Highly Ordered Mesoporous CuCo2O4 Nanowires, a PromisingSolution for High-Performance SupercapacitorsAfshin Pendashteh,†,‡ Seyyed Ebrahim Moosavifard,† Mohammad S. Rahmanifar,§ Yue Wang,⊥

Maher F. El-Kady,⊥,∥ Richard B. Kaner,*,⊥ and Mir F. Mousavi*,†,⊥

†Department of Chemistry, Tarbiat Modares University, Tehran 14115-175, Iran‡IMDEA Energy Institute, ECPU, Avenida Ramon de la Sagra 3, 28935 Mostoles, Madrid, Spain§Faculty of Basic Science, Shahed University, Tehran 18151-159, Iran∥Department of Chemistry, Faculty of Science, Cairo University, Giza 12613, Egypt⊥Department of Chemistry and Biochemistry, California NanoSystems Institute, University of California, Los Angeles (UCLA),Los Angeles, California 90095, United States

*S Supporting Information

ABSTRACT: The search for faster, safer, and more efficient energy storagesystems continues to inspire researchers to develop new energy storagematerials with ultrahigh performance. Mesoporous nanostructures areinteresting for supercapacitors because of their high surface area, controlledporosity, and large number of active sites, which promise the utilization ofthe full capacitance of active materials. Herein, highly ordered mesoporousCuCo2O4 nanowires have been synthesized by nanocasting from a silica SBA-15template. These nanowires exhibit superior pseudocapacitance of 1210 F g−1 inthe initial cycles. Electroactivation of the electrode in the subsequent 250 cyclescauses a significant increase in capacitance to 3080 F g−1. An asymmetricsupercapacitor composed of mesoporous CuCo2O4 nanowires for the positiveelectrode and activated carbon for the negative electrode demonstrates anultrahigh energy density of 42.8 Wh kg−1 with a power density of 15 kW kg−1

plus excellent cycle life. We also show that two asymmetric devices in series can efficiently power 5 mm diameter blue, green, andred LED indicators for 60 min. This work could lead to a new generation of hybrid supercapacitors to bridge the energy gapbetween chemical batteries and double layer supercapacitors.

■ INTRODUCTION

The rapidly growing demand for electric vehicles and portableelectronics has stimulated a great deal of research to develophigh-performance electric energy storage devices.1−3 Super-capacitors, also known as ultracapacitors or electrochemicalcapacitors, are considered one of the most reliable energy storagedevices mainly due to their capability of providing quick bursts ofenergy and long lifespan. Current supercapacitors use carbon-based materials and store charge through non-Faradaic electricdouble layers (EDL). Capitalizing on Faradaic redox reactions,4,5

metal oxide- or conducting polymer-based pseudocapacitors6,7

show considerably higher specific capacitances than carbon-based supercapacitors.8 Transition metal oxides are consideredespecially promising as electrode materials for the next genera-tion of supercapacitors due to their multiple oxidation states.9

However, their poor electrical conductivity and cycling stabilityhave so far hindered practical applications.10 Therefore, it is a greatchallenge to boost the electrochemical performance of pseudoca-pacitive materials by carefully controlling their structure at thenanoscale and by designing the cell structure.11−15

Since only the surface of metal oxides can effectively contributeto the total capacitance, the preparation of porous metal oxide

nanostructures represents a promising solution toward harvest-ing their full capacitance.16 In addition, pore sizes and theirdistribution directly affect the ability of a material to functioneffectively as a supercapacitor. Therefore, development of nano-porous materials, especially metal oxides (consisting of micro-pores, <2 nm; mesopores, 2−50 nm; and macropores, >50 nm)with an extended range of pore sizes, can provide a promisingmethod to enhance the capacitive performance due to enhancedsurface area and short electron-/ion-transport pathways.11

From a wide range of pseudocapacitive materials, spinelstructures containing binary or ternary mixtures of metal oxidesare of great interest for energy storage applications.17−19 Amongthe various types of these structures, spinel cobaltites (MCo2O4)are promising because of the presence of mixed valence metalcations that provide higher electronic conductivity and electro-chemical activity in comparison with single-componentoxides.18−20 This makes MCo2O4 a promising electrode materialnot only for supercapacitors but also for Li-ion batteries.20−23

Received: February 23, 2015Revised: April 16, 2015Published: April 20, 2015

Article

pubs.acs.org/cm

© 2015 American Chemical Society 3919 DOI: 10.1021/acs.chemmater.5b00706Chem. Mater. 2015, 27, 3919−3926

CuCo2O4 is an interesting cobaltite spinel because of its low costand nontoxicity. Here, in order to improve the kinetics of theelectrode toward fast ion insertion/deinsertion, CuCo2O4nanostructures, such as nanoparticles and nanowires, havebeen studied. Previously, Wang et al. synthesized CuCo2O4nanowire arrays on a carbon-fabric substrate and obtained arelatively low specific capacitance of 57.8 F g−1 at a currentdensity of 1.25 A g−1.20 They then boosted the capacitive per-formance by fabricating core−shell CuCo2O4@MnO2 hetero-structured nanowire arrays to achieve a maximum specificcapacitance of 327 F g−1. Recently, we fabricated cauliflower-likeCuCo2O4 nanostructures via a simple urea combustion method,which provided a maximum specific capacitance of 338 F g−1

(at 1 A g−1) with good rate capability and cycling stability.24

Although these studies have demonstrated the promise ofCuCo2O4 electrodes, the reported specific capacitances are stillfar from values obtained for other oxides, resulting in super-capacitors with low energy density. While the performance ofthese nanostructured CuCo2O4 is better than the bulk material,the preparation of CuCo2O4 nanostructures with aligned porosityand improved electronic and ionic conductivities has yet to berealized. Such materials would greatly improve the charge storagecapacity of CuCo2O4 electrodes and their rate capability.By combining the unique properties of nanowires with a

controlled porous structure, improved performance can beachieved, as illustrated in Figure 1. The nanowires offer large

accessible specific surface area and effective sites for redoxreactions, making it possible to fully utilize the charge storageability of CuCo2O4. In addition, the aligned and interconnectedpores provide effective contacts between the nanowires and theelectrolyte ions resulting in shortened ion diffusion pathways and

facilitated ion transport. This is advantageous to nanostructureswith poorly accessible pores in which only a fraction of thematerial can contribute to charge storage (Figure 1).

■ EXPERIMENTAL SECTIONAll chemicals were directly used as purchased without furtherpurification.

Synthesis ofMesoporous SBA-15 Silica Template. Amesoporoussilica template with hexagonal P6mm symmetry (SBA-15) was preparedaccording to previous reports.25,26 Tetraethyl orthosilicate (TEOS) wasused as the silica source, and a hydrothermal reaction was performed inan autoclave at 100 °C for 24 h. The final solid product was calcined at550 °C (2° min−1) for 3 h to obtain the SBA-15 silica template.

Synthesis of Highly OrderedMesoporous CuCo2O4.A 0.01 molamount of Cu(NO3)2·3H2O and 0.02 mol of Co(NO3)2·6H2O weredissolved in 10 mL of doubly distilled water. A 2 mL aliquot of thesolution was added to 2 g of SBA-15 and then dispersed in 80 mL ofn-hexane under stirring for 8 h. The mixture was then filtered and driedat 60 °C. The obtained powder was calcined at 400 °C (1°min−1) for 5 hand the resulting material treated two times with a hot aqueous KOHsolution (2.0 M) to remove the silica template, washed with water, andthen dried at 60 °C. This sample is denoted as HO−CuCo2O4.

Synthesis of Disordered Mesoporous CuCo2O4. A 0.0025 molamount of Cu(NO3)2·3H2O, 0.005 mol of Co(NO3)2·6H2O, and0.025 mol of urea were dissolved in 50 mL of distilled water. Thesolution was transferred to an autoclave and heated to 180 °C for 6 h.The obtained precipitate was filtered, washed several times with waterand ethanol, and then dried at 60 °C. Next, the powder was calcined at400 °C for 5 h and is denoted as DO−CuCo2O4.

Materials Characterization. The crystal phase of each sample wasexamined using powder X-ray diffraction (XRD, Philips X’pertdiffractometer with Co Kα radiation (λ = 0.178 nm) generated at40 kV and 40 mA with a step size of 0.02° s−1). The sample morphologywas characterized by scanning electron microscopy (SEM, Philips andJEOL-JSM-6700) and transmission electron microscopy (TEM, PhilipsEM 208 and FEI Technai G2 TF20 operated at 200 kV). The nitrogen(N2) sorption measurements were performed using a Belsorp instru-ment at 77 K. The specific surface area was calculated using the Brunauer−Emmett−Teller (BET)method, and the porosity distribution was obtainedfrom the desorption branch of the isothermusing Barrett−Joyner−Halenda(BJH) analysis.

Electrochemical Measurements. The electrochemical measure-ments of the prepared samples were performed in a three-electrodeconfiguration in 6 M KOH electrolyte. The working electrodes wereprepared by mixing active material, carbon black, and polyvinylidenefluoride (PVDF) (10% solution in N-methyl-2-pyrolidone) with a massratio of 75:20:5. A 5% solution of themixture in isopropanol was sprayedonto Ni foam as the current collector. The prepared electrodes weredried at 60 °C overnight. A Pt plate was used as the auxiliary electrodeand Hg/HgO as the reference electrode. An Autolab PGSTAT30 wasemployed to measure the electrochemical properties of the samplesthrough cyclic voltammetry (CV) and electrochemical impedancespectroscopy (EIS). The EIS measurements were conducted at opencircuit potentials. A Solartron battery test unit equipped with Cell Testsoftware (v. 3.5.0) was used for galvanostatic charge−discharge (GCD)measurements.

Asymmetric Cells. Asymmetric supercapacitors were assembled byintegrating a HO−CuCo2O4 positive electrode with an activated carbon(AC) negative electrode in 6 M KOH electrolyte solution. A cellulosicpaper was used as the separator. It should be mentioned that in order toutilize its full pseudocapacitance, the positive electrode was electro-activated before integration into the asymmetric cell by runningcontinuous charge/discharge for 250 cycles in a three-electrode setup.The as-fabricated HO−CuCo2O4//AC asymmetric supercapacitorswere then subjected to CV, GCD, and EIS measurements. Finally, thecycle life of this asymmetric supercapacitor was tested over 5000 charge/discharge cycles at a current density of 6 A g−1.

Figure 1. Schematic comparison of the pore accessibility in the highlyordered (HO) HO−CuCo2O4 and disordered (DO) DO−CuCo2O4mesoporous samples.

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■ RESULTS AND DISCUSSION

Highly orderedmesoporous CuCo2O4 nanowires (HO−CuCo2O4)were synthesized by nanocasting from a silica SBA-15 hard template(Figure 2). Moreover, a nontemplate hydrothermal route wasemployed to synthesize a disordered porous sample (DO−CuCo2O4).

The composition of the samples has been characterized via XRD(Figure 3a). All diffraction peaks can be indexed to the pure cubicspinel phase of copper cobaltite (JCPDS File No. 001-1155).17

The sharp peak at ∼1.07° in a low-angle XRD pattern forHO−CuCo2O4 (Figure 3b) can be indexed as the (100) reflection

Figure 2. Schematic preparation steps of highly ordered mesoporous CuCo2O4 nanowires via nanocasting from a silica template.

Figure 3. Powder XRD patterns of the as-prepared CuCo2O4 samples (a) and small-angle XRD pattern of the HO−CuCo2O4 sample (b). TEM imagesof the sample from different orientations: (c) view from the top showing the tips of the nanowires and (d) a side view of the nanowire bundle. (e)HRTEM image of the sample and (f) corresponding SAED pattern.

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of the P6mm space group anticipated for the replica supra-molecular structure of SBA-15,17 showing the complete replica-tion of the SBA-15 mesoporous system by CuCo2O4.Scanning electronmicroscopy was employed to investigate the

samples morphologically, showing uniform elongated-shapedparticles for theHO−CuCo2O4 sample (Supporting Information(SI) Figure S1a,b). Magnified SEM images zooming into theHO−CuCo2O4 particles from different viewing directions revealeach particle is further comprised of three-dimensional porousstructures (SI Figure S1c,d). In contrast, the DO−CuCo2O4sample is comprised of nonuniform particles with a broad particlesize distribution (i.e., 50 to >600 nm), with no internal porousstructures (SI Figure S1e,f).The highly ordered mesopores of the HO−CuCo2O4 sample

is evident through TEM (Figure 3c,d). A porous particle com-prised of oriented CuCo2O4 nanowires (as viewed from the top)shows the tips of the nanowires (Figure 3c) clearly corroboratingthe hexagonal arrangement of the replica in the orientation of(100) planes created by the template. In Figure 3d, a side-viewimage (i.e., from the elongated direction) of such a particleillustrates that the HO−CuCo2O4 sample is comprised of indi-vidual nanowires organized into parallel bundles with sizes in therange of a few hundred nanometers in length. The mesostructureregularity can be seen through all of the particle domains(SI Figure S2a,b), demonstrating that no obvious nonporousparticles were formed. As can be observed in Figure 3d, thediameter and the interwire spacing (i.e., pores) of the orientednanowires are estimated to be around 7.2 and 2.9 nm, respec-tively, in good agreement with the previous reported values forthe SBA-15 silica template.27 A high-resolution TEM (HRTEM)

image of the HO−CuCo2O4 sample (Figure 3e) reveals thatthe nanowires are comprised of nanocrystalline domains of thespinel cobaltite. Lattice fringes with a d-spacing of 0.47 nm areobserved, corresponding to the (111) crystallographic plane ofthe cubic spinel CuCo2O4. On the other hand, TEM images ofthe DO−CuCo2O4 (SI Figure S2c,d) reveal a disordered porousstructure, with pore sizes ranging from 3 to 15 nm. The dis-ordered characteristic of the pores hinders their access to theelectrolyte and feasibility of fast ion movements which sig-nificantly suppresses electrochemical reactions. This is especiallyimportant in supercapacitor applications.In order to further investigate the crystallographic character-

istics of themesoporous sample, selected area electron diffraction(SAED) was performed (Figure 3f). The ring diffraction patternillustrates the polycrystalline nature of the mesoporous nano-wires. The diffraction rings show d-spacings in agreement withreference values (SI Table S1) that can be indexed to the (111),(220), (311), (400), (422), (440), and (511) planes of the spinelCuCo2O4, which is consistent with the XRD results in Figure 3a.Nitrogen (N2) adsorption−desorption measurements were

conducted to evaluate the pore sizes, their distribution, and theBrunauer−Emmett−Teller (BET) surface area of the samples. SIFigure S3a,b clearly shows the mesostructural features of thesampletypical type IV isotherms including type H1 hysteresisloops.28 Accordingly, BET surface areas of 59.34 and 37.08 m2 g−1

(SI Table S2) were obtained for the HO−CuCo2O4 andDO−CuCo2O4 samples, respectively.The electrochemical performances of the samples as super-

capacitor electrodes were evaluated in a three-electrode con-figuration using 6 M KOH solution as the electrolyte. Figure 4a

Figure 4. Electrochemical characterizations of the highly orderedmesoporous (HO−CuCo2O4) and disorderedmesoporous (DO−CuCo2O4) samples assupercapacitor electrode material: (a) CV curves of the samples at a scan rate of 5 mV s−1; (b) charge−discharge profiles of the samples at a current densityof 2 A g−1; (c) charge−discharge profiles of the HO−CuCo2O4 at various current densities ranging from 2 to 20 A g−1; (d) calculated capacitance as afunction of current density; (e) cycling performance of the HO−CuCo2O4 sample at various current densities of 2, 5, 10, and 20 A g−1 during 4250 cycles;and (f) Nyquist plots of the HO and DO samples. The inset shows the proposed equivalent circuit: ESR, Rct,W, CPE1, and CPE2 refer to equivalent seriesresistance, Faradaic charge transfer resistance, Warburg impedance, double layer capacitance, and Faradaic pseudocapacitance, respectively.

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shows cyclic voltammetry (CV) of the HO−CuCo2O4 andDO−CuCo2O4 samples at a scan rate of 5 mV s−1. For bothsamples, the shape of the CV curves indicates pseudocapacitivebehavior arising from Faradaic reactions of the Co4+/Co3+ andCu2+/Cu+ redox pairs. The exact electrochemical behavior ofcopper and cobalt oxides in strong alkaline solutions needsfurther investigation due to several possible phases for thesecations and the complex nature of the system. A possiblemechanism is that by initiating the scan from cathodic potentials,CoOOH29 and CuOH may form at the outer surface of theCuCo2O4 electrode, according to the following equation:

+ + ↔ +−eCuCo O H O 2CoOOH CuOH2 4 2 (1)

Then, by sweeping the potential toward positive values, redoxreactions occur as follows:

+ ↔ + +− −eCoOOH OH CoO H O2 2 (2)

+ ↔ +− −eCuOH OH Cu(OH)2 (3)

The area under the CV curve is proportional to the chargestored during the anodic and cathodic scans, while the specificcapacitance (i.e., the capacitance per unit mass of electrode activematerial) can be calculated using the equation

∫ υ= ΔC i v m vd /s (4)

where ∫ i dv is the integration of the current during discharge(i.e., cathodic scan), m is the loading mass of the active material,and υ is the scan rate. Accordingly, a high specific capacitance ofabout 1150 F g−1 is obtained for the HO−CuCo2O4 sample,much larger than the value obtained for the DO−CuCo2O4sample (260 F g−1). Moreover, CV measurements for thesamples at various scan rates ranging from 5 to 80 mV s−1

(SI Figure S4a,b) showed a slight increase in peak separationsthat is likely due to a small ohmic resistance and polarization. Thelinear dependence of peak current density against the square rootof the scan rate in both samples (SI Figure S5) illuminates thediffusion control characteristic of the redox reaction that canbe understood by dependency of the reactions (eqs 2 and 3) onthe diffusion of the OH− ions.In order to evaluate applicability of CuCo2O4 as super-

capacitor electrode materials, charge−discharge measurementswere conducted at different current densities. Figure 4b showsthe real time curves for HO−CuCo2O4 and DO−CuCo2O4samples at a current density of 2 A g−1, in a potential range of0−0.5 V. The voltage plateau is characteristic of pseudocapaci-tance due to charge storage based upon Faradaic redox reactionsat the electrode−electrolyte interface (in good agreement withthe CV curves).The specific capacitance is calculated by the formula

= Δ ΔC I t m V( )/( )s (5)

where I is the discharge current,Δt is the discharge time,m is themass of the active material, and ΔV is the potential window.30

Here, a specific capacitance as high as 1210 F g−1 (arealcapacitance of 0.6 F cm−2) is obtained for the HO−CuCo2O4sample at a current density of 2 A g−1, which is more than 4 timesgreater than that achieved byDO−CuCo2O4 (270 F g

−1). Takingadvantage of its highly ordered mesoporous structure, theHO−CuCo2O4 sample provides thorough access of the elec-trode surface to the electrolytic ions, resulting in superior capacitanceperformance in comparison with the DO−CuCo2O4 sample. Inother words, the presence of abundant diffusion channels for

OH− anions remarkably enhances the utilization of the internaland external surfaces of the HO−CuCo2O4 electrode, a phe-nomenon that is significantly restricted to the external surfaces inthe DO−CuCo2O4 sample (Figure 1).

31,32

In order to test the rate capability of the electrodes, charge−discharge measurements were performed at various currentdensities ranging from 2 to 20 A g−1. The corresponding profilesfor the HO−CuCo2O4 electrode are shown in Figure 4c, demon-strating the maintenance of voltage plateaus even when operatedat a high current density of 20 A g−1. The calculated specificcapacitances for the HO−CuCo2O4 and DO−CuCo2O4 electro-des versus the applied current density are presented in Figure 4d.As can be seen, with a 10-fold increase in current density from2 to 20 A g−1, 64% of the initial capacitance was retained in theHO−CuCo2O4 electrode, indicating its high rate capability. Thiscontrasts with the DO−CuCo2O4 electrode, which shows onlyabout 35% capacitance retention after the increased currentdensity. This suggests that the interconnected channels in theHO−CuCo2O4 provide efficient ion transport within theelectrode even when tested at high current densities (Figure 1).To further demonstrate the advantages of the HO−CuCo2O4

electrode architecture on its electrochemical behavior, thecycling performance was tested using sequential charge/discharge curves and the results are presented in Figure 4e.The specific capacitance increases upon cycling reaching

an ultrahigh capacitance of 3080 F g−1 in the course of the first250 cycles at a current density of 2 A g−1. This remarkableincrease in capacitance may be attributed to structural activationand full pore opening during insertion/deinsertion of ionsthrough themesopore channels and possibly micropore channelsof the sample.33 Such an increase in capacitance during initialcycling has been previously observed.24,34 After 1000 cycles at2 A g−1, the electrode was tested at progressively increasedcurrent densities. Despite the sudden change of the appliedcurrent density, the electrode exhibits relatively stable capac-itance. We observe some fluctuations at higher current densities,which could be attributed to partial degradation of fragile wallsof smaller pores due to consecutive insertion/deinsertion of ionsat high rates. After 4000 continuous cycles at varying currentdensities, the current was turned back to 2 A g−1. Remarkably,93.5% of the capacitance after activation can still be deliveredand maintained for another 250 cycles without any noticeablechanges. This interesting cycling performance demonstrates thesuitability ofHO−CuCo2O4 electrodes for practical energy storage.Based on the preceding overall electrochemical performance,

the unique highly ordered mesoporous CuCo2O4 on Ni foamelectrode has been found to be at least comparable or superiorto other copper cobaltite electrodes, single-component metaloxides including CuO and Co3O4, and binary metal oxides as asupercapacitor electrode material (Supporting InformationTable S3).Electrochemical impedance spectroscopy (EIS) was per-

formed to provide further insight into the electrochemicalbehavior of the electrodes, respectively. Figure 4f shows Nyquistplots for HO−CuCo2O4 and DO−CuCo2O4 electrodes. Inaccordance with a CNLS fitting using the proposed equivalentcircuit (Figure 4f, inset), the diameter of the semicircle, whichcorresponds to the charge transfer resistance, is significantlysmaller in the HO−CuCo2O4 electrode (3.77 Ω) when com-pared to that of the DO−CuCo2O4 electrode (10.6 Ω). Thex-intercept of the Nyquist plot suggests a very small equivalentseries resistance (ESR) for the HO−CuCo2O4 electrode (0.54Ω)as opposed to 0.96 Ω for the DO−CuCo2O4 electrode. These

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results demonstrate that the HO−CuCo2O4 aligned nanowireshave lower interparticle resistance and better contact with thecurrent collector than the DO−CuCo2O4 electrode. Furtherinvestigations indicate that the slope of the linear section atlow frequencies is steeper (Table 1), a characteristic of pure

capacitive behavior. This structural activation behavior becomes evenbetter after 300 cycles of charge and discharge (SI Figure S6),indicating facilitated ion diffusion and electrolyte access to the surface

of the electrode.All of these observations demonstrate the importanceof mesoporous pseudocapacitive nanowires for supercapacitorelectrodes. The high surface area of the nanowires provides moreactive sites for charge transfer, thus increasing the overall capacity ofthe electrodes. In addition, the aligned mesopores facilitate iontransport kinetics during charge/discharge processes.Up to this point, we have only discussed the electrochemical

performance of HO−CuCo2O4 in a three-electrode config-uration. To further evaluate the HO−CuCo2O4 electrode for realapplications, asymmetric supercapacitors were made by integrat-ing the HO−CuCo2O4 electrode as the positive electrode (afteractivation by 250 cycles), activated carbon (AC) as the negativeone, and a cellulosic paper as the separator in 6 M KOH solution.This configuration combines a high-energy electrode with a high-power electrode to produce a hybrid energy storage system withthe best attributes of both. In order to achieve the maximumcapacitance and best cycle life, the masses of positive and negativeelectrodes were adjusted according to the following equation:

= → = × Δ× Δ+ −

+

−

− −

+ +Q Q

mm

C VC V

for(6)

Table 1. Electrical Parameters for CuCo2O4 from EISMeasurements at OCP

circuit elements HO−CuCo2O4 DO−CuCo2O4

ESR (Ω) 0.544 0.960Rct (Ω) 3.77 10.6n1a 0.860 0.821

n2a 0.982 0.890

aThe slope of the linear section at low frequencies is steeper for theHO−CuCo2O4 sample (phase elements, n, is 0.86 and 0.98 for CPE1 andCPE2, respectively), while these values obtained for the DO−CuCo2O4electrode are 0.82 and 0.89, characteristic of pure capacitive behavior.

Figure 5. (a) CVs and (b) charge−discharge curves for the HO−CuCo2O4//AC asymmetric supercapacitor; (c) specific capacitances and (d) Ragoneplot of the asymmetric supercapacitor at various current densities; (e−e″) Photographs showing two supercapacitors in series which can light up blue,green, and red LED indicators, respectively, during 60 min.

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According to this equation a charge balance between the twoelectrodes is reached when m+/m‑= 0.12. Figure 5a shows CVcurves of the asymmetric HO−CuCo2O4//AC supercapacitor atvarious scan rates. It is noticed that a maximum operating voltageof 1.5 V was obtained thanks to the asymmetric configurationwhich takes advantage of the different stable potential windowsof the positive and negative electrodes to maximize the overallcell voltage. Clearly observed broad redox peaks indicate thepseudocapacitance contribution from the positive electrode. Thispseudocapacitance feature can also be seen in galvanostaticcharge−discharge curves acquired at different current densitiesas shown in Figure 5b and SI Figure S7. The discharge curvesexhibit a small IR drop even at high current densities (Figure 5b),which reveal the low internal resistance of the asymmetric device.Additionally, the charge and discharge profiles are completelysymmetric which suggest an excellent electrochemical reversibilityand good Coulombic efficiency. We also tested the rate capabilityof the supercapacitor over a wide range of current densities from1 to 20 A g−1 as shown in Figure 5c. Interestingly, the specificcapacitance reaches a maximum of 137 F g−1 at 1 A g−1 andretains 67 F g−1 at a high current density of 20 A g−1. Thisexcellent rate performance can be attributed to the nanoporousHO−CuCo2O4 electrode whose surface is highly accessible tothe electrolyte and the rapid charge/discharge characteristics ofthe activated carbon negative electrode. Furthermore, the cyclelife, which is an important feature of supercapacitors, is excellentwith a capacitance retention of 86% after 5000 charge/dischargecycles at a relatively high current density of 6 A g−1, SI Figure S8.We also ran EIS experiments in order to gain a deeper under-standing of the electrochemical performance of the asymmetriccell. As shown in Nyquist plots in SI Figure S9, the small chargetransfer resistance at high frequencies verifies the facilepseudocapacitance feature of the device, whereas the linearpart observed at low frequencies reflects its excellent capacitorcharacteristics. In addition, the intercept of the Nyquist curve onthe real axis is only 0.33 Ω, manifesting the very low internalresistance of the asymmetric supercapacitor.Energy density and power density are considered the twomain

parameters used to characterize the performance of super-capacitors. Figure 5d depicts the Ragone plot which correlatesthe energy and power densities of the HO−CuCo2O4//ACasymmetric supercapacitor at various current loads. The max-imum energy density of the supercapacitor is 42.81 Wh kg1−

which decreases to 21.15 Wh kg−1 as the power density increasesfrom 0.75 to 15.0 kW kg−1. These values are much higher thanmost of the asymmetric aqueous supercapacitors reported in theliterature such as graphene/MnO2//graphene (30.4 Wh kg−1 at0.1 kWkg−1),35 CoO@PPy//AC (11.8Wh kg−1 at 5.5. kW kg−1),36

graphene−NiCo2O4//AC (7.6 Wh kg−1 at 5.6 kW kg−1),37 andnanoporous Ni(OH)2/ultrathin graphite film//microwave exfoli-ated graphite oxide (6.9 Wh kg−1 at 44.0 kW kg−1).38 Figure 5e−e″presents a quick demonstration for the viability ofHO−CuCo2O4//AC asymmetric supercapacitor for practical applications. In thisexperiment, two asymmetric supercapacitors assembled in seriessuccessfully powered 5 mm diameter blue, green, and red roundlight-emitting diode (LED) indicators, respectively, for almost60 min (see video file in Supporting Information).

■ CONCLUSIONSIn summary, we have fabricated highly ordered mesoporousCuCo2O4 electrode materials by replication from a silica hardtemplate. Owing to the unique structural features and thoroughelectrolyte access to the surfaces of the active material via

interconnected channels, the electrode exhibits an outstandingspecific capacitance of 1210 F g−1 at a current density of 2 A g−1

that increases rapidly upon cycling to exceed 3000 F/g−1. We alsoshow that the HO−CuCo2O4 electrode can be integrated inan asymmetric supercapacitor that delivers a high energy densityof 42.81 Wh kg−1, which is among the highest reported energydensities for aqueous-based asymmetric supercapacitors. Thiswork provides an effective energy storage solution to circumventthe energy density limitation of the current generation of carbonsupercapacitors.

■ ASSOCIATED CONTENT*S Supporting InformationSEM images of HO and DO samples (Figure S1), TEM images ofDO sample (Figure S2),N2 adsorption−desorption isotherms andpore size distributions (Figure S3), CV curves of the samples atvarious scan rates (Figure S4), linear dependency of anodic peakcurrent against square root of the scan rate (Figure S5), Nyquistplots of the HO sample before and after 300 cycles (Figure S6),GCD profiles of the asymmetric supercapacitor at various currentdensities (Figure S7), cycle performance (Figure S8) and Nyquistplot (Figure S9) of the asymmetric supercapacitor, experimentaland standard d-spacing values for the HO sample (Table S1),characterization parameters of samples from N2 adsorption−desorption isotherms (Table S2), comparison of performance ofthe current work with previous reports (Table S3), and a video fileshowing two series connected asymmetric supercapacitors lightingup the blue, green, and red LED indicators, respectively, for60min. The Supporting Information is available free of charge on theACSPublicationswebsite atDOI: 10.1021/acs.chemmater.5b00706.

■ AUTHOR INFORMATIONCorresponding Authors*(R.B.K.) E-mail: kaner@chem.ucla.edu.*(M.F.M.) E-mail: mousavi@chem.ucla.edu.Author ContributionsA.P. conceptualized the idea, designed and performed experi-ments, analyzed the data, and wrote the first draft of themanuscript. S.E.M. performed the two-electrode experimentsand analyzed the data. M.S.R., M.F.E.-K., and R.B.K. wereinvolved in discussions on the design and interpretation of theexperiments. Y.W. obtained and analyzed the TEM and SAEDdata. M.F.M. supervised the project and was involved indiscussions and interpretation of the obtained results. All authorsdiscussed the results, commented on the manuscript, and havegiven approval to the final version of the manuscript.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was made possible through financial support from aNational Science Foundation Graduate Research Fellowship(Y.W.), the Tarbiat Modares University Research Council, IranNanotechnology Initiative Council (M.F.M.), and NanotechEnergy, Inc. (R.B.K.).

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