Highly Ordered Mesoporous CuCo2O4 Nanowires ... - ACS Publications

Apr 20, 2015 - IMDEA Energy Institute, ECPU, Avenida Ramon de la Sagra 3, 28935 Mostoles, Madrid, Spain. §. Faculty of Basic Science, Shahed Universi...
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Highly Ordered Mesoporous CuCo2O4 Nanowires, a Promising Solution for High-Performance Supercapacitors Afshin 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 storage systems continues to inspire researchers to develop new energy storage materials with ultrahigh performance. Mesoporous nanostructures are interesting for supercapacitors because of their high surface area, controlled porosity, and large number of active sites, which promise the utilization of the full capacitance of active materials. Herein, highly ordered mesoporous CuCo2O4 nanowires have been synthesized by nanocasting from a silica SBA-15 template. These nanowires exhibit superior pseudocapacitance of 1210 F g−1 in the initial cycles. Electroactivation of the electrode in the subsequent 250 cycles causes a significant increase in capacitance to 3080 F g−1. An asymmetric supercapacitor composed of mesoporous CuCo2O4 nanowires for the positive electrode and activated carbon for the negative electrode demonstrates an ultrahigh 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, and red LED indicators for 60 min. This work could lead to a new generation of hybrid supercapacitors to bridge the energy gap between chemical batteries and double layer supercapacitors.

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INTRODUCTION The rapidly growing demand for electric vehicles and portable electronics has stimulated a great deal of research to develop high-performance electric energy storage devices.1−3 Supercapacitors, also known as ultracapacitors or electrochemical capacitors, are considered one of the most reliable energy storage devices mainly due to their capability of providing quick bursts of energy and long lifespan. Current supercapacitors use carbonbased materials and store charge through non-Faradaic electric double layers (EDL). Capitalizing on Faradaic redox reactions,4,5 metal oxide- or conducting polymer-based pseudocapacitors6,7 show considerably higher specific capacitances than carbonbased supercapacitors.8 Transition metal oxides are considered especially promising as electrode materials for the next generation of supercapacitors due to their multiple oxidation states.9 However, their poor electrical conductivity and cycling stability have so far hindered practical applications.10 Therefore, it is a great challenge to boost the electrochemical performance of pseudocapacitive materials by carefully controlling their structure at the nanoscale and by designing the cell structure.11−15 Since only the surface of metal oxides can effectively contribute to the total capacitance, the preparation of porous metal oxide © 2015 American Chemical Society

nanostructures represents a promising solution toward harvesting their full capacitance.16 In addition, pore sizes and their distribution directly affect the ability of a material to function effectively as a supercapacitor. Therefore, development of nanoporous materials, especially metal oxides (consisting of micropores, 50 nm) with an extended range of pore sizes, can provide a promising method to enhance the capacitive performance due to enhanced surface area and short electron-/ion-transport pathways.11 From a wide range of pseudocapacitive materials, spinel structures containing binary or ternary mixtures of metal oxides are of great interest for energy storage applications.17−19 Among the various types of these structures, spinel cobaltites (MCo2O4) are promising because of the presence of mixed valence metal cations that provide higher electronic conductivity and electrochemical activity in comparison with single-component oxides.18−20 This makes MCo2O4 a promising electrode material not only for supercapacitors but also for Li-ion batteries.20−23 Received: February 23, 2015 Revised: April 16, 2015 Published: April 20, 2015 3919

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

CuCo2O4 is an interesting cobaltite spinel because of its low cost and nontoxicity. Here, in order to improve the kinetics of the electrode toward fast ion insertion/deinsertion, CuCo2O4 nanostructures, such as nanoparticles and nanowires, have been studied. Previously, Wang et al. synthesized CuCo2O4 nanowire arrays on a carbon-fabric substrate and obtained a relatively low specific capacitance of 57.8 F g−1 at a current density of 1.25 A g−1.20 They then boosted the capacitive performance by fabricating core−shell CuCo2O4@MnO2 heterostructured nanowire arrays to achieve a maximum specific capacitance of 327 F g−1. Recently, we fabricated cauliflower-like CuCo2O4 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 of CuCo2O4 electrodes, the reported specific capacitances are still far from values obtained for other oxides, resulting in supercapacitors with low energy density. While the performance of these nanostructured CuCo2O4 is better than the bulk material, the preparation of CuCo2O4 nanostructures with aligned porosity and improved electronic and ionic conductivities has yet to be realized. Such materials would greatly improve the charge storage capacity of CuCo2O4 electrodes and their rate capability. By combining the unique properties of nanowires with a controlled porous structure, improved performance can be achieved, as illustrated in Figure 1. The nanowires offer large

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EXPERIMENTAL SECTION

All chemicals were directly used as purchased without further purification. Synthesis of Mesoporous SBA-15 Silica Template. A mesoporous silica template with hexagonal P6mm symmetry (SBA-15) was prepared according to previous reports.25,26 Tetraethyl orthosilicate (TEOS) was used as the silica source, and a hydrothermal reaction was performed in an autoclave at 100 °C for 24 h. The final solid product was calcined at 550 °C (2° min−1) for 3 h to obtain the SBA-15 silica template. Synthesis of Highly Ordered Mesoporous CuCo2O4. A 0.01 mol amount of Cu(NO3)2·3H2O and 0.02 mol of Co(NO3)2·6H2O were dissolved in 10 mL of doubly distilled water. A 2 mL aliquot of the solution was added to 2 g of SBA-15 and then dispersed in 80 mL of n-hexane under stirring for 8 h. The mixture was then filtered and dried at 60 °C. The obtained powder was calcined at 400 °C (1° min−1) for 5 h and the resulting material treated two times with a hot aqueous KOH solution (2.0 M) to remove the silica template, washed with water, and then dried at 60 °C. This sample is denoted as HO−CuCo2O4. Synthesis of Disordered Mesoporous CuCo2O4. A 0.0025 mol amount of Cu(NO3)2·3H2O, 0.005 mol of Co(NO3)2·6H2O, and 0.025 mol of urea were dissolved in 50 mL of distilled water. The solution was transferred to an autoclave and heated to 180 °C for 6 h. The obtained precipitate was filtered, washed several times with water and ethanol, and then dried at 60 °C. Next, the powder was calcined at 400 °C for 5 h and is denoted as DO−CuCo2O4. Materials Characterization. The crystal phase of each sample was examined using powder X-ray diffraction (XRD, Philips X’pert diffractometer with Co Kα radiation (λ = 0.178 nm) generated at 40 kV and 40 mA with a step size of 0.02° s−1). The sample morphology was characterized by scanning electron microscopy (SEM, Philips and JEOL-JSM-6700) and transmission electron microscopy (TEM, Philips EM 208 and FEI Technai G2 TF20 operated at 200 kV). The nitrogen (N2) sorption measurements were performed using a Belsorp instrument at 77 K. The specific surface area was calculated using the Brunauer− Emmett−Teller (BET) method, and the porosity distribution was obtained from the desorption branch of the isotherm using Barrett−Joyner−Halenda (BJH) analysis. Electrochemical Measurements. The electrochemical measurements of the prepared samples were performed in a three-electrode configuration in 6 M KOH electrolyte. The working electrodes were prepared by mixing active material, carbon black, and polyvinylidene fluoride (PVDF) (10% solution in N-methyl-2-pyrolidone) with a mass ratio of 75:20:5. A 5% solution of the mixture in isopropanol was sprayed onto Ni foam as the current collector. The prepared electrodes were dried at 60 °C overnight. A Pt plate was used as the auxiliary electrode and Hg/HgO as the reference electrode. An Autolab PGSTAT30 was employed to measure the electrochemical properties of the samples through cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The EIS measurements were conducted at open circuit potentials. A Solartron battery test unit equipped with Cell Test software (v. 3.5.0) was used for galvanostatic charge−discharge (GCD) measurements. Asymmetric Cells. Asymmetric supercapacitors were assembled by integrating a HO−CuCo2O4 positive electrode with an activated carbon (AC) negative electrode in 6 M KOH electrolyte solution. A cellulosic paper was used as the separator. It should be mentioned that in order to utilize its full pseudocapacitance, the positive electrode was electroactivated before integration into the asymmetric cell by running continuous charge/discharge for 250 cycles in a three-electrode setup. The as-fabricated HO−CuCo2O4//AC asymmetric supercapacitors were then subjected to CV, GCD, and EIS measurements. Finally, the cycle 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 highly ordered (HO) HO−CuCo2O4 and disordered (DO) DO−CuCo2O4 mesoporous samples.

accessible specific surface area and effective sites for redox reactions, making it possible to fully utilize the charge storage ability of CuCo2O4. In addition, the aligned and interconnected pores provide effective contacts between the nanowires and the electrolyte ions resulting in shortened ion diffusion pathways and 3920

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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 images of 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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RESULTS AND DISCUSSION Highly ordered mesoporous CuCo2O4 nanowires (HO−CuCo2O4) were synthesized by nanocasting from a silica SBA-15 hard template (Figure 2). Moreover, a nontemplate hydrothermal route was employed 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 cubic spinel phase of copper cobaltite (JCPDS File No. 001-1155).17 The sharp peak at ∼1.07° in a low-angle XRD pattern for HO−CuCo2O4 (Figure 3b) can be indexed as the (100) reflection 3921

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Figure 4. Electrochemical characterizations of the highly ordered mesoporous (HO−CuCo2O4) and disordered mesoporous (DO−CuCo2O4) samples as supercapacitor 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 density of 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 a function 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 series resistance, Faradaic charge transfer resistance, Warburg impedance, double layer capacitance, and Faradaic pseudocapacitance, respectively.

of the P6mm space group anticipated for the replica supramolecular structure of SBA-15,17 showing the complete replication of the SBA-15 mesoporous system by CuCo2O4. Scanning electron microscopy was employed to investigate the samples morphologically, showing uniform elongated-shaped particles for the HO−CuCo2O4 sample (Supporting Information (SI) Figure S1a,b). Magnified SEM images zooming into the HO−CuCo2O4 particles from different viewing directions reveal each particle is further comprised of three-dimensional porous structures (SI Figure S1c,d). In contrast, the DO−CuCo2O4 sample is comprised of nonuniform particles with a broad particle size distribution (i.e., 50 to >600 nm), with no internal porous structures (SI Figure S1e,f). The highly ordered mesopores of the HO−CuCo2O4 sample is evident through TEM (Figure 3c,d). A porous particle comprised of oriented CuCo2O4 nanowires (as viewed from the top) shows the tips of the nanowires (Figure 3c) clearly corroborating the hexagonal arrangement of the replica in the orientation of (100) planes created by the template. In Figure 3d, a side-view image (i.e., from the elongated direction) of such a particle illustrates that the HO−CuCo2O4 sample is comprised of individual nanowires organized into parallel bundles with sizes in the range of a few hundred nanometers in length. The mesostructure regularity can be seen through all of the particle domains (SI Figure S2a,b), demonstrating that no obvious nonporous particles were formed. As can be observed in Figure 3d, the diameter and the interwire spacing (i.e., pores) of the oriented nanowires are estimated to be around 7.2 and 2.9 nm, respectively, in good agreement with the previous reported values for the SBA-15 silica template.27 A high-resolution TEM (HRTEM)

image of the HO−CuCo2O4 sample (Figure 3e) reveals that the nanowires are comprised of nanocrystalline domains of the spinel cobaltite. Lattice fringes with a d-spacing of 0.47 nm are observed, corresponding to the (111) crystallographic plane of the cubic spinel CuCo2O4. On the other hand, TEM images of the DO−CuCo2O4 (SI Figure S2c,d) reveal a disordered porous structure, with pore sizes ranging from 3 to 15 nm. The disordered characteristic of the pores hinders their access to the electrolyte and feasibility of fast ion movements which significantly suppresses electrochemical reactions. This is especially important in supercapacitor applications. In order to further investigate the crystallographic characteristics of the mesoporous sample, selected area electron diffraction (SAED) was performed (Figure 3f). The ring diffraction pattern illustrates the polycrystalline nature of the mesoporous nanowires. The diffraction rings show d-spacings in agreement with reference values (SI Table S1) that can be indexed to the (111), (220), (311), (400), (422), (440), and (511) planes of the spinel CuCo2O4, 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 the Brunauer−Emmett−Teller (BET) surface area of the samples. SI Figure S3a,b clearly shows the mesostructural features of the sampletypical type IV isotherms including type H1 hysteresis loops.28 Accordingly, BET surface areas of 59.34 and 37.08 m2 g−1 (SI Table S2) were obtained for the HO−CuCo2O4 and DO−CuCo2O4 samples, respectively. The electrochemical performances of the samples as supercapacitor electrodes were evaluated in a three-electrode configuration using 6 M KOH solution as the electrolyte. Figure 4a 3922

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OH− anions remarkably enhances the utilization of the internal and external surfaces of the HO−CuCo2O4 electrode, a phenomenon that is significantly restricted to the external surfaces in the DO−CuCo2O4 sample (Figure 1).31,32 In order to test the rate capability of the electrodes, charge− discharge measurements were performed at various current densities ranging from 2 to 20 A g−1. The corresponding profiles for the HO−CuCo2O4 electrode are shown in Figure 4c, demonstrating the maintenance of voltage plateaus even when operated at a high current density of 20 A g−1. The calculated specific capacitances for the HO−CuCo2O4 and DO−CuCo2O4 electrodes versus the applied current density are presented in Figure 4d. As can be seen, with a 10-fold increase in current density from 2 to 20 A g−1, 64% of the initial capacitance was retained in the HO−CuCo2O4 electrode, indicating its high rate capability. This contrasts with the DO−CuCo2O4 electrode, which shows only about 35% capacitance retention after the increased current density. This suggests that the interconnected channels in the HO−CuCo2O4 provide efficient ion transport within the electrode even when tested at high current densities (Figure 1). To further demonstrate the advantages of the HO−CuCo2O4 electrode architecture on its electrochemical behavior, the cycling 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 first 250 cycles at a current density of 2 A g−1. This remarkable increase in capacitance may be attributed to structural activation and full pore opening during insertion/deinsertion of ions through the mesopore channels and possibly micropore channels of the sample.33 Such an increase in capacitance during initial cycling has been previously observed.24,34 After 1000 cycles at 2 A g−1, the electrode was tested at progressively increased current densities. Despite the sudden change of the applied current density, the electrode exhibits relatively stable capacitance. We observe some fluctuations at higher current densities, which could be attributed to partial degradation of fragile walls of smaller pores due to consecutive insertion/deinsertion of ions at high rates. After 4000 continuous cycles at varying current densities, the current was turned back to 2 A g−1. Remarkably, 93.5% of the capacitance after activation can still be delivered and maintained for another 250 cycles without any noticeable changes. This interesting cycling performance demonstrates the suitability of HO−CuCo2O4 electrodes for practical energy storage. Based on the preceding overall electrochemical performance, the unique highly ordered mesoporous CuCo2O4 on Ni foam electrode has been found to be at least comparable or superior to other copper cobaltite electrodes, single-component metal oxides including CuO and Co3O4, and binary metal oxides as a supercapacitor electrode material (Supporting Information Table S3). Electrochemical impedance spectroscopy (EIS) was performed to provide further insight into the electrochemical behavior of the electrodes, respectively. Figure 4f shows Nyquist plots for HO−CuCo2O4 and DO−CuCo2O4 electrodes. In accordance with a CNLS fitting using the proposed equivalent circuit (Figure 4f, inset), the diameter of the semicircle, which corresponds to the charge transfer resistance, is significantly smaller in the HO−CuCo2O4 electrode (3.77 Ω) when compared to that of the DO−CuCo2O4 electrode (10.6 Ω). The x-intercept of the Nyquist plot suggests a very small equivalent series resistance (ESR) for the HO−CuCo2O4 electrode (0.54 Ω) as opposed to 0.96 Ω for the DO−CuCo2O4 electrode. These

shows cyclic voltammetry (CV) of the HO−CuCo2O4 and DO−CuCo2O4 samples at a scan rate of 5 mV s−1. For both samples, the shape of the CV curves indicates pseudocapacitive behavior arising from Faradaic reactions of the Co4+/Co3+ and Cu2+/Cu+ redox pairs. The exact electrochemical behavior of copper and cobalt oxides in strong alkaline solutions needs further investigation due to several possible phases for these cations and the complex nature of the system. A possible mechanism is that by initiating the scan from cathodic potentials, CoOOH29 and CuOH may form at the outer surface of the CuCo2O4 electrode, according to the following equation: CuCo2O4 + H 2O + e− ↔ 2CoOOH + CuOH

(1)

Then, by sweeping the potential toward positive values, redox reactions occur as follows: CoOOH + OH− ↔ CoO2 + H 2O + e−

(2)

CuOH + OH− ↔ Cu(OH)2 + e−

(3)

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

Cs =

∫ i dv/mυΔv

(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 of about 1150 F g−1 is obtained for the HO−CuCo2O4 sample, much larger than the value obtained for the DO−CuCo2O4 sample (260 F g−1). Moreover, CV measurements for the samples at various scan rates ranging from 5 to 80 mV s−1 (SI Figure S4a,b) showed a slight increase in peak separations that is likely due to a small ohmic resistance and polarization. The linear dependence of peak current density against the square root of the scan rate in both samples (SI Figure S5) illuminates the diffusion control characteristic of the redox reaction that can be understood by dependency of the reactions (eqs 2 and 3) on the diffusion of the OH− ions. In order to evaluate applicability of CuCo2O4 as supercapacitor electrode materials, charge−discharge measurements were conducted at different current densities. Figure 4b shows the real time curves for HO−CuCo2O4 and DO−CuCo2O4 samples at a current density of 2 A g−1, in a potential range of 0−0.5 V. The voltage plateau is characteristic of pseudocapacitance due to charge storage based upon Faradaic redox reactions at the electrode−electrolyte interface (in good agreement with the CV curves). The specific capacitance is calculated by the formula

Cs = (I Δt )/(mΔV )

(5)

where I is the discharge current, Δt is the discharge time, m is the mass of the active material, and ΔV is the potential window.30 Here, a specific capacitance as high as 1210 F g−1 (areal capacitance of 0.6 F cm−2) is obtained for the HO−CuCo2O4 sample at a current density of 2 A g−1, which is more than 4 times greater than that achieved by DO−CuCo2O4 (270 F g−1). Taking advantage of its highly ordered mesoporous structure, the HO−CuCo2O4 sample provides thorough access of the electrode surface to the electrolytic ions, resulting in superior capacitance performance in comparison with the DO−CuCo2O4 sample. In other words, the presence of abundant diffusion channels for 3923

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of the electrode. All of these observations demonstrate the importance of mesoporous pseudocapacitive nanowires for supercapacitor electrodes. The high surface area of the nanowires provides more active sites for charge transfer, thus increasing the overall capacity of the electrodes. In addition, the aligned mesopores facilitate ion transport kinetics during charge/discharge processes. Up to this point, we have only discussed the electrochemical performance of HO−CuCo2O4 in a three-electrode configuration. To further evaluate the HO−CuCo2O4 electrode for real applications, asymmetric supercapacitors were made by integrating the HO−CuCo2O4 electrode as the positive electrode (after activation by 250 cycles), activated carbon (AC) as the negative one, and a cellulosic paper as the separator in 6 M KOH solution. This configuration combines a high-energy electrode with a highpower electrode to produce a hybrid energy storage system with the best attributes of both. In order to achieve the maximum capacitance and best cycle life, the masses of positive and negative electrodes were adjusted according to the following equation:

results demonstrate that the HO−CuCo2O4 aligned nanowires have lower interparticle resistance and better contact with the current collector than the DO−CuCo2O4 electrode. Further investigations indicate that the slope of the linear section at low frequencies is steeper (Table 1), a characteristic of pure Table 1. Electrical Parameters for CuCo2O4 from EIS Measurements at OCP circuit elements

HO−CuCo2O4

DO−CuCo2O4

ESR (Ω) Rct (Ω) n1a n2a

0.544 3.77 0.860 0.982

0.960 10.6 0.821 0.890

a

The slope of the linear section at low frequencies is steeper for the HO−CuCo2O4 sample (phase elements, n, is 0.86 and 0.98 for CPE1 and CPE2, respectively), while these values obtained for the DO−CuCo2O4 electrode are 0.82 and 0.89, characteristic of pure capacitive behavior.

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

for Q + = Q − →

m+ C × ΔV − = − m− C+ × ΔV+

(6)

Figure 5. (a) CVs and (b) charge−discharge curves for the HO−CuCo2O4//AC asymmetric supercapacitor; (c) specific capacitances and (d) Ragone plot 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. 3924

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Chemistry of Materials According to this equation a charge balance between the two electrodes is reached when m+/m‑= 0.12. Figure 5a shows CV curves of the asymmetric HO−CuCo2O4//AC supercapacitor at various scan rates. It is noticed that a maximum operating voltage of 1.5 V was obtained thanks to the asymmetric configuration which takes advantage of the different stable potential windows of the positive and negative electrodes to maximize the overall cell voltage. Clearly observed broad redox peaks indicate the pseudocapacitance contribution from the positive electrode. This pseudocapacitance feature can also be seen in galvanostatic charge−discharge curves acquired at different current densities as shown in Figure 5b and SI Figure S7. The discharge curves exhibit 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 completely symmetric which suggest an excellent electrochemical reversibility and good Coulombic efficiency. We also tested the rate capability of the supercapacitor over a wide range of current densities from 1 to 20 A g−1 as shown in Figure 5c. Interestingly, the specific capacitance reaches a maximum of 137 F g−1 at 1 A g−1 and retains 67 F g−1 at a high current density of 20 A g−1. This excellent rate performance can be attributed to the nanoporous HO−CuCo2O4 electrode whose surface is highly accessible to the electrolyte and the rapid charge/discharge characteristics of the activated carbon negative electrode. Furthermore, the cycle life, which is an important feature of supercapacitors, is excellent with a capacitance retention of 86% after 5000 charge/discharge cycles 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 understanding of the electrochemical performance of the asymmetric cell. As shown in Nyquist plots in SI Figure S9, the small charge transfer resistance at high frequencies verifies the facile pseudocapacitance feature of the device, whereas the linear part observed at low frequencies reflects its excellent capacitor characteristics. In addition, the intercept of the Nyquist curve on the real axis is only 0.33 Ω, manifesting the very low internal resistance of the asymmetric supercapacitor. Energy density and power density are considered the two main parameters used to characterize the performance of supercapacitors. Figure 5d depicts the Ragone plot which correlates the energy and power densities of the HO−CuCo2O4//AC asymmetric supercapacitor at various current loads. The maximum energy density of the supercapacitor is 42.81 Wh kg1− which decreases to 21.15 Wh kg−1 as the power density increases from 0.75 to 15.0 kW kg−1. These values are much higher than most of the asymmetric aqueous supercapacitors reported in the literature such as graphene/MnO2//graphene (30.4 Wh kg−1 at 0.1 kW kg−1),35 CoO@PPy//AC (11.8 Wh kg−1 at 5.5. kW kg−1),36 graphene−NiCo2O4//AC (7.6 Wh kg−1 at 5.6 kW kg−1),37 and nanoporous Ni(OH)2/ultrathin graphite film//microwave exfoliated graphite oxide (6.9 Wh kg−1 at 44.0 kW kg−1).38 Figure 5e−e″ presents a quick demonstration for the viability of HO−CuCo2O4// AC asymmetric supercapacitor for practical applications. In this experiment, two asymmetric supercapacitors assembled in series successfully powered 5 mm diameter blue, green, and red round light-emitting diode (LED) indicators, respectively, for almost 60 min (see video file in Supporting Information).

interconnected channels, the electrode exhibits an outstanding specific 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 also show that the HO−CuCo2O4 electrode can be integrated in an asymmetric supercapacitor that delivers a high energy density of 42.81 Wh kg−1, which is among the highest reported energy densities for aqueous-based asymmetric supercapacitors. This work provides an effective energy storage solution to circumvent the energy density limitation of the current generation of carbon supercapacitors.

CONCLUSIONS In summary, we have fabricated highly ordered mesoporous CuCo2O4 electrode materials by replication from a silica hard template. Owing to the unique structural features and thorough electrolyte access to the surfaces of the active material via

(1) El-Kady, M. F.; Strong, V.; Dubin, S.; Kaner, R. B. Science 2012, 335, 1326. (2) Chen, H.; Hu, L.; Chen, M.; Yan, Y.; Wu, L. Adv. Funct. Mater. 2014, 24, 934. (3) Dong, X.-C.; Xu, H.; Wang, X.-W.; Huang, Y.-X.; Chan-Park, M. B.; Zhang, H.; Wang, L.-H.; Huang, W.; Chen, P. ACS Nano 2012, 6, 3206.

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ASSOCIATED CONTENT

* Supporting Information S

SEM images of HO and DO samples (Figure S1), TEM images of DO sample (Figure S2), N2 adsorption−desorption isotherms and pore size distributions (Figure S3), CV curves of the samples at various scan rates (Figure S4), linear dependency of anodic peak current against square root of the scan rate (Figure S5), Nyquist plots of the HO sample before and after 300 cycles (Figure S6), GCD profiles of the asymmetric supercapacitor at various current densities (Figure S7), cycle performance (Figure S8) and Nyquist plot (Figure S9) of the asymmetric supercapacitor, experimental and standard d-spacing values for the HO sample (Table S1), characterization parameters of samples from N2 adsorption− desorption isotherms (Table S2), comparison of performance of the current work with previous reports (Table S3), and a video file showing two series connected asymmetric supercapacitors lighting up the blue, green, and red LED indicators, respectively, for 60 min. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b00706.

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AUTHOR INFORMATION

Corresponding Authors

*(R.B.K.) E-mail: [email protected]. *(M.F.M.) E-mail: [email protected]. Author Contributions

A.P. conceptualized the idea, designed and performed experiments, analyzed the data, and wrote the first draft of the manuscript. S.E.M. performed the two-electrode experiments and analyzed the data. M.S.R., M.F.E.-K., and R.B.K. were involved in discussions on the design and interpretation of the experiments. Y.W. obtained and analyzed the TEM and SAED data. M.F.M. supervised the project and was involved in discussions and interpretation of the obtained results. All authors discussed the results, commented on the manuscript, and have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was made possible through financial support from a National Science Foundation Graduate Research Fellowship (Y.W.), the Tarbiat Modares University Research Council, Iran Nanotechnology Initiative Council (M.F.M.), and Nanotech Energy, Inc. (R.B.K.).

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REFERENCES

DOI: 10.1021/acs.chemmater.5b00706 Chem. Mater. 2015, 27, 3919−3926

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DOI: 10.1021/acs.chemmater.5b00706 Chem. Mater. 2015, 27, 3919−3926