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A Binder-Free Hybrid of CuO-Microspheres and rGONanosheets as an Alternative Material for Next GenerationEnergy Storage ApplicationMohit Saraf,[a] Riyaz A. Dar,[d] Kaushik Natarajan,[a] Ashwini K. Srivastava,[e] and ShaikhM. Mobin *[a, b, c]

Herein, a facile and effective ultrasonication assisted approachhas been employed to integrate hydrothermally produced CuOmicrospheres (CMS) with chemically synthesized reduced gra-phene oxide conducting nanosheets (CRGO), yielding a newcomposite material (CSCO). The synthesized materials havebeen probed by various characterization techniques. The elec-trochemical performance of all the materials are thoroughlystudied by cyclic voltammetry (CV), galvanostatic charging-dis-charging (GCD) and electrochemical impedance spectroscopy(EIS) techniques. At a current density of 0.125 A g�1, CSCO dem-onstrates an excellent specific capacitance of 244 F g�1 (morethan six times higher than CMS and CRGO) and the maximumenergy and power density values of 8.47 Wh kg�1 and 73.49 W

kg�1, respectively. Additionally, it exhibits high rate perform-ance (retains 82.78 % of its initial capacitance even at a highcurrent density of 0.5 A g�1) and long cycle life (~ 90 % up to1000 cycles), confirming the robustness and high stability ofthe composite on the electrode surface. The superior electro-chemical performance can be attributed to the positive syner-gistic effects between CMS and CRGO, providing mechanicalrobustness and short diffusion path for charge transfer, leadingto high conductivity. Distinguishing features such as beingbinder-free, ease of preparation, high energy and power den-sity, make CSCO a potential candidate for high performance su-percapacitors.

Introduction

Due to global warming and fuel crisis, there has been a drasti-cally increasing demand in last few years for sustainable energybased on highly efficient renewable technologies. To meet rap-idly increasing energy demands, development of high perform-ance, low cost, environment friendly energy storage devices isessential and still remains a great challenge.[1–4] As one of themost promising types of energy storage devices, super-capacitors or ultracapacitors have drawn great interest owingto their distinguishable features such as higher power densityand longer cyclic life than conventional batteries, higher en-

ergy density than capacitor, high rate capacity, rapid chargedischarge mechanism and environment friendliness.[5–11]

Based on the charge storage mechanism, supercapacitorscan be divided into two types involving, surface faradaic re-actions (pseudocapacitors) and ion adsorption at the electro-lyte/electrode interface (electric double layer capacitors,EDLCs).[12–13] Studies have shown that electrode materials areprimarily responsible for overall supercapacitor performance. Inthis respect, conducting polymers (e. g. polyaniline, polypyrrole)and transition metal oxides such as NiO,[14] Co3O4

[15–18] andMnO2

[19] are among the most commonly employed electrodematerials for pseudocapacitors. However, these materials sufferfrom severe structural degradation during the cycling processas well as low electrical conductivity, deteriorating their powercapability at high rates which leads to poor cycling perform-ance, making them unfeasible for practical purposes.[20] On theother hand, EDLCs primarily involve carbon based materials(e. g., reduced graphene oxide or activated carbon) as the elec-trode material and accumulate charges in the electric doublelayer. Although, EDLCs can be operated at much higher ratesthan batteries, their energy densities are relatively lower. An al-ternative strategy to enhance the energy density can be toconstruct hybrid supercapacitors, where one pseudocapacitiveelectrode replaces carbon electrodes,[21] which can store morecharges via fast and reversible faradaic redox reactions andhigher energy density can be achieved due to the enhancedcapacitance. Such types of supercapacitor involve both physicalas well as chemical charge storage mechanism resulting in su-perior performance in terms of cyclic stability, energy density

[a] M. Saraf, K. Natarajan, Dr. S. M. MobinCentre for Material Science and EngineeringIndian Institute of Technology Indore, Simrol, Indore-452020, IndiaTel: + 91 731 2438 762E-mail: [email protected]

[b] Dr. S. M. MobinDiscipline of Chemistry, School of Basic SciencesIndian Institute of Technology Indore, Simrol, Indore-452020, India

[c] Dr. S. M. MobinCentre for Bioscience and Biomedical EngineeringIndian Institute of Technology Indore, Simrol, Indore-452020, India

[d] Dr. R. A. DarDepartment of Chemistry, Maharashtra College of Arts, Science andCommerce, Nagpada, Mumbai-400008, India

[e] Prof. A. K. SrivastavaDepartment of Chemistry, University of Mumbai, Vidyanagari, Mumbai-400098, India

Supporting information for this article is available on the WWW underhttp://dx.doi.org/10.1002/slct.201600481

Full PapersDOI: 10.1002/slct.201600481

2826ChemistrySelect 2016, 1, 2826 – 2833 � 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

http://dx.doi.org/10.1002/slct.201600481

and power density.[21–22] Recent literature reports several meth-ods for the fabrication process of CuO-carbon materials ascomposite for enhanced capacitance owing to promotional ef-fects of carbon nanomaterials.[23] For example, Zhao et al.showed the enhanced electrochemical throughput by the hy-drothermally synthesized leaf-like porous copper oxide-gra-phene nanostructure.[24] Similarly, Zhang et al. synthesized com-posite of CuO-single wall carbon nanotubes based flexibleelectrode and demonstrated high charge storage efficiency andcyclic stability than that of pure nanotube material.[25]

Among transition metal oxides, pseudocapacitive CuO hasbeen widely acknowledged due to its abundance in nature, fac-ile synthesis, low cost, environmental stability, low toxicity anddesirable electrochemical properties.[26–28] In past few years,CuO of different morphologies and sizes have been employedwidely for energy storage applications.[22–25] However, they suf-fer from several drawbacks like poor cyclic stability, power den-sity and rate capability. In this respect, micro/nano structures ofCuO can be a potential candidate for energy storage applica-tions, particularly due to high surface area and porosity whichis suitable for achieving high electrochemical performance.[29] Inthis concern, biomolecules such as glucose, sucrose and ty-rosine etc. have emerged as important templates owing totheir captivating self-assembling properties and unique fea-tures, suitable for the development of nanostructures.[30, 31] Re-cently, glucose was employed as a bio-template for con-struction of copper micropuzzles,[31] however glucosetemplated synthesis suffers from complex redox reactions dur-ing carbonization step between metal salts and glucose. Toavoid such undesired redox reactions throughout the carbon-ization process, sucrose can be a suitable template due to itsnon-reducing nature.[31, 32] On the contrary, reduced grapheneoxide (rGO) or graphene is a potential EDLC material encom-passing 2D single layer sheets of sp2 hybridized conjugated car-bon atom and has been proved to be one of the most essentialcarbon based materials for potential applications due to itshigh conducting nature, excellent mechanical strength andlarge surface area.[33–40] Further, carbon nanostructures providenot only high conductivity to improve power density and rateability at high current densities but also act as support matrixfor metal oxides to buffer the volume change during charge-discharge process.[23]

In the present work, we have fabricated a hybrid of bio-molecule sucrose templated, hydrothermally driven, pseudoca-pacitive CuO microspheres (CMS) and chemically synthesizedelectric double layer capacitive rGO nanosheets (CRGO) using afacile ultrasonication assisted method (Scheme 1). Further, thecomposite (CSCO) was explored as an electrode material to in-vestigate its potential capability for high-performance super-capacitors. To the best of our knowledge, this is the first in-stance, where CuO microspheres (CMS) were hydrothermallyproduced by introducing sucrose as a template and integratedwith rGO nanosheets (CRGO) for supercapacitor application.

Results and Discussion

The crystallinity and phase purity of CMS were analysed bypowder X-ray diffractometer (XRD) in the range of 10–808 (Fig-ure 1). The diffraction peaks for CMS (Figure 1a) at 2q= 32.508,

35.688, 38.658, 46.158, 48.898, 53.408, 58.178, 61.628, 65.688,66.228, 68.188, 72.278 and 75.088 correspond to (110), (-111),(111), (-112), (-202), (020), (202), (-113), (022), (-311), (113), (311)and (-222) lattice planes respectively, which can be readily in-dexed to monoclinic phase of CuO (JCPDS No. 01–089-5897)with the lattice parameters a = 4.6870 �, b = 3.4220 �, c =

5.1300 �, a=g= 908, b= 99.508. The average crystallite sizewas calculated to be 27.4 nm using Debye-Scherrer equation.Additionally, no diffraction peak was detected corresponding toany impurity signifying the high phase purity of material.[32] Fig-ure 1b presents the diffraction pattern for bare CRGO, where

Scheme 1. Flowchart of synthesis of CSCO.

Figure 1. PXRD patterns of (a) CMS (b) CRGO and (c) CSCO.

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peaks located at 2q= ~258 and ~438 correspond to (002) and(100) planes of graphite like structure verifies the formation ofreduced graphene oxide. Further, in the diffraction pattern ofCSCO (Figure 1c) a significant peak at 2q= ~258 can be seenconfirming the presence of CRGO. Additionally, the prominentcrystalline peaks of copper oxide were also retained confirmingthe presence of CMS in the CSCO.

The surface morphology of the prepared materials was pro-bed by using FESEM (Figure 2). Figure 2a presents the FESEM

image of hydrothermally produced copper oxide exhibitingspheres of diameters ranging from 1–6 mm.[32] Figure 2b pres-ents the FESEM image of rGO showing curly and corrugatedappearance, forming agglomerates. The curly morphology pre-vents restacking and provides porosity to the sheets. The wrin-kles present on these conducting sheets improve the electro-chemical performance by shortening the ion diffusion path,ultimately leading to enhanced ion transportation on electrodesurface.[39–43] Further, the surface morphology of composite ma-terial shows the proper embedment of CuO microspheres with-in rGO matrix, which in turn provides mechanical strength andimproves electrochemical performance (Figure 2c).

The obtained materials were further characterized usingTEM. (Figure S1a in the Supporting Information) shows the TEMimage of CMS, confirming the presence of smooth micro-spheres. These microspheres are composed of aggregated CuOnanoparticles, providing porosity to the material. [32] In (Fig-ure S1b in the Supporting Information), well defined reducedgraphene oxide thin sheets with wrinkled surface are observed,agreeing with FESEM results. (Figure S1c in the Supporting In-formation) shows the TEM image of composite material CSCOand reveals that individual CuO nanoparticles are well dis-tributed within CRGO matrix. These results show that this prop-er embedment of CMS with CRGO leads to positive synergism,

providing high mechanical strength as well as short diffusionlength for charge transfer and increases the conductivity ofcomposite, which fosters enhanced electrochemical output.(Figure S2, Supporting Information) presents the IR spectra of(a) CMS, (b) CRGO and (c) CSCO. The prominent peak located at3404 cm�1 in CRGO and CSCO is due to O�H stretching of re-duced graphene oxide. Further, the peaks at 2890 cm�1 and2850 cm�1 are attributed to the stretching vibrations of -CH3

and -CH2, respectively. The spectra show a stretching vibrationof C=O at 1737 cm�1, and the peak around 1673 cm�1 is therepresentative of aromatic C=C stretching vibration. The ab-sorption band at around 1060 cm�1 can be assigned to thestretching vibration of C–O. It was observed that, the peak in-tensities of C=O and C�O groups decreases significantly in theFTIR spectrum of CSCO, which describes the probable inter-action between CMS and CSCO (Figure S2c, Supporting In-formation).

Figure 3 presents the cyclic voltammograms (CVs) of CMS/GCE, CRGO/GCE and CSCO/GCE in 1 M Na2SO4 solution. The

comparison of CV profiles of all three electrodes (CMS/GCE,CRGO/GCE and CSCO/GCE) at a scan rate of 100 mV s�1 is pre-sented in Figure 3a. It is clear that CSCO/GCE demonstrates ex-cellent charge propagation and enhanced integrated area dueto cumulative interaction of CMS and CRGO. The CVs of all thethree electrodes at different scan rates are presented in Fig-ure 3 (b-d) and it is observed that the area under the CV curvesincreases with the increasing scan rate. As expected, CV ofCMS/GCE doesn’t show an ideal rectangular nature due to itspseudocapacitive nature. On the contrary, CV of CRGO/GCE ex-hibits nearly rectangle type behaviour indicating good chargepropagation at the electrode surface following electric doublelayer charging (EDLC) mechanism. In contrast, CSCO/GCE dem-

Figure 2. FESEM images of (a) CMS, (b) CRGO and (c) CSCO.

Figure 3. (a) Comparison of CV profiles of CMS/GCE, CRGO/GCE and CSCO/GCE at a scan rate of 100 mV s�1 and CV curves of (b) CMS/GCE, (c) CRGO/GCE and (d) CSCO/GCE at different scan rates using 1 M Na2SO4 as support-ing electrolyte..

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onstrates excellent charge propagation and enhanced in-tegrated area due to healthy alliance between CMS and CRGO.The combination of pseudocapacitive process due to CMS andelectric double layer charging mechanism due to CRGO greatlyimproves overall charge propagation process. Therefore, theoverall capacitance of CSCO is the result of synergy betweenCMS and CRGO. Here, CRGO not only serves as the physicalsupport of CMS but also provide the channels for charge trans-port. Additionally, the high electronic conductivity of CRGO im-proves rate capability and power density at a large charge/dis-charge current. The CMS is the main source that stores thecharge. The electro-activities of CMS contribute to high specificcapacitance and high energy density of the CSCO/GCE. A syn-ergy could be expected between CMS and CRGO, which leadsto the drastic enhancement in the charge storage capacity ofthe composite CSCO. Moreover, the CV curves for CSCO/GCEtry to achieve quasi-rectangular shape which ultimately resultsin better charge storage capacity. [22] From this, it can be con-cluded that at higher scan rates EDLC mechanism predom-inates due to faster diffusion of electrolyte ions into the CMSmaterial. The specific capacitance values calculated from CVloops are ~62 F g�1, ~49 F g�1 and ~247 F g�1 for CMS/GCE,CRGO/GCE and CSCO/GCE at a scan rate of 10 mV s�1, re-spectively.

Figure 4 shows the effect of scan rate on specific capaci-tance for all three electrodes ((a) CRGO/GCE (b) CMS/GCE and

(c) CSCO/GCE), specifying that specific capacitance decreaseswith increasing scan rate because of fast diffusion of electrolyteions. Further, CSCO/GCE is found to have the highest rate per-formance, as it retains 211 F g�1 (85.42 %), of its initial capaci-tance even at a higher scan rate of 200 mV s�1 (Figure 4c).

Figure 5 presents the galvanostatic charge-discharge studyof all three electrodes (CMS/GCE, CRGO/GCE and CSCO/GCE) in

1 M Na2SO4 solution. The comparison of charging dischargingprofiles for CMS/GCE, CRGO/GCE and CSCO/GCE at a currentdensity of 0.125 A g�1 and under open circuit potential isshown in Figure 5a. The charging-discharging time of CSCO/GCE is found to be greater than CMS/GCE and CRGO/GCE, sig-nifying its higher specific capacitance. Figure 5 (b-d) shows thegalvanostatic charging-discharging (GCD) profiles of CMS/GCE,CRGO/GCE, and CSCO/GCE, respectively at different currentdensities i. e. (i) 0.125, (ii) 0.16, (iii) 0.25, (iv) 0.33, (v) 0.41 and (vi)0.50 A g�1. A non-linear charging discharging curve is obtainedfor CMS/GCE, confirming the pseudocapacitive nature of theelectrode material. Furthermore, a small IR drop is also ob-served during the discharging process consisting of two sec-tions i. e. a fast voltage drop, followed by a slow potential de-cay. This potential drop is generated due to the internalresistance of material. The obvious non-ideal rectangular shapeof CV pattern as well as non-linear shape of discharge curve re-veals the pseudocapacitive behavior of CMS. Similarly, GCDprofiles of CRGO/GCE are obtained at different current densitiesand found to be non-linear. However, no potential drop is ob-served, further confirming that CRGO/GCE primarily involvesEDLC mechanism. In contrast, the charging-discharging profilesof CSCO/GCE are observed to be somewhat more linear com-pared to the rest and IR drop was also minimized, inferring thatthe internal resistance has been minimized considerably due topositive symbiosis between CMS and CRGO.

Figure 6a shows the effect of current density on the specificcapacitance for all three electrodes ((i) CRGO/GCE (ii) CMS/GCEand (iii) CSCO/GCE), describing that on increasing the currentdensity, specific capacity decreases, which can be associated to

Figure 4. Specific capacitance vs. scan rate (10-500 mV s�1) for (a) CRGO/GCE,(b) CMS/GCE and (c) CSCO/GCE.

Figure 5. (a) Comparison of galvanostatic charging-discharging profiles ofCMS/GCE, CRGO/GCE and CSCO/GCE at a current density of 0.125 Ag�1.Galvanostatic charging-discharging profiles of (b) CMS/GCE, (c) CRGO/GCE and (d) CSCO/GCE at different current densities ((i) 0.125, (ii) 0.16, (iii)0.25, (iv) 0.33, (v) 0.41 and (vi) 0.50 A g�1).

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the fact that at higher current densities very few electrolyteions can occupy space in the inner spaces of electrode material.The (iii) CSCO/GCE displays high rate ability by retaining82.78 % of its initial capacitance even at a high current densityof 0.5 A g�1, which agrees well with the CV results. The max-imum specific capacitance for CMS/GCE, CRGO/GCE and CSCO/GCE at a current density of 0.125 A g�1 are determined to be40.06 F g�1, 29.06 F g�1 and 244 F g�1, respectively.

The maximum energy and power density for CSCO/GCE arecalculated to be 8.47 Wh kg�1 and 73.49 W kg�1 at a currentdensity of 0.125 A g�1, respectively. The long term cyclic stabil-ity of the CSCO/GCE was also evaluated by repeating the GCDsbetween 0–0.5 V for 1000 cycles at a current density of 0.125 Ag�1. Figure 6b shows the change in specific capacitance withcycle numbers for CSCO/GCE. A high retention capability (~90 %) of specific capacitance is observed for CSCO/GCE up to1000 cycles. Inset of Figure 6b shows first 10 cycles of chargingdischarging profile. The presence of CRGO with CMS seems tosynergistically enhance the stability of the composite, resultingin the retention of specific capacitance for long time. The ex-cellent stability of CSCO reveals the potential capability to useit as an electrode material for charge storage devices. Figure 7

presents the Ragone plots for CMS/GCE, CRGO/GCE and CSCO/GCE, at different current densities. The CSCO/GCE delivers anenergy density of 8.47 Wh kg�1 with a corresponding powerdensity of 73.49 W kg�1, at a current density of 0.125 A g�1;whereas energy density values of 1.390 Wh kg�1 and 1.01 Whkg�1, are obtained for CMS/GCE and CRGO/GCE, respectively.

EIS was also performed to study the charge transfer kineticsand capacitive components of electrode material. In EIS, thedata is generally expressed by Nyquist plots where the semi-circle at higher frequencies correspond to charge transfer re-sistance (Rct) generated by faradaic reaction and electric doublelayer capacitance (Cdl) generated at the electrode/electrolyte in-terface, whereas the semicircle at lower frequencies corre-sponds to the capacitance at the grain boundary and interfacialedge of the electrode, which is typically a diffusion-controlledprocess.[44–45] In our study, the EIS was performed at an appliedpotential of 10 mV in the frequency range of 1 Hz to 50 KHzunder open circuit potential conditions. From the Nyquist plotsof (a) CRGO/GCE, (b) CMS/GCE and (c) CSCO/GCE as shown inFigure 8, it is clear that the semicircle diameter has been re-

duced for CSCO/GCE compared to the other two electrodes (Rct

(CRGO/GCE)>Rct(CSCO/GCE)), suggesting low charge transfer(Rct) as well as interfacial resistance (Rint) of CSCO/GCE. The ACequivalent circuit generated by fitting the data obtained fromEIS spectra is shown in the inset of Figure 8. A lowest value ofRct in case of CSCO/GCE indicates proper fusion between CMSand CRGO, increasing its conductivity and results in enhancedcharge transfer performance, which is indicated in the en-hanced currents observed in CV. The vertical line at lower fre-quencies appears to be almost parallel to the imaginary axis forCSCO/GCE, indicating positive capacitive behavior and repre-sentative of the ion diffusion in the electrode structure.[22] Ta-

Figure 6. (a) Specific capacitance of (i) CRGO/GCE, (ii) CMS/GCE and (iii)CSCO/GCE as a function of current density, (b) Cyclic stability test of CSCO/GCE at a current density of 0.125 A g�1, Inset shows the first 10 cycles ofcharging discharging profile of CSCO/GCE.

Figure 7. Ragone plots for CRGO/GCE, CMS/GCE and CSCO/GCE at differentcurrent densities.

Figure 8. Nyquist plots for (a) CRGO/GCE, (b) CMS/GCE and (c) CSCO/GCE atamplitude of 10 mV vs. Ag/AgCl over the frequency range of 1 Hz to 50 KHz.Here Z’ = real impedance and Z’’= imaginary impedance, Inset shows theequivalent circuit.

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ble 1 summarizes the data obtained after fitting of EIS data tothe circuit.

A comparative statement of some recently reported elec-trode materials is presented in Table 2. From the table, it can beconcluded that CSCO is a potential candidate over many othermaterials for achieving high electrochemical performance andcan be applied as a supercapacitor electrode material. Though,CSCO delivers lower electrochemical response compared to hy-drothermally synthesized CuO/rGO composite[51] this may bedue to absence of any conducting additives like activated car-bon (provides high surface area and increases conductivity)and lack of any binders (responsible for achieving high cyclabil-ity), the ultrasonication driven CSCO has an edge because offacile synthesis procedure. Moreover, the drastic enhancementin specific capacitance (more than six times higher than CMSand CRGO) on a 3 mm diameter electrode with very less massloading shows the perfect synergy between CMS and CRGO,which is highly suitable for a composite based supercapacitor.

Conclusions

In summary, we report a facile hydrothermal technique to fab-ricate CuO microspheres (CMS) and these spheres were in-tegrated with rGO nanosheets (CRGO) by a simple ultra-sonication assisted method. The fabricated composite (CSCO)was used as a modifier for glassy carbon electrode to make itwork as a working electrode for electrochemical testing. InCSCO, the faradaic current due to CMS was accompanied byelectric double layer charging process caused by CRGO. Thesynergy between CMS and CRGO in CSCO, induces high me-chanical strength and robustness, high conductivity, stabilityand short diffusion path for charge transfer, which are favor-able to generate a high charge storage capacity system, with aspecific capacitance of 244 F g�1 at a current density of 0.125 Ag�1, and energy and power densities of 8.47 Wh Kg�1 and 73.49W Kg�1, respectively. Additionally, it exhibits high rate abilityand long cycle life (~90 % up to 1000 cycles). The results showthat CSCO could be considered as a promising electrode mate-rial for energy storage devices. However, further attempts willbe made to improve the specific capacitance, energy and pow-

er density of the synthesized materials by tailoring the size,morphology or altering the reaction conditions.

Suppoting Information

Experimental section, TEM and FTIR images are given in sup-porting information file.

Acknowledgements

S. M. M would like to acknowledge CSIR, New Delhi and IIT In-dore for funding and Sophisticated Instrumentation Centre(SIC), IIT Indore for providing the characterization facility. M. S.would like to thank MHRD, New Delhi, India for providing fel-lowship. We gratefully acknowledge Advance Imaging Center,IIT Kanpur for TEM results. One of the authors (R. A. D.) ac-knowledges UGC-New Delhi for awarding Post-doctoral fellow-ship under its Dr. D. S. Kothari fellowship scheme..

Keywords: Copper oxide microspheres · reduced grapheneoxide · pseudocapacitors · electric double layer capacitors ·specific capacitance

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Table 1. Circuit parameters obtained from fitting of EIS spectra.

Parameter CRGO CMS CSCO

Rs (W) 13.33 11.17 9.179Rct (W) 43.06 39.17 36.38Cdl (F) 0.85x10�6 0.62x10�6 0.543x10�6

Rint (W) 216.4 220.5 204.7Cint (F) 0.1373x10�3 0.110x10�3 0.113x10�3

Where: Rs correspond to resistance due to test set up and electrolyte. Rint

corresponds to resistance at electrode/electrolyte interface. Cint is thecapacitance generated due to formation of depletion region Cdl is thecapacitance at electrode/electrolyte interface capacitance. Rct is the chargetransfer resistance inside the material.

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4863–4868.[42] Y. Bai, R. B. Rakhi, W. Chen, H. N. Alshareef, J. Power Sources 2013, 233,

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Table 2. A comparison of some recent supercapacitor electrode materials.

Electrode Material Binder SupportingElectrolyte

Specific ca-pacitance(F g�1)

CurrentDensity(A g�1)

Energydensity(Whkg�1)

Powerdensity(W kg�1)

Retention of Specif-ic Capacitance

Reference

3D graphenemacroforms

Binderfree

1 M H2SO4 230 0.1 8.3 24.5 98.5 % after 10000cycles

46

CuO nanoparticles on graphene oxidenanosheets

Binderused

1 M Na2SO4 245 0.1 - - 79 % after 1000 cy-cles

47

(P(NiPPAm-GO-AA)) one dimensional pho-tonic crystals

Binderused

1 M H2SO4 22.6 - - - 91.1 % after 3,000cycles

48

CuO nanoparticles on nitrogen-doped rGO Binderused

6 M KOH 340 0.5 - - 80 % after 500 cy-cles

49

Cu2O/CuO/RGO nanocomposite Binderused

6 M KOH 173.4 1 - - 98.2 % after 100,000cycles

50

CuO/rGO composite Binderused

0.5 M K2SO4 326 0.5 65.7 302 ~100 % after 1500cycles

51

MnO2/GrapheneHybrid

Binderfree

PVA/H3PO4 gelelectrolyte

267 0.2 18.64 12600 92 % after 7000cycles

52

Leaf-like CuO ongraphene sheets

Binderused

6 M KOH 305 2 - - 95.1 % after 1000cycles

53

CuO nanosheets/rGO hybrid lamellar films Binderused

6 M KOH 163.7 1 3.5 200 ~50.39 % after 1000cycles

54

TiO2�graphene�polypyrrole composite Binderfree

1 M H2SO4 201.8 - - - 76.5 % after 100 cy-cles

55

Composite of CoOOH nanoplates withmulti-walled carbon nanotubes

Binderused

0.5 M KOH 270 1 - - 100 % after 10000cycles

56

Polyaniline-h-MoO3 hollow nanorods Binderused

1 M H2SO4 270 1 - - ~80 % after 5000cycles

57

Activated carbon/ niobium doped hydro-gen titanate

Binderused

- 78.4 3 24.3 1794.6 91.4 % after 1000cycles

58

MoSe2 nanosheets Binderused

0.5 M H2SO4 199 - - - 75 % after 10000 cy-cles

59

GO–DE@MnO2

compositesBinderused

1 M Na2SO4 152.5 - - - 83.3 % after2000 cycles

60

Fluorine-Doped Fe2O3 Binderused

1 M LiPF6 71 2.25 28 550 90 % after 15000 cy-cles

61

Nitrogen-doped carbon/Mn3O4

hybridsBinderused

1 M Na2SO4 136 0.1 - - 94 % after 1000 cy-cles

62

Carbon-coated CuO nanocomposites Binderused

6 M KOH 207.2 1 - - 99.32 % after 1000cycles

63

Graphitizednanoporous carbons

Binderused

0.5 M H2SO4 238 - 19.6 700 - 64

CSCO Binderfree

1 M Na2SO4 244 0.125 8.47 73.49 90 % after 1000 cy-cles

Presentwork

Full Papers


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Submitted: May 6, 2016

Accepted: June 28, 2016

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