Journal of Electroanalytical Chemistry 787 (2017) 24–35

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Journal of Electroanalytical Chemistry

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Nanoporous copper-cobalt mixed oxide nanorod bundles as highperformance pseudocapacitive electrodes

Assumpta C Nwanya a, Chawki Awada b, Daniel Obi b, Kumar Raju c, Kenneth I. Ozoemena c,d, Rose U. Osuji a,e,f,Andreas Ruediger b, Malik Maaza e,f, Federico Rosei b, Fabian I. Ezema a,e,f,⁎a Department of Physics and Astronomy, University of Nigeria Nsukka, Nigeriab INRS Centre for Energy,Materials and Telecommunications and UNESCO Chair inMaterials and Technologies for Energy Conversion, Saving and Storage, 1650, Boulevard Lionel-Boulet, Varennes,QC J3X 1S2, Canadac Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Johannesburg 2050, South Africa.d Energy Materials, Materials Science & Manufacturing, Council for Scientific & Industrial Research (CSIR), Pretoria 0001, South Africae Nanosciences African Network (NANOAFNET), iThemba LABS-National Research Foundation, 1 Old Faure road, Somerset West 7129, P.O. Box 722, Somerset West, Western Cape Province,South Africa.f UNESCO-UNISA Africa Chair in Nanosciences/Nanotechnology, College of Graduate Studies, University of South Africa (UNISA), Muckleneuk Ridge, P.O. Box 392, Pretoria, South Africa

⁎ Corresponding author at: Department of Physics and ANsukka, Nigeria.

E-mail addresses: fabian.ezema@unn.edu.ng, fiezema@

http://dx.doi.org/10.1016/j.jelechem.2017.01.0311572-6657/© 2017 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 14 June 2016Received in revised form 11 January 2017Accepted 12 January 2017Available online 16 January 2017

We used a simple, cost effective and scalable chemical method to depositmixed oxides of copper and cobalt onindium tin oxide (ITO) and stainless steel (ss) substrates. The depositedmixed oxides of Cu-Co and Co-Cu exhibituniform surface morphology with nanoporous structure as obtained from scanning electron microscopy (SEM).The electrochemical properties were characterized by cyclic voltammetry (CV), galvanostatic charge-discharge(GCD) and electrochemical impedance spectroscopy (EIS). The Cu-Co oxide film on ITO yielded very high specificand volumetric capacitances of 919 Fg−1 and 616.1 Fcm−3 respectively with high energy (28.78 Wh kg−1) andpower (51.8W kg−1) densities. The same oxide on ss yields 195 Fg−1 and 236.8 Fcm−3 respectively for the spe-cific and volumetric capacitances. In addition, the Cu-Co oxide electrode shows superior rate capability and ex-cellent long-term cyclability. While the ss offers less internal resistance, the stability of the films is higher onITO substrates. The bundles of rod-like Cu-Co mixed oxide embedded with nanoporous structure exposedmore active surfaces withminimal ion diffusion length thereby enhancing the redox behavior and the binary ox-ides are synergistically responsible for superior rate capability and excellent durability. Our results indicate thatthese nanoporous electrodes are promising for use in pseudocapacitive applications.

© 2017 Elsevier B.V. All rights reserved.

Keywords:Copper-cobalt mixed oxidesSupercapacitorsSpecific capacitanceSuccessive ionic layer adsorption and reactionCyclic voltammetry

1. Introduction

Energy storage is considered as the grand challenge of all renewableenergy systems such as wind and solar power due to their intermittentnature. The development of flexible, cost effective and sustainable ener-gy storage materials is an urgent, necessary step to meet the increasingdemand for energy storage. Electrochemical capacitors (EC), otherwiseknown as supercapacitors, store and release energy very rapidly andhave excellent long-term cyclability [1]. As such, they are ideal forhigh-power applications such as hybrid electrical vehicles, portableelectronic devices, cranes and forklifts [1]. An EC is an energy storage de-vice that tends to bridge the gap between batteries and conventional ca-pacitors [2]. It consists mainly of two electrodes and an electrolyte and

stronomy, University of Nigeria

yahoo.com (F.I. Ezema).

based on the charge storage mechanism, EC is classified as non-faradaic(it stores energy by accumulating charges in the electrostatic doublelayer, and is also known as electric double-layer capacitor (EDLC)),and faradaic supercapacitors (FS) referred to as pseudocapacitors.In EDLCs, the electrode materials, which are mainly carbonaceous,are not electrochemically active and charges are stored at theelectrode/electrolyte interface. In pseudocapacitors, (mainly metaloxides/hydroxides and conducting polymers) the electrode materialsare electrochemically active and charges are stored by redox (faradaic)reactions [3]. Carbon basedmaterials such as activated carbon,mesopo-rous carbon, carbon nanotubes (CNTs), graphene etc. [4–8] that exhibitEDLC characteristics have been extensively studied for use as electrodematerials in supercapacitors due to the large specific surface area theyexhibit. However, the electrochemical capacitor performance of thesecarbonaceous materials cannot meet the rapidly increasing demand ofhigh power and energy densities, which are the major requirementsof a storage device. Hence, transition metal oxides like RuxOx, NiO,CoxOx, Ni(OH)2, MnxOx, CuxOx and conducting polymers which store

25A.C. Nwanya et al. / Journal of Electroanalytical Chemistry 787 (2017) 24–35

their charges by pseudo capacitance have also been extensively studiedas alternatives to carbon based electrodes in supercapacitors [9–16].

The emergence of hybrid electrodes incorporating simultaneouslyEDLCs and pseudo capacitance are believed to be the key to developingthe next generation of super capacitor devices. Such hybrid electrodesinclude NiO/MWCNTs prepared by Gund et al., [17] in a three electrodeconfiguration which gave a specific capacitance of 1727 F/g at a scanrate of 5 mAcm−2. Other notable examples are: polypyrole/CNT [18],polypyrole/graphene [19], graphene oxide/Mn3O4 [20], (Ni, Fe, Co)Oxide/SWCNT [21], Co(OH)2/graphene foams [22] andMWCNT/TAPcNi[23], electrodes etc. These hybrid electrodes yielded higher specific ca-pacitances and better stability than the individual electrode materials.

Single transition metal oxide electrodes have been shown to sufferfrom a high initial capacity loss and poor cycling performance [24].Charge (electrons or ions) transport within the bulk of these materialsis crucial to their performance since their pseudocapacitive activitiesrely on redox reactions. Unfortunately, most of the transition metal ox-ides provide insufficient charge transport, which limits their efficientuse. Most often, the high theoretically predicted pseudocapacitancesare seldom obtained in practical applications. To improve the electrodecapacity, cycling performance, and rate capability, some studies haveproposed the synthesis of transition metal oxide materials with morethan onemetal oxide structure for supercapacitor applications. In addi-tion, the incorporation of metals into active metal oxide materials wasfound to increase the charge transfer in electrode materials along withimproved ion diffusion [25]. Such developed binary and ternary metaloxides/hydroxides for supercapacitor applications include NiO-CoOcomposite films [26], Co-Ni mixed hydroxide films [27], Ni–Co oxidesfilms [28] and CoFe2O4 nano flakes [29].

Cobalt-copper oxide with different copper/cobalt molar ratios havebeen studied for various applications. Amria et al. [30] studied thesolar absorbance of copper-cobalt oxide thin films for solar absorber ap-plications. De Koninck et al. [31] and Zhang et al. [32] reported thephysico/chemical and electrochemical properties of CuxCo3is − xO4 andcopper doped cobalt oxide electrodes respectively for use as catalystin oxygen evolution reactions. The electrochemical and crystallographicproperties of copper doped cobalt oxide films were also studied by LaRosa-Toro et al. [33] and it was found that when the copper contentexceeds that of stoichiometric CuCo2O4 spinel, a new CuO phase segre-gates at the surface.

The electrochemical performance of electrodematerials is highly de-pendent on its nanoscale structure, such as particle size, surface area,pore volume and crystallinity [34]. These properties in turn depend onthe deposition methods and parameters such as temperature, pH, pre-cursor concentration etc. Engineering the size,morphology andporosityof the active material as well as developing core–shell structures andmodifying the contact between the activematerial and current collectorhave been identified as a possible strategy to enhance the pseudo capac-itive properties of these oxides [35]. Methods such as sol-gel, thermaldecomposition,molten salt, dcmagnetron sputtering, chemical bath de-position, hydrothermal synthesis [11,30–33,36,37] have been used todeposit Co3O4 and CuO and their mixed oxide for energy storage appli-cations. Despite the reported enhancement in the charge storage prop-erties,most of thesemethods have drawbacks such as high temperaturerequirements, high costs and synthesis complexity rendering it difficultto scale up for commercialization.

Here we present the pseudocapacitive properties of Cu-Co and Co-Cu mixed oxide films deposited by successive ionic layer adsorptionand reaction (SILAR) method on indium tin oxide (ITO) and stainlesssteel (ss) substrates. SILAR is a simple chemical method used for largearea growth of metal oxides in which the thin films are obtained by im-mersing a substrate into separately placed cationic and anionic precur-sor solutions [38,39]. By changing the deposition cycle, temperature, pHand salt concentration, the deposition rate, thickness, particle size andmorphology of the film can be easily controlled. There is no restrictionon the substrate material, dimensions, or surface profile to be used.

We developed cobalt oxide-copper oxide electrodes composited withcopper-cobalt bundles with rod like backbone, and demonstrated thatsuch embedded nanostructures enhance the electrochemical propertiesand stability of the single oxide electrodes during repeated cycling. Wealso show that the choice of substrates used in the deposition affects theelectrochemical properties of the films to a very good extent.

2. Experimental details

All chemicals were of analytical grade and were used without fur-ther purification. Cu-Co and Co-Cu mixed oxide films were depositedon indium tin oxide (ITO) and stainless steel (ss) substrates. The ss sub-strates were polished with zero grade emery polish papers to a roughfinish, then the ss and ITOwerewashedwith plenty of water and deter-gent solution, rinsed with water and ultrasonicated in a mixture ofwater and acetone for 10 mins. The cationic precursors used were0.1 M cobalt chloride hexahydrate (CoCl2·6H2O) and 0.1 M cuprous ac-etate (CH3COO)2Cu·H2O) of equal volume and alkaline (pH ~ 10.5)withammonium hydroxide (NH4OH) and maintained at room temperature.The anionic precursor used was distilled water with 1% H2O2 main-tained at a temperature of 333–343 K. A complete SILAR cycle involvesthe alternate immersion of the substrates vertically into the alkalineCo salt bath (source of Co2+) for 20s (or the Cu salt bath (source ofCu2+) for 20s), the 1% H2O2 for 20s and rinsing in distilled water keptat room temperature for another 20s. This cycle was repeated 20times and at the end of the complete cycle, the films were rinsed thor-oughly with distilled water. Another 10 cycles were repeated usingthe obtained films and dipping in Cu salt bath (or Co salt bath) to obtainthe Co-Cu or Cu-Co mixed oxide film, respectively. The films wereannealed in air at 300 °C for 1 h to remove any hydroxide phase thatmay be present. Scanning electron micrographs (SEM) of the filmswere recorded using a Carl Zeiss Ma-10 field emission electron micro-scope operating at 20 keV. Raman spectroscopy was performed with aconfocal optical microscope coupledwith amodular Raman spectrome-ter from Horiba (iHR320). The light source is a continuous wave diode-pumped solid-state (CW-DPSS) laser with a wavelength of 473 nm(Cobolt Inc.). The laser was focused onto the sample with a 100× objec-tive (N.A 0.9) and a power of 10 mW. The Raman signal was dispersedinto the spectrometer with a grating of 2400 lines/mm, then, a charge-coupled detector (CCD) (Synapse, Horiba Inc.) was used to collect theRaman spectra. X-ray diffraction (XRD) measurements were donewith anXPERTPROdiffractometer. TheX-rayphotoelectron Spectrosco-py (XPS) spectrumwas obtained with a VG Escalab 220i XL XPS systemusing the polychromatic Al source at a resolution of 1 eV. Silver (Ag) ref-erence sample was used with the source. Energy Dispersive X-ray spec-troscopy (EDX) measurement were performed with the Link ISISsystem of Oxford Instrument with a resolution of 132 eV.

Thefilm thickness and furthermorphological studieswas carried outvia atomic force microscopy (AFM) imaging operated in contact modeconfiguration. AFM technique is a convenient and efficient means ofobtaining the thickness of nanostructured thin films, no more than1 μm thick, by using contact mode large area scans. In our case,10 × 10 μm scans were carried out across multiple channels createdon the films by using soft probes that were known not to affect the sub-strates. AFM software allows determining the depth profile of suchchannels. The topography of the samples was determined using5 × 5 μ m contact mode scan in which the AFM images are recorded atthe sample surface. The electrochemical properties of thefilmswere de-termined using a potentiostat (Princeton Applied Research VersaSTATMC) in a three-electrode system, which consists of the working elec-trode (deposited films on ITO and ss), large graphite counter electrodeand satd. Ag/AgCl reference electrode. The electrolyte used in all studieswas 0.5 M Na2SO4. EIS was performed using a three electrode system in0.5 M Na2SO4 electrolyte and at a freqency range of 10 mHz – 100 kHz,scanning in AC mode with a constant voltage amplitude of 10 mV.

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3. Results and discussion

3.1. Film formation

Upon adding aqueous ammonia (NH4OH) to the CoCl2.6H2O and(CH3COO)2Cu·H2O solution, the ionic product becomes greater thanthe solubility product [39], Co(OH)2 and Cu(OH)2 are precipitatedmak-ing the solution turbid. Complex cobalt [Co(NH3)4]2+ and copper[Cu(H2O)2(NH3)4]2+ ions are formed on addition of excess ammoniadue to ligand exchange reactions. These ions avoid further precipitationand make the solution clear and transparent. This can be explained byEqs.s 1 and 2: [15,40]

Co H2Oð Þ2 OHð Þ2 þ 4NH3→ Co NH3ð Þ4� �2þ þ 2H2Oþ 2OH− ð1Þ

Cu H2Oð Þ4 OHð Þ2 þ 3NH3→ Cu NH3ð Þ3� �2þ þ 4H2Oþ 2OH− ð2Þ

Film deposition involves the ion-by-ion growth of the Co2+ andCu2+ at nucleation sites on the surface of the immersed substrates.When the substrates are immersed in the above solution, complex co-balt [Co(NH3)4]2+ and copper [Cu(H2O)2(NH3)4]2+ ions becomeadsorbed on the substrate due to attractive force between the ionsand the substrates. The forces responsible may be cohesive forces,Van-der Waals forces, or chemical attractive forces. The oxidant, H2O2

together with heat tends to unstabilize adsorbed complexes by remov-ing ammonia and facilitate reaction with solution OH−. Hence, immer-sion of the ITO and sswith the adsorbed complex Co2+ or Cu2+ into the1% H2O2 causes OH– to be attracted to them from the solution formingthe Co or Cu oxide on the ITO and ss. These can be represented by thefollowing reactions [41,42]:

H2O2 þ H2O→2OH− þ 2Hþ þ O− ð3Þ

3Co NH3ð Þ42þ þ 2OH−→Co3O4 þ 12NH3 þ H2O ð4Þ

Cu NH3ð Þ32þ þ 2OH−→CuOþ 3NH3 þ H2O ð5Þ

Further alternate immersion into the solutions causes additionalgrowth at the nucleation sites, which eventually forms a thin film onthe substrate. Using either the Co or Cu oxide electrode as the substrateand repeating the process for 10 cycles led to the formation of cobalt-copper hydroxide or copper-cobalt oxide. Annealing at 300 °C for 1 hremoves any hydroxide phase thatmay be present. The sameprocedureis followed using the separate salt baths for the deposition of CuO andCo3O4 separately on the substrates. The films adhered well to the sub-strates while the films produced without H2O2 did not adhere to thesubstrates, hence were not further characterized.

Fig. 1. EDX spectrum of the mixed oxide films depo

3.2. Energy dispersive X-ray spectroscopy (EDX)

The EDX spectrum of the mixed oxide films deposited on ITO sub-strates is shown in Fig. 1(a, b). The atomic percentage of the elementsshowed a ratio Cu:Co of 3 for the Cu-Co mixed oxide while the Co-Cumixed oxide showed a ratio Cu:Co of 1.4. The high percentage of Cu incomparison to Co in the mixed oxides could possibly be due to higherattractive force between the Cu ions and the substrates. The spectrumexhibits strong peaks of Cu, Co and O at the appropriate energy levels.Cl emanated from the precursor used while In, Si, Mg, Na and Ca origi-nates from the ITO substrate.

3.3. X-ray photoelectron spectroscopy (XPS)

The electronic structure and chemical composition of the depositedmixed oxide films on ITOwere further investigated by X-ray photoelec-tron spectroscopy (XPS). As shown in Fig. 2a, the spectra reveal thepresence of carbon, oxygen, copper and cobalt. Indium peaks originatefrom the substrate used. Fig. 2(b and c) shows the O 1s XPS spectra ofthe Co-Cu and Cu-Co mixed oxide films respectively. The O 1s spectraexhibit a strong peak with a shoulder at a higher binding energy forthe Co-Cu mixed oxide film while the shoulder of the Cu-Co mixedoxide film is at a lower binding energy. The positions of the shouldersare consistentwithwhat is obtained separately for copper oxide and co-balt oxide respectively [30] and this is consistent with the depositionmethod. The deconvolution of the two O 1s spectra shows three distinctcurve-fitting components in each spectrum of the samples. The peaks(denoted by “i”) at binding energy (BE) around 529.2–529.4 eV is duemost likely to lattice O2– (Cu–O, Co–O) [31,43]. The peaks (denoted byii) at BE around 530.9–531.2 eV may be attributed to surface oxygenspecies such as adsorbed oxygenO– and/or OH– like species like hydrox-yl groups [44,45]. The third peak (denoted by iii) at BE around 531.9–532.1 eV is most likely due to water (H-O-H). The peak of non-oxygen-ated carbon (C 1s) is detected at a binding energy of about 285.0 eV.

The XPS spectra of the Cu 2p core level for the films are shown in Fig.2d. The twomain peaks of Cu 2p3/2 and Cu 2p1/2 and the satellites on thehigh energy side of each Cu 2p3/2 and Cu 2p1/2, respectively, are found inall spectra. The oxidation state of copper is confirmed from the splittingof the Cu 2p peak. The presence of Cu2+ on the sample is confirmed bythe existence of satellite peaks at 943.95 and 962.55 eV on the high en-ergy side of each of Cu 2p3/2 andCu2p1/2 respectively. The peaks are dueto the existence of the unfilled Cu3d9 shell [30]. The binding energy dif-ference between Cu 2p1/2 and Cu 2p3/2 peaks which is around 19.8 eVand the satellite peak between Cu 2p3/2 and Cu 2p1/2 confirm the pres-ence of Cu2+ ions in the sample [30].

Fig. 2e shows the Co 2p spectrum showing the two main peaks cen-tered around780 and 795 eV,which are assigned as Co 2p3/2 and Co2p1/2 respectively. Peak separation of the Co 2p3/2 and Co 2p1/2 peaks by aspin–orbit splitting of around 15.1 eV indicates the presence of themixed Co(II) and Co(III) [30]. A low intensity satellite peak centeredaround 789 eV between the two main Co 2p3/2 and Co 2p1/2 peaks

sited on ITO (a) Cu-Co (b) Co-Cu mixed oxide.

Fig. 2. (a) Full XPS of the Cu-Co and Co-Cumixed oxide films deposited on ITO (b) O 1s spectrum of the Co-Cu oxidefilm (c) O 1s spectrum of the Cu-Comixed oxide film (d) core level XPSspectra of Cu 2p (e) core level XPS spectra of Co 2p.

27A.C. Nwanya et al. / Journal of Electroanalytical Chemistry 787 (2017) 24–35

suggests that the cobalt ions are present in a partial spinel-type latticearrangement [30]. Comparing the Co 2p and the Cu 2p peaks for theCo-Cu and Cu-Co mixed oxide films, it is observed that the Cu 2p peakis stronger for the Co-Cumixed oxide filmwhile the Co 2p peak is stron-ger for the Cu-Co film.

3.4. XRD analysis

The structural properties of the deposited filmswere determined byXRD in the 2θ range of 10 to 90 degrees. Fig. 3 displays the XRD patternof the Cu-Co and Co-Cu mixed oxide films. Strong peaks corresponding

Fig. 3. XRD pattern of the Cu-Co and Co-Cu oxide films.

to the (−111) and (−202) diffraction planes of the monoclinic CuO(JCPDS no. 80-0076) are observed at 2θ angles of 35.42° and 50.79° re-spectively. Another peak of CuO occurred at 40.70° corresponding to dif-fraction planes of (111). A strong peak of Co3O4 occurred at the 2θ anglevalue of 30.48° corresponding to the (220) diffraction plane of theCo3O4

(JCPDS card no. 42-1467). Other identifiable peaks of Co3O4 occurred atangle values (2θ) 37.58° and 60.40° and correspond to the (311) and(511) diffraction planes of Co3O4 respectively. There is no significant dif-ference in the XRD pattern of the films except that the peaks of the Co-Cu mixed oxide appear slightly more prominent than that of Cu-Comixed oxide. This shows that the two electrodes have the structure ofboth the CuO and the Co3O4.

3.5. Raman spectroscopy

Raman spectroscopy was performed to obtain additional informa-tion on the crystal structure of mixed copper-cobalt oxide films. Fig. 4shows three Raman peaks of Cu-Co mixed oxide respectively at282 cm−1, 330 cm−1 and 614 cm−1. The peak at 282 cm−1 is attributedto the Ag mode while the two latter peaks at 330 and 614 cm−1 areassigned respectively to the Bg

1 and Bg2 modes of CuO [46,47]. However,

we observe seven Raman peaks of Co-Cu mixed oxide respectively at188 cm−1, 282 cm−1, 330 cm−1, 465 cm−1, 510 cm−1, 601 cm−1 and662 cm−1. The two peaks at 282 cm−1 and 330 cm−1 are attributedto the CuO modes mentioned above, whereas the other five peaks arerelated to the phonon modes of Co3O4 respectively F2g, Eg, A1g [48–50].Raman spectra confirm what we observed in the EDX, XPS and XRD,specifically that the films are composed of a mixing of monoclinic crys-tal structure phase of CuO attributed to a space group of C2h6 [51] and anormal spinel structure Co2+(Co3+)2O4

2− which belongs to the spacegroup Oh

7 [48].

Fig. 4. Room-temperature Raman spectra of the two films of copper-cobalt oxide, Cu-Co(red color) and Co-Cu (black color. (For interpretation of the references to color in thisfigure legend, the reader is referred to the web version of this article.)

28 A.C. Nwanya et al. / Journal of Electroanalytical Chemistry 787 (2017) 24–35

3.6. Surface morphology by scanning electron microscopy (SEM)

Fig. 5 shows SEM images of the Cu-Co and Co-Cu mixed oxide filmsdeposited on an ITO substrate. SEM micrographs of the Cu-Co mixedoxide film show spherical grains interspersed with nano-porous ag-glomerates of short nano-rods on the surface. The film is observed togrow uniformly as shown in the lower magnification images (inset ofFig. 5a and b). The Co-Cu mixed oxide film shows the outgrowth ofnano-porous short spherical granules from the nucleation centre em-bedded between nano-rod agglomerates. The nano-rods are longerthan that of Cu-Co oxide. The film also shows uniform growth as seenin the inset of Fig. 5c and d). On the ss substrates, the morphology ofthe Cu-Co oxide (Fig. 6a and b) looks similar with that on ITO but thatof Co-Cu oxide (Fig. 6c and d) looks blurred.

3.7. Surface morphology by atomic force microscopy (AFM)

The morphology of the mixed oxide electrodes as obtained fromAFM is shown in Fig. 7(a–d). The measured thicknesses are 43.2 and167.0 nm for Cu-Co mixed oxide film on ITO (Cu-Co/ITO) and Cu-Comixed oxide film on ss (Cu-Co/ss) respectively while the values are

Fig. 5. SEMof the deposited films on ITO (a and b) Cu-Co and (c and d) Co-Cumixed oxideat two different magnifications 50 K and 100 K.

22.2 and 259.0 nm respectively for Co-Cu mixed oxide film on ITO(Co-Cu/ITO) and Co-Cu mixed oxide on ss (Co-Cu/ss). This shows thatthe films have higher affinity for the ss substrates due most likely tohigher attractive force between the films and the ss that led to thickerfilms on the ss. Preferential alignment of nucleated species is observedfor the films on ITO substrate. Irrespective of substrate, Co3O4 tends tofill the voids in CuO in both mixed oxide and reduces the roughness tothe same level. In order words, Co3O4 tends to nucleate on the substratethrough the voids on CuO rather than grow on the top of existing CuO.

3.8. Electrochemical study

3.8.1. Cyclic voltammetry (CV)Cyclic voltammetry (CV) measurements were conducted in a three-

electrode configuration and used to determine the accessibility of theelectrode material to the electrolyte and hence the capacitive behavior.The CVmeasurements of the electrodes on ITO and ss substrates record-ed in 0.5 M Na2SO4 aqueous solution in potential window of −0.2 to+1.2 V/Ag/AgCl at 5 mVs−1 are shown in Fig. 8. The shape of the CVcurves indicates that the electrochemical capacitance is due mainly topseudocapacitance rather than electric double layer capacitance(EDLC) that is almost rectangular. The CV curve for CuO on ITO showstwo anodic peaks centered at 0.25 V and 1.0 V and a cathodic peak at0.95 V. On the ss, the cathodic peak centered around 0.95 V broadensout while another cathodic peak around 0 V becomes visible. In addi-tion, the anodic peaks shift to the more positive potential (from 0.25 Vand 1.0 V observed for the electrode on ITO substrate to 0.3 V and1.1 V on ss respectively). Upon introducing cobalt oxide into the CuOstructure to form the Cu-Co mixed oxide film, the pair of redox peaksbecomesmore visible for the films on ITOwhile the peaks on ss becomeless pronounced.

For the Co3O4 film, the CV curve shows some form of reversibility.The redox couples are not clearly visible. This is attributable to oxygenevolution reaction (OER) [52]. The redox peaks became more promi-nent with the addition of CuO into the Co3O4 spinel. The redox peaksfor the copper oxide correspond to the Faradaic redox reactions attrib-uted to Cu (0) and Cu(II) of porous CuO. The cathodic peaks of theCuO electrode centered around 0.0 V (ss) and−1.8 V (ITO) correspondto the reduction of CuO to Cu (I)while the broad peaks around 0.9 V (forboth the ss and ITO) results from the reduction of Cu(I) to Cu(0). The an-odic signal with peaks at 0.3 V (ss) and 0.25 V (ITO) represents the ox-idation of Cu (0) to Cu (I) and while the peaks around 1.1 V (ss) and1.0 V (ITO) correspond to combination of two processes: oxidation ofCu(0) to Cu(I) and Cu(0) to Cu(II) [53,54]. This explains why the oxida-tion peak at 1.0 V is larger in current with respect to the other oxidationpeak especially for the CuO on ITO. The redox peaks of the cobalt oxidemay be assigned to theCo (II)/Co (III) andCo (III)/Co (IV) redoxprocess-es during the intercalation/deintercalation of smaller H+ or the biggerNa+ into the matrix of the material. Likewise the peaks of the mixedoxide films arises from the reduction/oxidation processes of the elec-trode films. The Cu-Co and the Co-Cu mixed oxide samples on ss showa very sharp current rise at potentials after the last anodic peaks. Thishas been attributed to OER, which is associated with electrocatalytic ef-fect [33]. This indicates that the films are also good candidates for appli-cations in electrocatalysis.

The specific and volumetric capacitances were calculated from theCV curves using Eqs. 6 and 7 [53]:

Cs ¼∫V f

ViIdV

V f−Vi� �

m dV.

dt

� � ð6Þ

Cv ¼∫V f

ViIdV

V f−Vi� �

Aτ dV.

dt

� � ð7Þ

Fig. 6. SEM of the deposited films on ss (a and b) Cu-Co and (c and d) Co-Cu mixed oxide at two different magnifications 50 K and 100 K.

29A.C. Nwanya et al. / Journal of Electroanalytical Chemistry 787 (2017) 24–35

where Cs and Cv are the specific and volumetric capacitance in Fg−1 andFcm−3 respectively, I is the average current obtained from the integra-tion of the the area under the CV curve in either direction, (where, Vf

and Vi are the integration potential limits of the voltammetric curve),m is the activemass of the depositedmaterial in g dipped in the electro-lyte (obtained by measuring the weight difference of the substrates be-fore and after deposition), dV/dt is the potential scan rate inmVs−1, A isthe area of the electrode in and τ is the thickness. At 5mVs−1, the values

Fig. 7. AFM of the deposited films (a) Cu-Co mixed oxide on ITO (b) Cu-Co mix

of the Cs for the films on ITO are 522, 919, 385 and 446 Fg−1 for CuO, Cu-Co mixed oxide, Co-Cu mixed oxide and Co3O4 respectively. On the sssubstrates, the Cs are 235, 195, 141 and 487 Fg−1 respectively for CuO,Cu-Comixed oxide, Co-Cumixed oxide and Co3O4. The scan rate depen-dence of the gravimetric (specific) (Cs) and volumetric (Cv) capacitancesobtained using Eqs.s 6 and 7 respectively for themixed oxide electrodesare shown in Fig. 9(a and b) for the Cu-Co mixed oxide and the Co-Cumixed oxide respectively. Fig. 9(c and d) compares the scan rate

ed oxide on ss (c)Co-Cu mixed oxide on ITO (d)Co-Cu mixed oxide on ss.

Fig. 8. The cyclic voltammograms of the electrodes on ITO and ss.

Fig. 9. The scan rate dependence of the gravimetric (specific) (Cs) and volumetric (Cv) capacitances (a) Cu-Co mixed oxide film (b) Co-Cu mixed oxide film (c) films on ITO and(d) films on ss.

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Table 1Internal resistance associated with the current collectors for the various samples.

Electrode CuO Cu-Co oxide Co-Cu oxide Co3O4

Current collector ITO ss ITO ss ITO ss ITO ssRi (Ω) 1.23 0.241 1.98 0.14 0.27 0.14 4.32 0.52

Fig. 10. Galvanostatic charge-discharge curve (a) samples on ITO (at 4.0 Ag−1 current density) (b) samples on ss (at 2.5 Ag−1 current density) (c) Variation of capacity retention of themixed oxide electrode films with number of cycles.

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dependence of capacitances of themixed oxide electrodes on ITO and ssrespectively. Generally, both the Cs and Cv decreased with increase inscan rates. This is because at low scan rates, electrolyte ions and elec-trons have enough time to access the interior reaction sites of the elec-trodes while at high scan rates, charge movements within theelectrodes are not synchronized with the rapid transfer of electrons inthe external circuit. This leads to a decrease in the capacitance valuesas the scan rate increases.

Generally, themixed oxidefilm electrodes on ITO gave higher capac-itances (both Cs and Cv) than their counterparts on ss (Fig. 9a and b).This could be attributed to the higher thickness of the films on ss sinceonly the surface/near surface layer of the film that is close to the currentcollector substrate is electrochemically active. Comparing the differentmixed oxide film electrodes on the same substrate, the Cu-Co/ITO elec-trode gave higher Cs than Co-Cu/ITO electrode. On the ss the values fromCu-Co/ss are a bit higher than Co-Cu/ss (Fig. 9c and d) electrode. The Cvfor the Cu-Co mixed oxide film on ss is also higher over the entire scanrate range. However, on ITO, the two electrodes have very close valuesof Cv at lower scan rates but at scan rate ˃20 mVs−1, the Co-Cu mixedoxide electrode gave slightly higher values. These relatively highervalues exhibited by Cu-Co mixed oxide film could be due to improvedsynergistic interaction of the films with the substrates as well as thenanoporous nature of the electrode that exposed more active surfaceswith minimal ion diffusion length thereby enhancing the redox behav-ior. The improved synergistic interaction could possibly be due to alower surface energy between the substrate (ITO) and grain aggregationthat leads to formation of Cu-Comixed oxidefilm. These led to easier in-sertion/de-insertion of ions into its matrix.

The values of the specific capacitances obtained for both CuO andCo3O4 are much higher than what most previous studies reported. Forexample, Kandalkar et al. [55], obtained 165 Fg−1 for low temperature

deposited cobalt oxide at 10 mVs−1. Jagadale et al. [11,40], using elec-trodeposited Co3O4 thin films, obtained maximum values of 248 Fg−1

and 365 Fg−1 respectively, at 5 mVs−1. Our value of the specific capac-itance for CuO films on ITO obtained at 5mVs−1 scan rate is comparablewith the highest value (535 Fg−1) obtained by Gund et al., [56] for sur-factant assisted CuO films but much higher than the highest values(396 Fg−1, 43 Fg−1) obtained by Dubal et al., [37,57] and 37 Fg−1 byPatake et al., [58]. The Cu-Comixed oxidefilm on ITO gave exceptionallyhigh specific and volumetric capacitances of 919 Fg−1 and 616 Fcm−3

respectively at a scan rate of 5 mVs−1. The value of the volumetric ca-pacitance is very high compared to other electrodes described in the lit-erature [59,60]. This comparative analysis shows that our surfactant-free nanostructured films are very good potential electrodes forpseudocapacitor applications.

3.8.2. Galvanostatic charge discharge (GCD)Galvanostatic charge-discharge (GCD) measurements were carried

out in the same electrolyte condition as the CV. GCD is used to under-stand the supercapacitive properties of the electrodes better, and thecurves are shown in Fig. 10. The GCD of the electrode filmswere carriedout at 4.0 Ag−1and 2.5 Ag−1 current densities respectively for the elec-trodes on ITO and ss in a potential window of−0.2 to +1.2 V/Ag/AgCl.Three variation ranges can be observed in the charge-discharge curves

Fig. 11.Nyquist plots (a,c) and Bode plots (b,d) for Cu-Co (a,b) and Co-Cumixed oxide samplesplots of the single oxides.

Table 2Specific power and specific energy of the electrode materials.

Current density(mAcm−2)

Parameter Electrode materials

Co-Cu/ITO Co-Cu/ss Cu-Co/ITO Cu-Co/ss

1.00 Specific power(kWkg−1)

15.05 5.88 51.8 6.930.125 1.88 0.74 6.66 0.871.00 Specific Energy

(Whkg−1)25.08 6.53 28.78 7.70

0.125 65.84 30.64 106.12 36.58

32 A.C. Nwanya et al. / Journal of Electroanalytical Chemistry 787 (2017) 24–35

of the electrodes. The first is the sudden drop in current at the onset ofdischarge, which is due to internal resistance. The second is the linearvariation of the time dependence of the potential, which is a character-istic of double layer capacitancewhile the third is the slope dependenceof the potential, which is due to redox reactions resulting in pseudo ca-pacitance at the electrode/electrolyte interface [59]while the second re-gion is characteristic of EDLC. We observed that the second region ismore pronounced for the composite films on ss. This is also evident intheir CV curve where the current is relatively constant between 0 and0.8 V. This implies that the composite films on ss show some EDLC

(c,d). (e) The Voigt electrical equivalent circuit used infitting the Nyquist plots. (f) Nyquist

Table 3EIS data of the Cu-Co and Co-Cu mixed oxides on ITO and stainless steel fitted the Voigtequivalent circuit.

Parameter Electrode materials

Co-Cu/ITO Co-Cu/ss Cu-Co/ITO Cu-Co/ss

Rs/Ω 44.15 ± 2.8 6.68 ± 0.74 42.55 ± 0.46 2.73 ± 0.22Q1/mF·s(α-1) 5.9 ± 1.03 1.66 ± 0.23 2.86 ± 0.38 2.48 ± 0.29n1 0.9 ± 0.01 0.81 ± 1.05 0.81 ± 0.13 0.70 ± 0.19Rct1/Ω 49.51 ± 3.82 10.06 ± 0.41 45.67 ± 5.8 10.71 ± 3.44Q2/mF·s(α-1) 58.09 ± 1.38 1.78 ± 0.2 4.82 ± 1.24 1.93 ± 0.64n2 0.82 ± 0.2 0.71 ± 0.16 0.68 ± 1.94 0.68 ± 0.25Rct2/Ω 448.9 ± 8.12 2243 ± 30.08 1653 ± 19.35 1178 ± 28.8Phase angle –80° –75° –78° –78°Knee frequency 79.4 Hz 3.2 kHz 126 Hz 10 kHz

33A.C. Nwanya et al. / Journal of Electroanalytical Chemistry 787 (2017) 24–35

characteristics than their counterparts on ITO. The internal resistance iscalculated using Eq. 8 [60].

Ri ¼ IRdrop�2Iconsm

ð8Þ

where Ri is the internal resistance, IRdrop is the voltage drop at the dis-charge side, Icons is the applied current and m is the mass of the activematerial. The calculated values as shown in Table 1 indicate that theelectrodes with ITO as current collectors showed higher internal resis-tances than those with stainless steel.

Next, we studied the long-term cyclability of the composite films byperforming repetitive charge–discharge cycling for 2000 cycles. The ca-pacitance retention is shown in Fig. 10c. The specific capacitance wascalculated using Eq. 9:

Cs ¼ I � tdΔV �m

ð9Þ

where Cs is the specific capacitance, I is the charge/discharge current, tdis the discharge time, ΔV is potential window and m is the mass of theactive material within the film.

The results show that the capacity retention of the films on ITO isbetter than that on ss. The Cu-Co mixed oxide on ITO retained close to100% of its capacitance up to the 700th cycle and about 75% of its initialcapacitance from the 800th to the 2000th cycle. The same film on ss re-duced to 75% of its capacitance by the 100th cycle though it remained atthis value until the 2000th cycle. By the 2000th cycle, the Co-Cu mixedoxide films on ITO have about 73% of their initial capacitance availablewhile the same film on ss has about 50%. This shows that while ssoffer less internal resistance, the stability of the films is better on ITO.Our result also shows that the Cu-Co mixed oxide electrode on ITOshows excellent capacitive values and high stability for applications inelectrochemical capacitive applications. The higher stability Cu-Co/ITOis due most likely to improved synergistic interaction of the Cu-Comixed oxide films with the ITO substrates. All the electrodes exhibit ap-preciable stability to be applied as pseudocapacitor electrodes.

We compared the energy content and rate capabilities for energystorage devices using energy density and power density respectively.The relationship between these two parameters is demonstrated byusing the Ragone plot. The specific power (SP) and specific energy(SE) were calculated using the following equations [61–63]:

SP ¼ I � ΔVm

ð10Þ

SE ¼ I � td � ΔVm

ð11Þ

Coulombicefficiency η%ð Þ ¼ tdtc

� 100 ð12Þ

The values of the SP and SE at current densities of 1 mAcm−2 and0.125 mAcm−2 are listed in Table 2. The SE increases with decrease inthe current density as a result of electrode polarization effect whilethe SP increases with increase in the current density. This trend indi-cates that the discharging capacity of the pseudocapacitor is directlyproportional to the discharge current. The coulombic efficiency obtain-ed is 100% for both the Cu-Comixed oxide and Co-Cumixed oxide filmson ITO while the values are 85% and 83% respectively for Cu-Co mixedoxide and Co-Cumixed oxide films on ss. These values further corrobo-rate the high stability of the films on ITO compared with ss as the lossesthat reduce the coulombic efficiency are minimal in the films on ITO.

3.8.3. Electrochemical impedance spectroscopy (EIS)Fig. 11 compares the Nyquist and Bode plots of the Co-Cu and Cu-Co

mixed oxides systems on the ITO and ss, which are satisfactorily fittedwith the Voigt RC electrical equivalent circuits (EEC, Fig. 11e). An ideal

EDLC should exhibit curves parallel with the imaginary impedanceaxis (y-axis) or perpendicular to the real (x-axis). Thus, the slight devi-ation from the perpendicularity observed in this work is ascribed to thepseudocapacitive properties of the electrodes. The EEC used in fittingthe raw EIS data consists of the electrolyte resistance (Rs), charge-trans-fer resistance (Rct) and constant-phase elements (CPE or Q). As shownin Table 3, the Rs values for the Co-Cu mixed oxide films on ITO (Co-Cu/ITO) and Co-Cu mixed oxide on ss (Co-Cu/ss) systems are approxi-mately 44 and 6.7 Ω, respectively.

For the Cu-Co/ITO and Cu-Co/ss systems, the Rs values are approxi-mately 43 and 2.7 Ω, respectively. The combined Rct values(Rct1 + Rct2) for the Co-Cu/ITO and Co-Cu/ss systems are 0.5 and2.253 kΩ, respectively. For the Cu-Co/ITO and Cu-Co/ss systems, theequivalent total Rct values are 1.699 and 1.189 kΩ, respectively. The re-sult shows that the series resistance is lowest at the ss steel substratebut the electrode system that offers the least charge-transfer resistanceis the Co-Cu/ITO with ca. 0.5 kΩ. In addition, the n values are between0.7 and 0.9, which are lower than unity (expected for an ideal EDLC).The deviation from the EDLC further proves the pseudocapacitive prop-erties of the Co-Cu and Cu-Comixed oxide systems. From the Bode plots(Fig. 11b, d), the phase angles are ca. –80°, which is lower than –90° foran ideal EDLC, further confirming the pseudocapacitive nature of theelectrodes. The maximum frequency at which the pseudocapacitorshows its best power-generating properties is known as its knee fre-quency (fo, ϕ = 45°). The higher the value of the knee frequency, thefaster the pseudocapacitor can be charged and discharged (i.e. highpower density). From the Bode plots, the Co-Cu/ss gave the highest fovalue of 10 kHz (time constant = 10 μs) which confirms the highpower capability of this electrode system.

For comparison, the Nyquist plot of the single oxides is shown inFig. 11f while the fitted circuit elements are shown in Table 4. Thetable shows that the equivalent total charge transfer resistance(Rct1+ Rct2) values for CuO/ITO (5.2 kΩ) and CuO/ss (0.5 kΩ) electrodesrespectively reduced to 1.7 kΩ for Cu-Co/ITO electrodes and increasedto 1.2 kΩ for the Cu-Co/ss. This may have contributed to the increasein the value of Cs for Cu-Co/ITO (919 Fg−1) compared to the value forCuO/ITO (522 Fg−1) and the lower value of the Cs for Cu-Co/ss(195 Fg−1) compared to that of CuO/ss (235 Fg−1). The combinedcharge transfer resistance of the Co-Cu/ss (2.3 kΩ) and Co-Cu/ITO(0.5 kΩ) are much lower than that of Co3O4/ss (4.3 kΩ) and Co3O4/ITO (2.3 kΩ) respectively. However, the values of Cs for Co3O4/ss andCo3O4/ITO are higher than that of Co-Cu/ss and Co-Cu/ITO. This impliesthat the combined charge transfer resistance did not play a significantrole in the determination of the Cs for these electrodes.

4. Conclusions and perspectives

In summary, we used a facile, cost effective, binderless, additive freeand scalable chemical method to deposit nanoporous mixed oxides ofcopper and cobalt on indium tin oxide and stainless steel substrates.The morphology of the nanoporous Cu-Co and Co-Cu mixed oxide

Table 4EIS data of the copper oxide and cobalt oxides on ITO and stainless steel fitted with theVoigt equivalent circuit.

Parameter Electrode materials

CuO on ITO CuO on ss Co3O4 on ITO Co3O4 on ss

Rs/Ω 31.35 ± 2.84 6.27 ± 0.56 204 ± 0.83 0.41 ± 0.19Q1/mF·s(α-1) 0.48 ± 0.16 11.84 ± 2.01 1.75 ± 0.18 0.23 ± 0.13n1 0.90 ± 0.19 0.88 ± 0.11 0.89 ± 0.27 0.81 ± 0.13Rct1/Ω 51.56 ± 2.96 11.98 ± 4.31 209.8 ± 5.33 16.6 ± 2.58Q2/mF·s(α-1) 46.82 ± 3.11 79.71 ± 2.84 94.95 ± 0.73 53.40 ± 1.39n2 0.59 ± 0.18 0.61 ± 0.24 0.48 ± 0.21 0.78 ± 0.15Rct2/Ω 5154 ± 28.71 481 ± 6.71 2145 ± 8.53 4340 ± 5.55

34 A.C. Nwanya et al. / Journal of Electroanalytical Chemistry 787 (2017) 24–35

electrodes provides sufficient active sites for redox reactions and trans-port routes for electrolyte ions. The Cu-Co oxide film on ITO delivers ex-ceptionally high specific and volumetric capacitances of 919 Fg−1 and616 Fcm−3 respectively at a scan rate of 5 mV/s together with superiorrate capability and excellent long-term cyclability. While the ss offersless internal and series resistance, the stability of the films is better onITO. Our results suggest that the Cu-Co oxide film electrode with ITOas current collector may provide a low cost and effective solution for fu-ture high performance supercapacitors. Based on these encouraging re-sults, future efforts will focus on studying the electrochemicalproperties of these electrodes in either symmetric or asymmetric twoelectrode configurations.

Acknowledgements

The UNN group thanks the US Army Research Laboratory for finan-cial support given to this research (under contract number W911NF-12-1-0588). Alsowe thank Engr. EmekaOkwuosaMDOilserv Ltd Groupof Companies for the generous sponsorship of April 2014 and August2016 Nano conferences/workshops on applications of nanotechnologyto energy, health &.Environment conference and for providing some re-search facilities. F.I.E. is grateful to the American Physical Society for atravel grant to visit INRS in Canada and the UNESCO Chair MATECSSfor a matching visiting fellowship. F.R. is grateful to NSERC for fundingand partial salary support through an EWR Steacie Memorial Fellow-ship. F.R. and A.R. are supported by NSERC Discovery Grants and FRQNTteam grants.

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