R E S E A R CH AR T I C L E

Estimation of electrochemical charge storage capabilityof ZnO/CuO/reduced graphene oxide nanocomposites

Hassnain Asgar1 | Kashif Mairaj Deen2 | Waseem Haider1

1School of Engineering and Technology,Central Michigan University, Mt.Pleasant, MI, 48859, USA2Department of Materials Engineering,University of British Columbia,Vancouver, BC, V6T 1Z4, Canada

CorrespondenceWaseem Haider, School of Engineeringand Technology, Central MichiganUniversity, Mt. Pleasant, MI 48859 USA.Email: haide1w@cmich.edu

Summary

Nanocomposites of ZnO/CuO with rGO were synthesized using sonochemical

and thermal treatment methods. The formation of ZnO and CuO phases on

the rGO sheets and both ZnO/CuO in the ternary component nanocomposites

were confirmed from IR spectroscopy and X-ray photoelectron spectroscopy.

The microstructure of the nanocomposites as revealed by TEM and FE-SEM

suggested that CuO was decorated as nano-rods, whereas ZnO particles

retained the spherical shape. In case of ternary nanocomposite, both nano-rods

and spherical particles were agglomerated on the rGO sheets. The

pseudocapacitive behavior of ZnO-rGO nanocomposite corresponded to rela-

tively higher specific capacitance (344.6 F/g) compared with other

nanocomposites. The galvanostatic cyclic charge/discharge (GCD) tests also

confirmed that ZnO-rGO nanocomposite could provide the highest specific

energy (21.7 Wh/kg) and power density (129.8 kW/kg) at 0.4 A/g compared

with CuO-rGO and ZnO/CuO-rGO.

KEYWORD S

supercapacitance, specific energy, reduce graphene oxide, charging/discharging

1 | INTRODUCTION

Among the technological challenges of the 21st century,development of sustainable energy storage devices withhigh efficiency is the most critical issue at present. Tocope with this challenge, the conversion, storage and uti-lization of electrochemical energy is under rigorousresearch.1-8 Within the electrochemical energy landscape,supercapacitors have attracted considerable attention dueto their widespread utilization in hybrid electric vehicles,emergency power supply systems, portable electronicdevices, and memory backup.2,8 Advantages of super-capacitors include; fast charge-discharge characteristics,high power density, and cyclic stability.8

Supercapacitors are generally categorized into two typesbased on their charge storage mechanisms: purely based onthe electrical double-layer capacitance (EDLC), also called

non-faradic capacitance and through pseudocapacitivebehavior (faradic reactions). The additional charge storagecapability in the double layer due to specific adsorption ofionic species from electrolyte and due to reversible chargetransfer processes occurring on the surface of electrodematerials which are termed as pseudocapacitive reactions.5

Usually, carbon-based materials (activated carbon, carbonnanotubes, graphene) give rise to EDLC whereas, compositeelectrodes composed of graphitic materials mixedwith metal oxides, sulfides or conducting polymers couldappreciably enhance the energy-storage capability viapseudocapacitance.4

Among other carbonaceous materials, an outstandingcandidate for electrode material is graphene oxide(GO) due to its exceptionally high specific surface areaand electrical conductivity.5 However, the unavoidableagglomeration of the graphene and hindrance in

Received: 6 September 2019 Accepted: 5 October 2019

DOI: 10.1002/er.4961

Int J Energy Res. 2019;1–14. wileyonlinelibrary.com/journal/er © 2019 John Wiley & Sons, Ltd. 1

electrolyte penetration within the micropores couldrestrict the full utilization of electrode surface area,which resulted in appreciably low energy storage capabil-ity.9 Although the graphene-based electrodes exhibit highpower and have long cyclic life, but the low specificcapacitance due to stacking layers greatly limit theirnon–faradaic capacitance and cyclic performance.10

The faradaic; capacitive redox reactions (pseudo-capacitance) on the surface of graphitic materialsi.e. GO can provide high specific capacitance due toadsorption/desorption of ionic species by the surfacefunctional groups. In this way, relatively high specificenergy in synergism with high power delivering capa-bility is expected compared with other materials onwhich EDLC is the sole mechanism of charge storage.11

Usually, transition metal oxides4,12-15 or conductivepolymers16,17 are mixed with GO to achieve high pseu-docapacitance and large specific energy. However,these materials exhibit low electrical conductivity, slowelectrochemical response and poor stability during fastcharge/discharge process, which could deleteriouslyaffect their cyclic performance.8 Therefore, mixing GOwith these materials could be attractive to achieve bet-ter charge storage capability. The composite materialsfor electrochemical capacitors are the emerging candi-dates which combine both non–faradaic and faradaiccapacitive reactions for obtaining high specific energyand power simultaneously.18

Due to high specific surface area supported with fastcharge transfer and ionic diffusion characteristics, thegraphene-based nanocomposites have attracted greatattention for their utilization in energy conversion andstorage applications. The nanocomposite of GO andmetal oxides could give combinatorial effect of high sur-face area and electrical conductivity which could facili-tate fast electron transfer within the composite electrode.In addition, the composite of metal oxides with GO couldalso inhibit the agglomeration of GO sheets, which couldalso assure the exposure of high surface area. In this way,the high specific capacitance is attributed to largeamount of charge storage through both faradaic andnon–faradaic processes at the electrode/electrolyte inter-face. Among various metal oxides, transition metal oxidesand conducting polymeric materials mixed with GO havebeen rigorously studied in the past.2,3,7,12-21

GO is highly hydrophilic owing to the abundance ofoxygen-containing functional groups on the basal plane(hydroxyl and epoxide) and at the edges (carbonyl andcarboxyl).1 The presence of these functional groups allowGO to readily exfoliate in water and to form stable disper-sions. The functional groups, in exfoliated suspensions,can act as nucleation centers to anchor nanoparticles onthe graphene sheets.

Among several transition metal oxides, ZnO and CuOhave exhibited a potential for supercapacitor applica-tions.8,9,12-14,20-23 Both CuO and ZnO have an inherentpseudocapacitive property. However, the effect of pres-ence of both the metallic oxides on the electrochemicalresponse of each other has not been explored.

In this work, we synthesized and characterized threedifferent nanocomposites, composed of reduced grapheneoxide (rGO) and metal oxides i.e. CuO-rGO, ZnO-rGOand ZnO-CuO-rGO. The electrochemical response andthe charge storage capability of these nanocompositeshad also been investigated.

2 | MATERIALS AND METHODS

2.1 | Synthesis of graphene oxide (GO)

Graphene oxide was synthesized from graphite (AsburyCorporation Inc.) using the Improved Hummer's methodas mentioned elsewhere.24 Figure 1 shows the schematicof the synthesis process. The powder was retrieved fromthe suspensions via centrifugation at 3500 rpm using theRotofix 32A Benchtop Centrifuge (Helmer® Scientific).The powder was then dried in vacuum oven at 80�C for12 hours.

2.2 | Synthesis of CuO-rGOnanocomposite

Figure 1 also illustrates the schematic of nanocompositessynthesis. Briefly, CuO-rGO nanocomposite was pro-duced by a single-pot synthesis method. The GO suspen-sion with a concentration of 1.5 mg/mL was prepared inDI water and ultrasonicated for 1 hour before heating at70�C. The Cu (II) sulfate pentahydrate was added in thesuspension by keeping the GO and Cu (II) weight ratio as1:6. The 0.5 M NaOH solution was mixed with this sus-pension and stirred vigorously for 15 minutes. Finally,hydrazine hydrate (10 mL per liter) was added drop-wiseto complete the reaction. After the reaction, the powderwas filtered, washed thoroughly with DI water and driedin vacuum oven at 80�C for 12 hours.

2.3 | Synthesis of ZnO-rGOnanocomposites

ZnO nanoparticles were decorated on the GO sheet byusing Zn (NO3)2 as a precursor. Initially, 0.02M of Zn(NO3)2 was dissolved in 100 ml DI water at 2�C. To exfo-liate GO, GO suspension (1 mg/mL) was prepared in DI

2 ASGAR ET AL.

water and ultrasonicated for 1 hour. The Zn (NO3)2 solu-tion was mixed with GO suspension (Zn (NO3)2 and GO)under vigorous stirring. The pH was maintained at 8 byadding 1 M KOH in the suspension. The stirring was car-ried out for 12 hours and the product was then filteredand dried at 80�C for 12 hours under vacuum. The driedproduct was heat-treated at 200�C in the air for 2 hoursto facilitate the crystallization of nanoparticles.

2.4 | Synthesis of ZnO-CuO-rGOnanocomposites

The ZnO-CuO-rGO nanocomposite was prepared byadding Zn (NO3)2 and Cu (II) sulfate in GO. The GO sus-pension in DI water (1mg/ml) was ultrasonicated for1 hour prior to addition of Zn (NO3)2 and Cu (II) sulfate.The weight ratio of precursors was maintained as GO: Zn(NO3)2: Cu (II) Sulfate = 1:1.8:3. The pH of the mixturewas maintained constant at 11 by adding 1 M KOH solu-tion. The mixture was stirred for 12 hours followed bydrying at 60�C in a vacuum oven. Finally, the powderwas heat treated at 200�C in the air for 2 hours.

2.5 | Characterization

Thermogravimetric analysis (TGA) and differential scan-ning calorimetry (DSC) was carried out up to 800�C with asweep rate of 20�C/min in an Argon environment (purgedat 20mL/min) using a simultaneous TGA/DSC by TAInstruments (Q600). Infrared (IR) spectra were acquired inan Attenuated Total Reflection (ATR) mode using anFTIR–ATR spectrometer (NicoletTM iSTM 50). The chemical

states of the elements in the nanocomposites were investi-gated by X-ray photoelectron spectroscope (XPS), (KratosAxis Ultra XPS) using a monochromatic Al source with10 mA and 15 kV to produce X–rays in the chamber at 1 ×10-9 torr.

The morphology of the synthesized powdered sam-ples was observed using transmission electron micro-scope (Hitachi HT7700 S/TEM) operated at 120 kV in thehigh-resolution mode coupled with the energy dispersivespectroscopy (EDS) facility and using the field emissionscanning electron microscope (Hitachi Regulus8230 FE–SEM) at 30 kV. For TEM imaging, the samples were pre-pared by drop-casting the suspension of powders onformvar coated Ni grids. However, to obtain FE–SEMimages, nanocomposite powders were sampled on laceycarbon grids (usually used for TEM).

2.6 | Electrochemical measurements

To investigate the electrochemical charge storage capa-bility of these nanocomposites, the carbon paste work-ing electrodes were prepared by mixing the synthesizedpowder samples in carbon black and poly (vinylidenefluoride) (PVDF) (17: 2: 1). Finally, 1–methyl–2-pyrrolidinone was added dropwise to make a homo-geneous slurry. This slurry was coated onto the nickelfoam (1 × 1 cm2) followed by drying at 80�C for 6 h in avacuum oven. Finally, the nanocomposite coated Nifoams were pressed using a hydraulic press with anapplied load of ~ 4500 psi for 30 sec. The total massloading of the active material (CuO–rGO, ZnO–rGOand/or CuO–ZnO–rGO) was approximately 5 mg.0.5 M Na2SO4 solution was used as an electrolyte in the

FIGURE 1 Schematic

illustrations of the synthesis process

of graphene oxide (GO) and

nanocomposites

ASGAR ET AL. 3

three-electrode cell assembly containing Pt coil andsaturated calomel electrode (SCE) as auxiliary and ref-erence electrodes, respectively. Gamry Interface–1000Potentiostat and Echem Analyst (version 6.30) wereused for all the electrochemical measurements andanalyses, respectively. Before each test, N2 gas (99.9%)was sparged for 30 min to eliminate the effect of anydissolved oxygen. All the experiments were conductedat constant temperature (25 ± 1�C).

The cyclic voltammograms were obtained at variousscan rates (100, 75, 50, 25, 10, 5, 2, and 1 mV/s). Scanswere initiated from the open circuit potential (OCP) to-0.2 mV vs. SCE followed by a forward scan to maxi-mum 0.5 mV vs. SCE. The scans were reproducible,and the data was retrieved to calculate the specificcapacitance from the CV curves by using theequation (1):25

Csp =1

2mυ Va−Vcð ÞðVc

Va

I Vð ÞdV ð1Þ

where, Csp is the specific capacitance (F/g), Va-Vc is theapplied potential range (V), m is the mass of electroactivematerial (g), υ is scan rate (V/s), and I(V) is the currentresponse (A).

The galvanostatic charge/discharge (GCD) tests ofthese nanocomposite electrodes materials were con-ducted at various specific current densities: 400, 500,600, and 700 mA/g. The charge-discharge profiles wereobtained within the potential range from –0.2 to +0.5Vvs. SCE. At each current density, the specific capacityand rate capability were determined from the GCD pro-files. The specific capacitance was calculated from theGCD profiles by using equation (2):25

Csp =IΔtΔVm

ð2Þ

where, Csp refers to the specific capacitance (F/g), I is thecurrent (A), m is the mass of electroactive material (g),ΔV is the potential window (V) and Δt is the dischargetime (s) excluding the IR drop at the onset of each dis-charge cycle.

In addition to the specific capacitance, energy density(E) and power density (P) are the most important param-eters to determine the operational performance/efficiencyof the electrochemical capacitors. The values of Csp fromequation (2) were used to calculate the energy and powerdensity from the GCD trends obtained at various currentdensities. E (Wh/kg) and P (kW/kg) were calculated byusing the relations as given in equation (3) and (4),respectively.

E=CspV2

max

2× 3:6ð3Þ

E× 3600Δt

ð4Þ

In the above equations, the Vmax is the potential win-dow (V) excluding the IR drop and Δt is the time for dis-charge curve (s).25

Additionally, to understand the charge transportbehavior at the electrode-electrolyte interface, thepotentiostatic electrochemical impedance spectra (EIS)were also obtained. The 5 mV AC perturbation over OCP(0V DC bias) was exerted within 100 kHz to 10 mHz fre-quency range. The experimental spectra were simulatedwith the equivalent electrical circuits and fitted toobtain the quantitative information about electrochemi-cal parameters.

3 | RESULTS AND DISCUSSION

3.1 | Characterization of nanocomposite

TEM images of as synthesized GO, CuO–rGO, ZnO–rGO,and ZnO–CuO–rGO nanocomposites are shown in Figure2. The morphology of the as–synthesized GO (Figure 2A)exhibited the stacking of graphene layers. Whereas, incase of CuO–rGO nanocomposite, the rod shaped CuOnanoparticles were formed on the reduced grapheneoxide (rGO) sheets (Figure 2B). The average length of theCuO nano–rods was found to be ~ 200 nm. In case ofZnO–rGO nanocomposite (Figure 2C), it can be seen thatspherical shaped ZnO nanoparticles were attached to therGO sheets and their size was in the range of 10-20nm. However, in ZnO–CuO–rGO nanocomposite (Figure2D), both CuO nano–rods and ZnO nanoparticles weregrown simultaneously on the rGO sheets. The clusters ofCuO nano–rods were more evident than ZnO sphericalparticles in the ternary phase ZnO–CuO–rGOnanocomposite. However, this morphology suggested therandom distribution of both the nanoparticles decoratedon the rGO sheets. It is also interesting to note that theaverage length of CuO nano-rods is almost less than50 nm in this ternary phase nanocomposite. This is mostlikely associated with the heterogeneous nucleation pro-cess where simultaneously nucleating ZnO nanoparticlesmay influence the growth of CuO nano-rods during syn-thesis process. Furthermore, the elemental compositionwas determined by EDS analysis in the TEM as given inTable S1 in the Supporting Information. Clearly, the EDSanalysis confirmed the presence of Cu, Zn and oxygen inthe respective nanocomposite materials which can be

4 ASGAR ET AL.

used to predict the existence of CuO and ZnO phases inthe final product. The presence of Ni in the elementalcomposition was associated with the background signalsoriginating from the Ni–grid used for sample preparation.

The IR spectroscopy results are presented in Figure3A. In the IR spectra, the vibrational peaks in the rangeof 2325–1981 cm–1 (shaded in grey) were associated withthe diamond crystal used in ATR mode.26 The spectrumof GO showed the presence of an excessive oxygen-containing functional groups, as presented by the TGAcurves. The broad absorption band at ~ 3200 cm–1 in thespectrum of GO was attributed to the O–H bondstretching vibrations mostly originating from C–OHand/or intercalated moisture. Similarly, the peaks at1576 cm–1 and 1348 cm–1 were ascribed to the stretchingof C–O and C=O bonds from the epoxide and carboxylicfunctional groups, respectively.24 The IR spectra of CuO–rGO, ZnO–rGO, and ZnO–CuO–rGO presented peaks ofCu–O, Zn–O, and both Zn–O and Cu–O at 593, 600,and 611 cm–1, respectively. Moreover, the signals ofC=O (~1340 cm–1) are evident in the spectra ofnanocomposites which are considered due to the migra-tion of functional groups (C–O or C–OH) to the edges ofgraphene sheet during thermal treatment.27 The IR spec-tra also supported the % weight loss behavior of the GO

and nanocomposites trends mostly associated with thesesurface functional groups.

IR results were further supported by the ther-mogravimetric (TG), and derivative thermogravimetric(DTG) results (Figure 3B). The highest weight lost wasobserved in case of GO sample during heating upto800�C. The initial ~10 % weight loss at 90�C was associ-ated with the removal of physically adsorbed moistureand/or by the dissociation of OH surface functionalgroups. The significant weight loss (~35%) in the range of200 to 350�C was attributed to the pyrolysis of oxygen-containing functional groups (C=O, C–O), resulting inthe subsequent evolution CO, CO2, and moisture.27,28

The significant loss in the range 350 to 600�C in all thepowders indicated the pyrolysis of the carbon skeleton.In the nanocomposite powders, the initial weight loss(~ 5 %) up to 350�C was mostly associated with the disso-ciation of surface functional groups. However, among allthe powder samples, the thermal stability of CuO-rGOwas appreciably high and the total weight loss wasfound to be ~ 17 %. Whereas, 28 % and 27% weightloss were registered by ZnO-rGO and ZnO-CuO-rGOnanocomposites, respectively, up to 800�C. It is interest-ing to note that the pyrolysis of graphitic structure in thetemperature range of 350 to 600�C for CuO-rGO was

FIGURE 2 Transmission electron

micrographs of (A) GO, (B) CuO-rGO (inset:

magnified CuO nanorod), (C) ZnO-rGO and (D)

ZnO-CuO-rGO

ASGAR ET AL. 5

limited compared with other nanocomposite samples,which is most likely attributed to its improved thermalstability. On the other hand, the extent of sp2–carbonmobilization is enhanced due to its reaction with the dec-orated nanoparticles as revealed by the exothermicresponse of nanocomposites within this temperaturerange. The less weight loss in the nanocomposites alsopredict the removal of surface functional groups from thegraphene sheets with minimum dissociation of its coreskeleton structure attributing to the improved thermalstability of rGO in the presence of decoratednanoparticles.

The surface morphology and distribution of nano-particles in the nanocomposites were observed in theFE–SEM as shown in Figure 4. The x–ray elemental map-ping of the as–synthesized nanocomposites is also pro-vided for verification. It was observed that in CuO-rGO,the CuO nanorods were agglomerated and grew as clus-ters on the rGO sheets. However, some particles were dis-tributed over the rGO sheets. Figures 4C–4E present theelemental mapping of O, Cu, and C. Moreover, the pres-ence of oxygen (Figure 4C) and Cu in the synthesizedproduct assured the clustering of CuO nano–rods on therGO sheets. These nano–rod shape particles wereagglomerated due to their high surface area and maybind with the surface functional groups (i.e. C=O) on therGO sheets which have been confirmed through IR spec-troscopy. Figure 4F shows the microstructural features ofZnO–rGO nanocomposite. The random distribution ofZnO nanoparticles (as presented in TEM image; Figure2C) is evident in the micrograph (Figure 4F). Addition-ally, the elemental maps of O (Figure 4H) and Zn (Figure4I) suggested the formation of ZnO particles which werebound with rGO sheets. Figure 4h exhibits the presenceof oxygen over the graphene sheet as supported byIR and TGA results. In case of ZnO–CuO–rGO

nanocomposite, both the CuO nano–rods and ZnOnanoparticles were agglomerated and were randomly dis-tributed on the graphene sheet as shown in Figures4K–4O. The elemental maps of these species andtheir morphology further confirmed the synthesis ofZnO–CuO–rGO nanocomposite. In the elemental map-ping of carbon (Figures 4E, 4J, and 4N) in all syntheticmaterials, the strong signals were originated from therGO sheets and more obviously from the backgroundlacy-carbon grid used for the samples preparation duringFE–SEM analyses.

XPS is one of the most important tools to identify theoxidation states and composition of the functional mate-rials, especially graphene-based materials. Figure S1 pre-sents the XPS survey profiles of the nanocompositematerials. All the spectra were corrected with referenceto C 1s advantageous peak at 284.8 eV. The fullsurvey profile showed the characteristic bands of C 1s(284.8 eV), and O 1s (530.8 eV). The characteristicspeaks related to the Cu 2p (933.59 eV) were observed inthe survey scans of CuO-rGO and ZnO-CuO-rGOnanocomposites. Similarly, the peaks corresponding toZn 2p (1021.8 eV) were also evident in the XPS spectra ofZnO-rGO and ZnO-CuO-rGO nanocomposites. The pres-ence of these characteristic peaks also validated the for-mation of nanocomposites.

To confirm the chemical states of the elements presentin the nanocomposites, high-resolution core level spectra ofeach element were also acquired and deconvoluted intofractional peaks by using Lorentzian–Gaussian function.Figures 5 and 6 represent the high-resolution XPS profilesof nanocomposites, whereas, Table S2 provides the quanti-tative information about the peaks position on the bindingenergy scale. In the high-resolution spectra of CuO–rGO(Figure 5A) the characteristic peaks of Cu 2p3/2 and Cu2p1/2 were observed at 933.74 and 953.63 eV, respectively.

FIGURE 3 (A) XRD pattern, (B) TGA curves, (C) IR spectra of GO and CuO-rGO, ZnO-rGO, ZnO-CuO-rGO nanocomposites and

(D) XPS survey spectra of nanocomposites

6 ASGAR ET AL.

These peaks corresponded to the presence of Cu2+ speciesand the spin–orbit splitting between these two peaks was ~20 eV, which further recommended the formation ofCuO.29 Moreover, two satellite peaks were also observed athigher binding energies compared with the mainpeaks. These satellite peaks at 941.69 and 961.69 eV demon-strated the presence of an unfilled Cu 3d9 shell, indicatingthe presence of Cu2+ in the CuO–rGO nanocomposite.30

The high-resolution spectrum of O1s (Figure 5B) showed aband for Cu-O at 531.57 eV.

In Figure 5D, the high-resolution spectra of Zn 2pdepicted the signature peaks of Zn 2p3/2 and Zn 2p1/2 at1022.19 eV and 1045.23 eV, respectively. The differencein the binding energy of these peaks was ~ 23 eV whichindicated the presence of Zn2+ species thus strengtheningthe claim of ZnO formation.31 Furthermore, the O1sspectra (Figure 5E) at 530.1 eV was related with the ZnOphase in the ZnO–rGO nanocomposite (Figure 5E) and itmay be referred to as chemisorbed oxygen on the ZnO.32

Similarly, the high-resolution spectra of ZnO–CuO–rGOalso depicted the Zn 2p3/2 (1021.97 eV), Zn 2p1/2 (1045.05eV), Cu 2p3/2 (934.48 eV) and Cu 2p1/2 (954.37 eV) peakswith spin-orbital splitting of ~ 23 and ~ 20 eV, respec-tively as shown in Figure 6. Also, the splitting of O 1score peak at 530.39 and 531.26 eV validated the co-

existence of both Zn-O and Cu-O phases, respectively.Based on the XPS, XRD, IR, and EDS analyses, the for-mation of CuO and ZnO phases on the rGO sheets havebeen confirmed. Additionally, the satellite peaks emergedwithin the core C 1s and O 1s high resolution peaks ofCuO–rGO, ZnO–rGO, and ZnO–CuO–rGO in the Figures5C, 5F, and 6B, respectively also suggested the presenceof surface functional groups on the rGO sheets assuggested in the IR spectra of these materials. The deco-nvoluted peaks in the high-resolution spectra also indi-cated the signatures of residual oxygen functional groupsin the nanocomposites, even after the heat-treatment.This feature can be used to predict the strong binding ofoxygen containing functional groups with the carbonbackbone structure and the similar feature were evalu-ated from the IR spectra as elaborated in the precedingdiscussion.

3.2 | Electrochemical measurements

The cyclic voltammograms of nanocomposite electrodeswere acquired at various scan rates in 0.5M Na2SO4 solu-tion as shown in Figure 7. Typically, the charge stored inthe electrochemical double layer capacitor (EDLC) as a

FIGURE 4 FESEM images of (A) CuO-rGO, (F) ZnO-rGO and (K) ZnO-CuO-rGO in secondary electron mode. FESEM images of (B)

CuO-rGO, (G) ZnO-rGO and (k inset) ZnO-CuO-rGO in back-scattered mode. EDS mapping of elements (C) O, (D) Cu, (E) C in CuO-rGO,

(H) O, (I) Zn, (J) C in ZnO-rGO and (L) O, (M) Cu, (N) C, (O) Zn in ZnO-CuO-rGO

ASGAR ET AL. 7

function of applied potential is purely associated withthe surface limited reactions and should represent therectangular CV curve.29 However, in our case, thevoltammograms did not follow a rectangular trend at

large scan rates which indicated the delayed currentresponse with increase in potential during both chargingand discharging process. This behavior was associatedwith the polarization effects due to the growth of the

FIGURE 6 XPS high-resolution

spectra for ZnO-CuO-rGO

nanocomposite

FIGURE 5 XPS high-resolution profiles for CuO-rGO and ZnO-rGO nanocomposites

8 ASGAR ET AL.

diffusion layer and the limited approach of the ionicspecies at the surface. However, the possibility of the IRdrop across the electrode/electrolyte interface cannot beneglected. It is therefore to ensure minimum IR, the posi-tive feedback mode was turned on by the potentiostatduring testing. At slow scan rate the voltammograms rep-resented almost rectangular shape current responsewhich indicated the capacitive behavior of the electrodes.However, the potential dependent current responsedepicted by these nanocomposite materials included thepseudocapacitive behavior of the electrode materialsi.e., current contribution arising from faradic reactions(adsorption/desorption and conversion etc.) in additionwith charge stored in the double layer. The reversibilityof the surface limited reactions is also dependent on thetype of electrolyte.33 In this study, the 0.5M Na2SO4 elec-trolyte facilitated the reversible adsorption/desorption ofintermediate species which were involved during charg-ing/discharging. The charge storage in the double layerand through pseudocapacitive reactions at the electrode/electrolyte interface increased the overall capacity of theelectrode materials. The reversible charge transfer pro-cesses at the electrode surface due to the formation ofdouble layer and by the migration of ionic species withinthe layered structure of rGO could facilitate the chargestorage at large scale.29 In this case the adsorption of cat-ionic species, predominantly H+ and Na+ at the surfaceor within the bulk of rGO layered structure occur duringcharging process. Clearly, the current response given bythe ZnO–rGO electrode was much higher than that ofCuO-rGO. However, the current response by the ZnO–CuO–rGO nanocomposite was slightly higher than theCuO–rGO but appreciably lower than the ZnO–rGO sam-ple. This behavior indicated the least progress of revers-ible faradaic (pseudocapacitive) reactions on the CuOnanoparticles. The relatively larger hysteresis betweencharging/discharging curves and high current densitygiven by the ZnO-rGO than CuO-rGO and ZnO-CuO-rGO samples at 25 mV/s (Figure 7D) further validatedthis behavior.

The specific capacitance (Csp) from the CV curveswas also calculated by using equation (1), at various scanrates as shown in Figure 8A. It can be observed that theCsp decreased at high scan rates in all nanocompositematerials. This typical behavior is an indication of thediffusion controlled electrochemical processes on theelectrode surfaces. In comparison, the Csp of ZnO–rGOwas much higher than other electrodes which represen-ted its relatively better charge storage capability. Figure8B is presented to understand the possible reactionsequence on the rGO during charging and dischargingprocess. The cations, i.e. Na+ and H+, would migratetoward the electrode upon reversible charging/

discharging process and may contribute to electrochemi-cal charge storage either through adsorption/desorptionon the surface and/or by intercalation/de–intercalationwithin the rGO interlayered structure. In other words, atlow scan rate, both outer and inner surfaces were avail-able to the charged species and the large surface area ofthe porous electrodes was effectively utilized. Moreover,the diffusion and migration of ionic species within theporous structure is time dependent and at high scan ratethe current response always lags the applied potential asshown by the inclined lines in the CV curves. At low scanrates, the ionic species had sufficient time to diffuse intothe internal porous structure and hence provided largespecific capacitance.34 Moreover, the CV curves wereused to estimate the nature of electrochemical reactionsoccurring on the electrode surface by using the followingrelation (equation (5)).35

ip Vð Þ= aυb ð5Þ

where, ‘ip(V)’ is the current response at a given potentialand ‘a’ and ‘b’ are the adjustable parameters. The peakcurrent response (ip) vs. the log (scan rate) is plotted forthe nanocomposite materials and the values of slope ‘b’from the resultant straight lines were calculated (FigureS2A). The well-defined values of ‘b’ i.e., b = 1 and b = 0.5can be attributed to the charge storage either purely bythe double layer or through faradaic processes involvingthe semi-infinite diffusion of reacting species, respec-tively. The b-values for the CuO–rGO, ZnO–rGO, andZnO-CuO-rGO nanocomposites were calculated to be0.49, 0.46, and 0.54, respectively. The b-values in all thecases were close to 0.5 indicating the dominance of fara-daic reactions giving rise to pseudocapacitance. The sepa-rate charge storage contribution by the faradaic and non-faradaic (double layer formation) processes was also cal-culated by using the current partitioning equation. Theplot of ip/υ vs. 1/υ1/2 (υ = scan rate) was used to calculatethe percent contribution of each process and is given inFigure S2B. It is evident that the charge storage via fara-daic reactions (pseudocapacitance) was dominant in allnanocomposite materials. It is determined that maximum12.6 % of the total charge was associated with the doublelayer in the ZnO-CuO-rGO electrode. However, thepseudocapacitive behavior of CuO-rGO and ZnO-rGOwas 96.97% and 94.08%, respectively.

Galvanostatic charge-discharge (GCD) curves of thesynthetic nanocomposites were obtained at various cur-rent densities (as depicted in Figure 9). In synergism withthe CV curves, the GCD profiles also revealed the non-linear charging/discharging trends, which validated theoccurrence of faradaic reactions at the electrode surface.Moreover, the initial potential drop at the start of each

ASGAR ET AL. 9

discharged cycle was associated with the internal resis-tance of the electrode and the IR drop within the electro-lyte.29 The larger time span for discharging at all specificcurrents is directly related with the relatively higher spe-cific capacity of the ZnO-rGO compared with othernanocomposites. Moreover, the CuO-rGO and ZnO-CuO-rGO nanocomposites demonstrated larger potential drop(~50 mV) in the GCD plots compared with ZnO-rGO. InFigure 9D, ZnO-rGO exhibited the large time span fordischarge curve at 400 mA/corresponding large specificcapacity. The GCD plots, obtained at various specific cur-rents, were also used to calculate the Csp values as shownin Figure 10A. It is shown that maximum 371 F/g Csp

was achieved at 400 mA/g by the ZnO-rGO which was

approximately two times higher than ZnO-CuO-rGOnanocomposite (141 F/g). The Csp was decreased almostlinearly with increase in charge/discharge current, vali-dating the occurrence of diffusion-controlled electro-chemical processes hence validating the results obtainedfrom the CV analyses. The large pseudocapacitive behav-ior of the ZnO-rGO was associated with the stability ofthe ZnO nanospheres under the applied conditions andtheir effective electrochemical response toward reversibleadsorption/desorption of ionic species. As presented inTEM (Figure 2) and FE-SEM (Figure 4) images, the veryfine ZnO particles could also increase the electrochemicalactive surface area and may provide more sites for theprogress of pseudocapacitve processes compared with

FIGURE 7 Cyclic

voltammograms of (A) CuO-

rGO, (B) ZnO-rGO, (C) ZnO-

CuO-rGO at different scan rates

and (D) comparison of the

nanocomposites at 25 mV/s

scan rate

FIGURE 8 (A) Specific capacitance values calculated from CV curves at various scan rates (b) schematic illustration of charge-

discharge mechanism on the rGO sheets

10 ASGAR ET AL.

reactions which may occur on the relatively larger CuOparticles. The specific energy and power was also calcu-lated for these materials by using equation (3) and (4)and the results are presented in the Ragone plot (Figure10B). It is shown that ZnO-rGO nanocomposite candeliver high specific power (198.5 kW/kg) at 700 mA/gwhereas the energy storage capability was 7.9Wh/kg. The maximum 21.7 Wh/kg sprcific energy canalso be stored with appreciably high specific power (129.8kW/kg) at 400 mA/g as depicted by the ZnO-rGO. Thespecific energy of the CuO-rGO and ZnO-CuO was muchlower than the ZnO-rGO which is possibly related withthe non-capacitive faradaic reactions taking place on theCuO nanoparticles.

The Nyquist plots from the electrochemical imped-ance spectra of the nanocomposite materials were alsoobtained (as shown in Figure 11). Both CuO-rGO and

ZnO-CuO-rGO nanocomposites exhibited almost simi-lar impedance behavior. In the high frequency region,the appearance of small semicircle highlights thecharge transfer process (Rct) at the interface of elec-trode and the formation of electrical double layer, rep-resented as constant phase element CPEdl.

29 It was alsodetermined that the real impedance (Zreal) componentwas relatively low. The double layer capacitance wasreplaced by the CPEdl to compensate the non-homogeneous charge relaxation on the porous elec-trode surfaces. In the medium frequency region, theincreasing imaginary impedance component was asso-ciated with the diffusion-controlled processes,i.e., diffusion of ionic species (H+ or Na+) which wasdepicted as Warburg coefficient (W) in the equivalentelectrical circuit (EEC), as shown in the inset of Figure11. The non-capacitive faradaic contribution by the

FIGURE 9 Cyclic charge-

discharge curves of (A) CuO-rGO,

(B) ZnO-rGO, (C) ZnO-CuO-rGO at

different current densities and (D)

comparison of nanocompositesat

the current density of 400 mA/g

FIGURE 10 (A) Specific

capacitance from cyclic charge-

discharge curves at different

current densities and (B)

Ragone plot for nanocomposites

ASGAR ET AL. 11

conversion product formed on the electrode surfacewas simulated with an additional capacitor element(Cp) in the EEC. The quantitative values of the parame-ters in the EEC were determined through iterative pro-cess and by simulating the experimental impedancespectra with the equivalent model. As listed in TableS3, that Rct of the ZnO-rGO was much higher (65.17 Ω-cm2) than the CuO-rGO (5.05 Ω-cm2) and ZnO-CuO-rGO (21.12 Ω-cm2). This indicated the least contribu-tion of the non-capacitive faradaic reactions on theZnO-rGO compared with others. Similarly, an order ofmagnitude high CPEdl of this nanocomposite (0.0195 S-sa/cm2) was in accordance with the CV and GCDresults. This element represented the greatest chargestorage capability of the ZnO-rGO nanocompositemostly affiliated with the faradaic but capacitive(reversible) redox reactions. Apparently, the admit-tance of the ZnO-rGO in the medium frequency range(indicated by W in the EEC) was much higher thanthose of CuO-rGO and ZnO-CuO-rGO, suggesting pro-nounced diffusion of ionic species into the layeredstructure of rGO. In simple words, the relatively largeW (0.0553 S–s1/2) by the ZnO-rGO sample was attrib-uted to the relatively fast kinetics of the redox reactionsduring intercalation/de-intercalation (charging/dis-charging) process as discussed above.36

4 | CONCLUSION

In this work we described the synthesis, characterizationand charge storage capability of CuO-rGO, ZnO-rGO andZnO-CuO-rGO nanocomposites and following conclu-sions were drawn from the results:

1. The microstructural features in FE-SEM and TEMimages of the CuO-rGO and ZnO-rGO compositepowder samples revealed the formation of nano–rodand spherical shape CuO (~200 nm) and ZnO (10-20nm) particles, respectively, on the stacked sheets ofrGO. However, in the ZnO-CuO-rGO, both rod andspherical particles were agglomerated on the rGOsheets.

2. The EDX analysis of the nanocomposites validatedthe formation of CuO and ZnO phases on the rGOsheets. The IR spectra and the deconvoluted high res-olution C 1s and O 1s peaks in the XPS spectra indi-cated the presence of oxygen containing surfacefunctional groups on the rGO sheets.

3. The high resolution XPS peaks of Cu 2p and Zn 2pin the respective composite materials confirmed thepresence of typical Cu2+ and Zn2+species. The exis-tence of shake-up satellite peaks in the higher reso-lution spectra of Cu 2p, and Zn 2p also assured theformation of CuO and ZnO phases in thenanocomposite materials.

4. From the CV analyses it was evaluated that thecharge storage in these nanocomposite materials waspredominantly pseudocapacitive in nature (> 90 %)in addition with the minor charge associated withthe electrical double layer (non–faradaic). Thispseudocapacitive response was associated withthe occurrence of reversible adsorption/desorptionprocesses on the electrode surface during charging/discharging process. However, the ZnO-rGO nano-composite presented relatively large current responseand its specific capacitance (344.6 F/g) was approxi-mately two times higher than CuO-rGO and ZnO-CuO-rGO at 1 mV/s sweep rate.

5. The GCD tests obtained at various specific current densi-ties validated the higher specific capacitance of theZnO–rGO nanocomposite compared with CuO-rGO andZnO-CuO-rGO. Moreover, the specific energy and powerdensity of ZnO-rGO was 21.7 Wh/kg and 129.8 kW/kg,respectively, which was found to be higher than CuO-rGO and ZnO-CuO-rGO nanocomposites.

ACKNOWLEDGEMENTSThe authors would like to acknowledge the MichiganCenter for Materials Characterization at University ofMichigan at Ann Arbor for providing us the opportunityto avail their lab facilities.

ORCIDHassnain Asgar https://orcid.org/0000-0002-8427-5188Kashif Mairaj Deen https://orcid.org/0000-0002-3619-2599Waseem Haider https://orcid.org/0000-0003-4235-3560

FIGURE 11 Nyquist plots (inset: EEC to simulate

experimental data) of nanocomposites in 0. M Na2SO4

12 ASGAR ET AL.

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SUPPORTING INFORMATIONAdditional supporting information may be found onlinein the Supporting Information section at the end of thisarticle.

How to cite this article: Asgar H,Deen KM, Haider W. Estimation ofelectrochemical charge storage capability ofZnO/CuO/reduced graphene oxidenanocomposites. Int J Energy Res. 2019;1–14.https://doi.org/10.1002/er.4961

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