite (i.e the electrode
ma-terial) is promising for future supercapacitor applications
Declaration of Competing Interest
There are no conflicts to declare
Acknowledgments
Thefinancial 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
Appendix A Supplementary data Supplementary data to this article can be found online at
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