Ultralayered Co3O4 for High-Performance Supercapacitor

ARTICLE pubs.acs.org/JPCC

Ultralayered Co3O4 for High-Performance Supercapacitor Applications Sumanta Kumar Meher and G. Ranga Rao* Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India

bS Supporting Information ABSTRACT:

Ultralayered Co3O4 structures with high porosity have been synthesized by a facile homogeneous precipitation process under hydrothermal conditions. The superstructures consist of well-arranged micrometer length rectangular 2D flakes with high specific surface area, pore volume, and uniform pore size distribution. The electrochemical measurements demonstrate that charge storage occurs in ultralayered Co3O4 due to reversible redox reactions. The charge
discharge study shows that the material is capable of delivering very high specific capacitance of 548 F g
1 at a current density of 8 A g
1 and retains 66% of capacitance at 32 A g
1. The charge
discharge stability measurements show excellent specific capacitance retention capability, ca. 98.5% after 2000 continuous charge
discharge cycles at high current density of 16 A g
1. The exceptional cyclic, structural, and electrochemical stability at higher current rate with ∼100% Coulombic efficiency, and very low ESR value from impedance measurements promise good utility value of ultralayered Co3O4 material in fabricating a wide range of high-performance electrochemical supercapacitors.

1. INTRODUCTION New renewable energy sources and energy storage materials are the two major challenges in electrochemical technology. In this context, the potential application of supercapacitors from simple consumer electronics to Airbus 380 planes has been methodically explored due to their functional ability to link the performance gap between batteries and dielectric capacitors with significantly better power and energy density.1
3 Among the array of electrode materials for supercapacitor applications, transition metal oxides are widely studied due to variable oxidation states of metal ions which facilitate redox transitions and higher charge storage within the potential range of water decomposition.4 RuO2 has been studied as highly suitable material with very good capacitive response,5 but its relatively high cost and environmentally poisonous nature have induced the exploration of lower cost complementary materials with comparable/superior performance and environmental affability. Among the transition metal oxides, Co3O4 is found to be one of the better alternate materials due to its higher surface area, good redox property, controllable size and shape, and structural identities.6
12 As the supercapacitance phenomenon is directly associated with surface properties, any change in the surface morphology of electroactive materials greatly influences their electrochemical performance.10,13
15 Various surface morphologies of Co3O4 show clear difference in their electrochemical performance due to dissimilarity in the material
electrolyte interface properties and ion transfer rate during the charge storage process.10
12,14,16 r 2011 American Chemical Society

Although various structures/morphologies such as nanoparticles, fibers, and wires have been demonstrated to be quite feasible for high-performance and stable electrodes,10,16
18 materials with layered morphologies are shown to be electrochemically preferable because of their structural tolerance toward the ions of electrolyte to move in and out of the interlayer region, without causing any major structural rearrangement during the redox reactions.19,20 Further, layered structures can be flake-off resistant and better tolerant to high rate redox reactions for highperformance supercapacitor applications. A recent report in the literature reveals better electrochemical performance of layered Co3O4.14 However, such materials exhibit lower capacitance value and rate capability due to the short-range structures (nm length flakes) which suffer internal resistance and structural modification/degradation under rigorous reaction conditions.7 So, long-range (micrometer length) 2D layered structures can be an alternative approach for high capacitance and better rate capability due to the possibility of lower internal resistance. The micrometer length layered oxide structures possess long-range layered electroconducting oxide frameworks as compared to the nanometer length flakes or nanoparticles composed of noncontinuous oxide framework.8 Furthermore, the layered morphology due to enhanced ordered structure Received: February 5, 2011 Revised: June 2, 2011 Published: July 05, 2011 15646

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The Journal of Physical Chemistry C possesses well-defined long-range, uniform, stable and smooth oxide conductivity paths as compared to the nanometer length flakes or nanoparticles of highly disordered microstructures.9 While concentration polarization is expected to be high for longrange structures, a suitable porosity of the material can ease the mass transfer of electrolytes within the pores for fast redox reactions and improve the charge
discharge rate.2,21,22 In this work, we report the synthesis of highly layered Co3O4 structures with uniform porosity under mild hydrothermal conditions. The layered Co3O4 is composed of uniform rectangular and micrometer length 2D flakes. The material displays the highest ever specific capacitance value and excellent cycling stability at high rate operating conditions than those reported in the literature.6,10,14,16 For better practicality, we have performed the activity study of this material at higher loading condition (∼1 mg/cm2)6 as compared to the other imitate systems.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Co3O4 Architectures. Analytical grade Co(NO3)2 3 6H2O, Triton X-100, and urea were used as received. One hundred milliliters of 20 mmol aqueous cobalt salt solution was added dropwise to 100 mL of 10 mmol of Triton X-100 aqueous solution and after stirring for 1 h, 40 mmol of urea was added and the mixture was subsequently stirred for additional 3 h. The resulting solution was then transferred to a Teflon lined stainless steel autoclave of 250 mL capacity and was subjected to heating at 120 °C for 24 h. After the autoclave was cooled to room temperature, a light pink solid precipitate was separated by centrifugation at 3000 rpm with successive repeated washing with water, ethanol
water mixture, and pure ethanol. After the precipitate was dried at 60 °C in vacuum, the sample was heated to 300 °C at a rate of 1 °C min
1, in flowing air. 2.2. Characterization. Thermogravimetric analysis of the precursor sample was carried out on a TA-make TGA Q500 V20.10 Build 36 instrument in air flow (20 mL per min) with a linear heating rate of 20 °C per min. The powder X-ray diffraction (PXRD) patterns are recorded from 10 to 80° using a Bruker AXS D8 Advance diffractometer at room temperature employing Cu KR (λ = 0.15408 nm) radiation generated at 40 kV and 30 mA with a scan rate of 0.01° s
1. The crystallite size of the Co3O4 sample was approximated applying the Debye
Scherrer equation; D = Kλ/(β cos θ), where D is the linear dimension of the particle (particle size), K is the spherical shape factor (0.89), and β is the full width at half-maximum height (fwhm) of the respective peaks measured graphically. The average crystallite size was obtained from (311), (400), (511) and (440) peaks. Multipoint N2 adsorption
desorption experiment is carried out on an automatic Micromeritics ASAP 2020 analyzer using the Brunauer
Emmett
Teller (BET) gas adsorption method, at 77 K. The sample is outgassed at 100 °C for 2 h followed by 150 °C for 10 h in a dynamic vacuum before physisorption measurements. The specific surface area was calculated by BET method and the pore size distribution plots were generated from the desorption branch of the isotherm by the Barrett
Joyner
Halenda (BJH) and Horvath
Kawazoe (HK) methods. The pore volume was obtained from the pore size distribution data. Surface morphologies of the uncalcined and calcined samples were obtained using an FEI Quanta FEG 200 high-resolution scanning electron microscope (HRSEM). The powder samples were dispersed in ethanol by sonication and deposited on a conducting

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carbon tape before being mounted on the microscope sample holder for analysis. 2.3. Fabrication of Electrode and Electrochemical Measurements. For evaluation of the electrochemical properties of layered Co3O4 sample, the working electrode was fabricated by mixing 80 wt % of Co3O4 powder and 15 wt % of acetylene black in an agate mortar. To this mixture, 5 wt % poly(vinylidene difluoride) (PVdF) binder dissolved in 1-methyl-2-pyrrolidinone (NMP) was added, making a slurry. The slurry was then coated on one side of a pretreated battery grade Ni foil (0.2 mm thick, 1 cm2 area, polished with 991A softflex grade abrasive paper (Silicium Carbid P 2000) followed by sonication in ethanol for 1 h) and dried in a vacuum oven at 60 °C for 8 h. The approximate mass of coated Co3O4 was ∼1.0 mg. Cyclic voltammetry (CV), chronopotentiometry (CP), and electrochemical impedance spectroscopy (EIS) measurements were carried out using conventional three-electrode configuration on a CHI 7081C electrochemical workstation. The Ni plate coated with Co3O4 served as working electrode while a square Pt foil (1  2 cm2) and Hg/ HgO (1.0 M KOH) electrodes served as counter and reference electrodes, respectively. All the experiments were carried out using freshly prepared 1.0 M aqueous KOH electrolyte in triple distilled water. The ac EIS measurements were performed between 0.01 and 105 Hz. Five working electrodes were tested simultaneously to check the reproducibility of the results of the electrochemical measurements.

3. RESULTS AND DISCUSSION Figure 1a shows the wide angle PXRD profile of the uncalcined and calcined samples. The PXRD pattern of uncalcined sample can be well indexed as the monoclinic cobalt hydroxide carbonate (Co2(OH)2CO3) phase (JCPDS card, no. 29-1416). In aqueous condition, decomposition and successive hydrolyzation of urea at elevated temperature leads to in situ release of OH
and CO32
. The generation of OH
and CO32
initiates the precipitation of Co2+ ions in the solution to produce cobalt hydroxide carbonate species. The reactions in the system can be expressed as hydrolysis

NH 2 CONH 2 sf NH 4 þ þ NCO
NCO
þ 3H2 O f HCO3
þ NH 4 þ þ OH
3HCO3
f CO3 2
þ Hþ 2Co2þ þ 2OH
þ CO3 2
f Co2 ðOHÞ2 CO3 The slow rate of hydrolysis results in controlled generation of OH
ions and crystal growth processes in the medium. Additionally, hydrothermal condition adds the advantage of better control over the structure and crystallization. The thermal behavior (Figure S1 in Supporting Information) of the Co2(OH)2CO3 precursor shows a sharp weight loss of ∼24.8% at ∼300 °C due to the thermal decomposition and loss of CO2 and H2O molecules. There is no additional weight loss observed at higher temperatures which confirms the formation of monophasic compound. The calcined (300 °C, in O2) product displays a characteristic PXRD pattern with detectable broad peaks at d = 4.67, 2.86, 2.44, 2.02, 1.65, and 1.56 Å, assigned to the respective 111, 220, 311, 400, 511, and 440 planes of the nanostructured FCC type Co3O4 (JCPDS Card no. 43-1003), with calculated 15647

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Figure 1. (a) XRD pattern of uncalcined precursor (Co2(OH)2CO3), and the product (Co3O4) obtained by calcining the precursor at 300 °C. (b) N2 adsorption
desorption isotherm of the product, Co3O4; inset shows Horvath
Kawazoe (HK) and Barrett
Joyner
Halenda (BJH) pore size distributions of Co3O4.

Figure 2. (a) Low and (b) high magnification FESEM images of precursor Co2(OH)2CO3 sample.

Figure 3. (a) Low and (b) high magnification FESEM images of product Co3O4 sample.

crystallite size of ∼15 nm. The absence of any secondary peaks in the PXRD patterns demonstrates the stoichiometric purity of the samples. The N2 adsorption
desorption isotherm of the Co3O4 sample measured at 77 K is shown in Figure 1b and the profile illustrates rapid uptake in the p/p0 region of 0 to 0.04 followed by a representative “type IV” hysteresis in the mesopore region. This reveals that the sample essentially contains mesopores with minor fraction of micropores as demonstrated by the BJH and HK pore size distribution graphs, respectively, presented in the insets of Figure 1b.23 The characteristic narrow and single pore size distribution at ∼1.85 nm is quite unique (mostly observed for samples prepared using templates). The uniform porosity and pore size distribution of this sample are better suited for supercapacitor applications.2 The BET specific surface area and pore volume of the sample are 97 m2 g
1 and 0.2 cm3 g
1, respectively. The uptake of N2 at higher relative pressure (p/p0 > 0.9) in the BET isotherm demonstrates the existence of macropores in the samples.23 From the BET isotherm of Co3O4 sample in Figure 1b, it is noticed that the uptake of N2 in the macropore region is considerably lower than that in the mesopore region. Therefore, it has been realized that the contribution of mesopores to the total surface area and pore volume is significantly higher than that of macropores. 3.1. FESEM Study and Possible Formation Mechanism. To demonstrate the formation mechanism of the layered Co3O4, FESEM images of the dried cobalt
hydroxide
carbonate sample were obtained and are shown in Figure 2. The sample shows layered morphology with well arranged 2D microsheets, but with less porosity. Further, the lower and higher magnification FESEM images of the Co3O4 sample in Figure 3 clearly show that the sample has highly layered morphology consisting of well arranged rectangular 2D microsheets.

The higher magnification image (Figure 3b) of the sample reveals long-range structure with almost uniform micrometer length interlayer spacing. Therefore it is noteworthy that the dried precursor sample with thermal treatment at elevated temperature transforms to Co3O4 phase keeping its morphology intact. However, the decomposition process involves removal of CO2 and H2O species from the precursor crystals resulting in generation of added porosity in the Co3O4 product. The formation and reproducibility of 3D architectures of Co3O4 have been ensured by optimizing experimental parameters such as concentration of metal salt solution, urea, reaction temperature, and time which are the known factors that influence the hydrolysis rate and control the nucleation as well as crystal growth processes.24 It is well-known that the crystal growth of nanomaterials is modulated extrinsically in accordance with the selective adsorption of solvents, inorganic additives, and surfactants to certain crystallographic planes by modifying the relative order of the surface energies.24
26 The selective adsorption inhibits the growth of certain crystal planes leading to specific shape of the material. In this work, the role of the neutral surfactant, Triton X-100, in generating the layered structure under hydrothermal conditions is essentially 2-fold, i.e., selective adsorption and prevention of agglomeration of 2D sheets. The direct evidence for the shape controlling mechanism of Triton X-100 is not yet clear, but the kinetic growth of layered structures under various factors like van der Waals forces, hydrophobic interactions, crystal face attraction, electrostatic and dipolar fields, hydrogen bonding, intrinsic crystal contraction, and Ostwald ripening immensely contribute to the final morphology of the material.26
28 Although the inference of exact formation mechanism of layered morphology is not yet experimentally established, from the strong literature survey, we have proposed 15648

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Scheme 1. Schematic Representation of Possible Formation Mechanism of Ultralayered Co3O4

Figure 5. CV of Co3O4 at different scan rates in 1 M KOH. The inset shows the linearity of anodic current density with scan rate.

Figure 4. Optical band gap energy of layered Co3O4 obtained by extrapolation to R = 0. Inset shows the representative UV
vis absorption spectrum of layered Co3O4.

the possible mechanism for the evolution of layered Co3O4 in Scheme 1.24,25,29 The final porous morphology of Co3O4 involves several intermediate processes like coalescence, hydrothermal, and Ostwald ripening, self-assembling, and thermal decomposition. 3.2. Optical Study. The optical absorption study was carried out to determine the band edge energy levels of layered Co3O4. The optical absorption profile of layered Co3O4 in the inset of Figure 4 shows two bands centered at 360 and 665 nm. Tauc plot method was applied to determine the optical band gap energies, by extrapolating the value of hν to R = 0, which shows two optical transitions at 1.75 and 2.55 eV. The optical transition at 1.75 eV has been ascribed to a band gap transition of O2
f Co2+ charge transfer process and the transition at 2.55 eV is assigned to O2
f Co3+ charge transfer process. The optical band gap energy is calculated to be 0.8 eV (semiconductor).30,31 The optical band gap energy value of ultralayered Co3O4 sample is marginally higher than the bulk Co3O4 samples reported in the literature.32 Since the band gap energy increases with decrease in crystallite size due to quantum confinement effect, the optical band gap energy of layered Co3O4 in the quantum confinement regime can be fine-tuned for specific applications by altering the synthesis methods adopted in this study. However, from the nature of the Tauc plot profile (Figure 4), it is apparent that the observed transitions are not solely from direct band gaps but from the discrete band gap and midgap transitions. This is attributed to the presence of defect sites in layered Co3O4.30,31

3.3. Electrochemical Study. In view of the highly layered structural arrangement of Co3O4, which is the ideal morphological foundation for fast redox reaction due to better electrochemical accessibility of OH
ions, its potential application in high rate electrochemical supercapacitor has been investigated by cyclic voltammetry (CV), galvanostatic charge
discharge (CP), and impedance measurements. Figure 5 shows typical CV curves of Co3O4 in the potential range of
0.2 to 0.5 V at different scan rates. The multiple peaks at different potentials correspond to the formation of number of cobalt oxide phases with different oxidation states.30 The following reversible reactions (eqs 1 and 2) take place during the redox process in the cyclic voltammetry study33,34 Anodic scan (toward positive potential): 9 3CoðOHÞ2 þ 2OH
f Co3 O4 þ 4H2 O þ e
> > > > > Co3 O4 þ OH
þ H2 O f 3CoOOH þ e
= ð1Þ > CoOOH þ OH
f CoO2 þ H2 O þ e
> > > > ; 4OH
f O2 þ 2H2 O þ 4e


Cathodic scan (toward negative potential):

9 > > =

CoO2 þ H2 O þ e
f CoOOH þ OH
3CoOOH þ e
f Co3 O4 þ OH
þ H2 O

> > Co3 O4 þ H2 O þ 2e f 3CoðOHÞ2 þ 2OH ;


ð2Þ




The anodic peaks (at positive current density) and cathodic peaks (at negative current density) in the CV curves are due to the oxidation and reduction processes, respectively. So the layered Co3O4 undergoes electrochemical charge transfer reactions Co(II) T Co(III) T Co(IV) in basic electrolyte, making it a potential candidate for pseudocapacitor application.34,35 Literature shows that there are three redox couples generally involved in this system, Co(OH)2/Co3O4, Co3O4/CoOOH, and CoOOH/CoO2. The CV patterns, however, vary from sample to sample and strongly depend on the morphology and surface properties.6,12,14,17,36 In the present study the Co(OH)2 in the first oxidation step during the anodic scan is from the reduced Co3O4 species in the last step of the cathodic scan during the CV cycling. The peaks corresponding to each redox couple are not clearly distinguishable and some are elusive. This may be due to the surface modification of the layered Co3O4 by conductive carbon which can induce appreciable broadening of some redox peaks.35b By and large, the CV characteristics of the ultralayered Co3O4 demonstrate the overall redox reactions (1) and (2) involved in the charge storage mechanism. The nonrectangular shapes of the CV curves reveal that 15649

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Figure 6. (a, left) Charge
discharge plots of Co3O4 electrode at different current densities (Id). (b, right) Specific capacitance and Coulombic efficiency (η) of Co3O4 electrode as a function of cycle number.

the charge storage is a characteristic of the pseudocapacitance process originating from reversible redox reactions.1,13 It is seen that, with an increase in the scan rate, the anodic and cathodic peaks shift to higher and lower potentials, respectively. Again, the positive sweeps of CV curves are not symmetric to their corresponding negative sweeps as observed in thin films of Co3O436 which indicates the kinetic irreversibility in the redox process due to polarization and Ohmic resistance during the Faradaic process in highly porous layered Co3O4.1,13,37 The asymmetrical CV curves become more prominent at higher scan rates indicating better high-rate response of the layered Co3O4 material.13,38 The almost linear (quasi-linear) relationship of the plot of anodic peak current density versus the scan rate (inset in Figure 5) indicates the occurrence of surface redox reactions. These characteristics of the CV patterns substantiate the pseudocapacitance behavior of the Co3O4 electrode.1,13 The specific capacitance (Cs) values of Co3O4 sample estimated from CV curves are shown graphically in Figure S2 in Supporting Information. The Cs values are calculated graphically by integrating the area under the I
V curves and then dividing by the sweep rate v (V s
1), the mass of Co3O4 (w), and the potential window (Va to Vc), using the equation39 Z Vc 1 ð3Þ Cs ¼ IV dV νwðVa
Vc Þ Va

Scheme 2. Schematic Representation of Easy Accession of OH
Ions to Ultralayered Co3O4

The ultralayered Co3O4 sample shows very high specific capacitance values of 1085, 961, and 836 F g
1 at scan rates of 5, 10, and 20 mV s
1, respectively. The potential window used for this calculation is
0.2 to 0.5 V. The layered morphology provides larger electroactive surface area and easy accessibility of OH
ions for highly feasible redox reactions. This leads to higher specific capacitance value of layered Co3O4 even at higher scan rates. Better cyclic performance and high rate discharge capability are crucial for an electrode material to be used efficiently for supercapacitor applications. The layered Co3O4 has been investigated for charge discharge cyclic study at current rates of 4, 8, 16, 32, and 64 A g
1 within the potential range of
0.05 to +0.45 V, and the results are shown in Figure 6a. The nonlinearity in the discharge curves, unlike battery40 and double-layer capacitors,3 shows representative pseudocapacitance behavior of Co3O4 resulting from the electrochemical adsorption/desorption or redox reaction at the electrode
electrolyte interface.1,13 The specific capacitance values have been calculated from the applied current density (i), mass of the Co3O4 sample (m), discharge time (Δt), and operating potential (ΔV) using the equation13

Figure 7. CV of Co3O4 electrode before and after 500 charge/discharge cycles.

Cs ¼

i mðΔV =ΔtÞ

ð4Þ

The specific capacitance values are found to be 604, 548, 474, 359, and 167 F g
1 at specific current densities of 4, 8, 16, 32, and 64 A g
1, respectively. To our knowledge, the specific capacitance values from CV and charge
discharge measurements reported here are the best values obtained so far for noncomposite Co3O4 materials in the given potential range, under higher loading conditions (∼1 mg/cm2) and high current rates.6 The reported Co3O4 composite materials prepared by tedious experimental procedures and using carbonious materials like CNT, graphene, or 1D nanoporous carbon show comparable or lower specific capacitance values.9,12,14,18,38,41 Generally an increase in the discharge current leads to a large voltage (iR) drop which results in decrease in capacitance value.13,37,38 The specific capacitance value of the layered Co3O4 shows ∼66% retention even at 32 A g
1 as compared to the value at 8 A g
1. This suggests prominent capacitive performance of layered Co3O4 under very high current density operation conditions which is significant in practical supercapacitor applications. The considerable enhancement of the supercapacitive 15650

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Figure 8. Complex plane impedance plots (Nyquist plots) of Co3O4 electrode at different bias potentials (a) 0.2, (b) 0.3, and (c) 0.4 V and (d) comparison plots at different potentials.

performance is essentially attributable to better utilization of electroactive surface of layered Co3O4 material than the other structures reported in the literature.6,7,10,14,16,16
18 The microscopic layered structure undertakes quicker permeation process of electrolyte by reducing the diffusion time of OH
ions and also accommodates the strain arising due to high rate insertion and extraction of OH
ions.42 Even under high current density operation conditions, most of the Co3O4 flakes participate efficiently in the electrochemical reaction because of comparable accession of electrolyte solution to every structural unit of uniform size and shape.43 The ultralayered structure also behaves as an ‘‘ion-buffering reservoir”,13 which results in enhancing the specific capacitance by shortening of the OH
diffusion distance, better penetration of electrolyte into the layered matrix, and occurrence of sufficient Faradaic reactions even at very high current density conditions (Scheme 2). The specific capacitance values of ultralayered Co3O4 derived from charge
discharge are lower than those from cyclic voltammetry measurements. This difference is essentially due to insufficient Faradic redox reactions arising from the reduced surface accession by OH
ions under higher discharge current density conditions. The scan rates at which the cyclic voltammetry have been performed do not hinder the full surface utilization of active material and therefore induce superior Faradic redox reactions and higher resulting specific capacitance. However, the current densities at which the charge
discharge measurements have been performed are extremely high. At very high current density condition, the full surface utilization of a porous material is practically not possible which results in reduced specific capacitance values. Some voltage (iR) drop and irreversibility in the redox couples in Co3O4 because of 15% conductive carbon as well as binder may have some contribution to the capacitance loss under extreme current density conditions. Since charge
discharge performance at higher current density is more important

Figure 9. Frequency-dependent specific capacitance of layered Co3O4 at different bias potentials.

for practical viability of an electrode material for supercapacitor application, the charge
discharge measurements were preferably carried out under high current density conditions. Retention of the specific capacitance for long cycles at higher current density operation condition is essential for practical viability of a supercapacitor. Figure 6b shows charge
discharge assessment of layered Co3O4 electrodes for 2000 cycles under a very high current density of 16 A g
1. Clear enhancement in the specific capacitance value is observed during the first few charge
discharge cycles which is possibly due to the structural activation and pore opening of the sample. The CV measurement analysis before and after 500 charge
discharge cycles shown in Figure 7 provides clear evidence of initial structural activation of Co3O4.13 The specific capacitance value remains nearly constant with further cycling. By and large, only ∼1.5% loss in specific capacitance is observed after 2000 charge
discharge cycles at a current density of 16 A g
1. The Coulombic effeciency (η), which is a measure of 15651

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Figure 10. (a) Nyquist plots of layered Co3O4 electrode before and after 2000 charge
discharge cycles at a current density of 16 A g
1. (b) Frequencydependent specific capacitance of layered Co3O4, before and after high rate charge
discharge cycles.

feasibility of the redox process, has been calculated for each charge
discharge cycle using equation13 tD η ¼  100 ð5Þ tC where tD and tC represent discharging and charging time, respectively, in galvanostatic charge
discharge measurements, as shown in Figure S3 in the Supporting Information. The ∼100% Coulombic efficiency during the 2000 charge
discharge cycles at higher current density (Figure 6b) clearly demonstrates the electrochemical suitability of the layered Co3O4 material with higher feasibility of the redox process. All these results reveal that the charge
discharge processes certainly do not induce major microstructural and phase changes in the layered Co3O4 electrode material contributing to its long-term electrochemical stability. Supercapacitors being power devices, lower resistance of the electrode materials is electrochemically preferred for better applicability. The complex-plane impedance plots of the of the Co3O4 electrode in the frequency range of 0.01
105 Hz, derived from the electrochemical impedance spectra (EIS), carried out at various bias potentials are shown in Figure 8. The Nyquist plots (imaginary part, Z00 , versus real part, Z0 ) of Co3O4 electrode at different potentials reveal dissimilar electrochemical characteristics, which suggest occurrence of various electrochemical processes on the electrode. Two major characteristic features observed in the high and low frequency regions are attributed to various resistance phenomena during different interfacial processes in Faradaic reactions. The impedance characteristics were analyzed by the CNLS fitting method based on a Randles equivalent circuit, as depicted below6,9

where Rs and Rct are solution and charge-transfer resistances, respectively. Cdl and Cps in the circuit represents double layer and pseudocapacitance, respectively. The interfacial diffusive resistance (Warburg impedance) in the process has been designated as “W”. The impedance behavior at the high frequency region is characteristic of the resistance at the oxide
electrolyte interface due to discontinuity in the charge transfer process at the solid oxide/liquid electrolyte interface which is a result of difference in

the conductivity between solid oxide (electronic conductivity) and the aqueous electrolyte phase (ionic conductivity). The impedance behavior in this region also involves contributing resistance from Faradaic redox processes, which is associated with the surface phenomena of the porous Co3O4 electrode. The linear parts in the impedance plots at lower frequencies correspond to the Warburg impedance, W, which is described as a diffusive resistance of the OH
ion within the Co3O4 electrode pores. With increase in applied potential, there is a gradual change in the linearity, and at 0.4 V, the Nyquist plot displays nearly a vertical line along the imaginary axis. This deviation of linearity along the imaginary axis at different applied potentials is ascribed to the pseudocapacitance, Cps, due to facile and reversible Faradaic redox reactions and accession of the OH
ions into the layered Co3O4 electrode under such low frequencies. The near-zero slope at higher operating voltage (0.4 V) and nearabsence of a semicircle are attributed to the minor effect of interfacial impedance, thus an ideal capacitive situation.6,44 At different biased potentials at higher frequencies, the intercepts at real part (Z0 ) are the same which corresponds to the identical combination of (i) ionic and electronic charge-transfer resistances, (ii) intrinsic charge-transfer resistance, and (iii) diffusive as well as contact resistance at the active material/current collector interface. This feature suggests explicit contribution of pseudocapacitance in the excellent energy storage performance of ultralayered Co3O4.1,13 The experimental impedance data has been converted to specific capacitance (Cs) using the equation1,13 Cs ¼

1 2πfZ00

ð6Þ

and plotted against frequency, “f” in Figure 9. The specific capacitance values inevitably decreases at lower bias potentials in the entire frequency region. Noticeably higher specific capacitance values have been observed at elevated operating frequency in a bias potential of 0.4 V (Figure 9). This demonstrates high power performance and excellent rate response of the layered Co3O4 material.13 The impedance behavior was also studied to verify the electrochemical stability and pseudocapacitive performance of Co3O4 electrode after long cycling at higher current density (2000 cycles at 16 A g
1). The Nyquist plots of Co3O4 electrode at a bias potential of 0.4 V, before and after 2000 charge
discharge cycles at 16 A g
1 is presented in Figure 10a. It is known that the equivalent series resistance (ESR), which is a combination of ionic 15652

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The Journal of Physical Chemistry C resistance of electrolyte, intrinsic resistance of active materials, and contact resistance at the active material/current collector interface, is an important parameter of supercapacitor measured at the high frequency region, where the impedance curve intercepts on the real axis. The higher ESR value indicates the lower electrical conductivity of the sample and vice versa.45 It can be seen that the ESR values in the Nyquist plot of the Co3O4 electrode are very small (Figure 8) and remains almost same before and after long charge
discharge cycling (Figure 10a). This observation suggests very high organizational durability as well as electrochemical stability of the layered Co3O4 structure even after long charge
discharge cycles at high current density operation condition and significant feasibility of redox reactions even after the long charge
discharge process. In addition, after 2000 cycles the slope at the low frequency region is still close to 90°, and there is near absence of semicircles at the high frequency region. This indicates that the capacitive behavior of layered Co3O4 is almost identical and suggests no appreciable change in structural and electrochemical characteristics of the layered Co3O4 during high rate cycling. The overall impedance characteristics suggest very good accessibility of the OH
ions through the layered structure and explicit contribution of the pseudocapacitance to energy storage performance of ultralayered Co3O4 material. Further, the calculated specific capacitance value of layered Co3O4 electrode before and after 2000 charge
discharge cycles at 0.4 V, as a function of frequency, is presented in Figure 10b. The specific capacitance values before and after charge
discharge cycles are found to be almost the same at all frequency regions. However a minimum of ∼1.0
1.5% decrease in specific capacitance has been observed at the lower frequency region (Figure 10b) which well corroborates with the observation from the charge
discharge study. The surprisingly no decrease in specific capacitance value at higher operating frequency region after long charge
discharge cycles at elevated current density condition is very significant for the layered Co3O4 as a possible electrode material for next generation high-performance supercapacitor applications.

4. CONCLUSION In conclusion, the present work demonstrates a combination of facile hydrothermal technique and homogeneous precipitation method to synthesize highly layered Co3O4 with excellent surface area and porosity. The electrochemical studies show the highest supercapacitance value reported so far for the single phase Co3O4 at very high current density due to the facile electrolyte penetration and better utilization of the electroactive surface for Faradaic reactions. The excellent cyclic, structural, and electrochemical stability at higher current rate with ∼100% Coulombic effeciency may be very useful for possible utilization of this layered Co3O4 material in fabricating a wide range of electrochemical supercapacitors. It is further suggested that, to use this material in real supercapacitor application for large voltage operation, the layered Co3O4 material can be desirably coupled with a suitable counter electrode material and can be assessed in a nonaqueous solvent. ’ ASSOCIATED CONTENT

bS

Supporting Information. TGA of uncalcined precursor, specific capacitance versus potential graph derived from CV measurements, and initial charge
discharge cycles from CP

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measurement showing tC and tD. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors gratefully acknowledge financial support by DRDO, New Delhi, through Grant No. ERIP/ER/0600319/M/01/1052. ’ REFERENCES (1) Conway, B. E. Electrochemical Supercapacitors, Scientific Fundamentals and Technological Applications; Kluwer Academic/Plenum Press: New York, 1999; p 11. (2) Simon, P.; Gogotsi, Y. Nat. Mater. 2008, 7, 845. (3) Winter, M.; Brodd, R. J. Chem. Rev. 2004, 104, 4245. (4) (a) Hall, P. J.; Mirzaeian, M. S.; Fletcher, I.; Sillars, F. B.; Rennie, A. J. R.; Bey, G. O. S.; Wilson, G.; Cruden, A.; Carter, R. Energy Environ. Sci. 2010, 3, 1238. (b) Naoi, K.; Simon, P. Electrochem. Soc. Interface 2008, 34. (c) K€otz, R.; Carlen, M. Electrochim. Acta 2000, 45, 2483. (5) Hu, C. -C.; Chang, K. -H.; Lin, M. -C.; Wu, Y. -T. Nano Lett. 2006, 6, 2690. (6) Wei, T. -Y.; Chen, C. -H.; Chang, K. -H.; Lu, S. -Y.; Hu, C. -C. Chem. Mater. 2009, 21, 3228. (7) Wang, G.; Liu, H.; Horvat, J.; Wang, B.; Qiao, S.; Park, J.; Ahn, H. Chem.—Eur. J. 2010, 16, 11020. (8) Sugimoto, W.; Iwata, H.; Yasunaga, Y.; Murakami, Y.; Takasu, Y. Angew. Chem., Int. Ed. 2003, 42, 4092. (9) Hsu, Y. -K.; Chen, Y. -C.; Lin, Y. -G.; Chen, L. -C.; Chen, K. -H. Chem. Commun 2011, 47, 1252. (10) Zhu, T.; Chen, J. S.; Lou, X. W. J. Mater. Chem. 2010, 20, 7015. (11) Zheng, M.; Cao, J.; Liao, S.; Liu, J.; Chen, H.; Zhao, Y.; Dai, W.; Ji, G.; Cao, J.; Tao, J. J. Phys. Chem. C 2009, 113, 3887. (12) (a) Lin, C.; Ritter, J. A.; Popov, B. N. J. Electrochem. Soc. 1998, 145, 4097. (b) Cao, L.; Lu, M.; Li, H.-L. J. Electrochem. Soc. 2005, 152, A871. (13) (a) Justin, P.; Meher, S. K.; Ranga Rao, G. J. Phys. Chem. C 2010, 114, 5203. (b) Meher, S. K.; Justin, P.; Ranga Rao, G. Electrochim. Acta 2010, 55, 8388. (c) Justin, P.; Ranga Rao, G. Int. J. Hydrogen Energy 2010, 35, 9709. (d) Meher, S. K.; Justin, P.; Ranga Rao, G. Nanoscale 2011, 3, 683. (e) Meher, S. K.; Justin, P.; Ranga Rao, G. ACS Appl. Mater. Interfaces 2011, 3, 2063. (14) Xiong, S.; Yuan, C.; Zhang, X.; Xi, B.; Qian, Y. Chem.—Eur. J. 2009, 15, 5320. (15) Balaya, P. Energy Environ. Sci. 2008, 1, 645. (16) Xia, X.-H.; Tu, J.-P.; Wang, X.-L.; Gu, C.-D.; Zhao, X.-B. Chem. Commun. 2011, 47, 5786. (17) Gao, Y.; Chen, S.; Cao, D.; Wang, G.; Yin, J. J. Power Sources 2010, 195, 1757. (18) Wei, T. -Y.; Chen, C. -H.; Chien, H. -C.; Lu, S. -Y.; Hu, C. -C. Adv. Mater. 2010, 22, 347. (19) Ghodbane, O.; Pascal, J. L.; Favier, F. ACS Appl. Mater. Interfaces 2009, 1, 1130. (20) Devaraj, S.; Munichandraiah, N. J. Phys. Chem. C 2008, 112, 4406. (21) An, K. H.; Kim, W. S.; Park, Y. S.; Choi, Y. C.; Lee, S. M.; Chung, D. C.; Bae, D. J.; Lim, S. C.; Lee, Y. H. Adv. Mater. 2001, 13, 497. (22) Zhou, H.; Li, D.; Hibino, M.; Honma, I. Angew. Chem., Int. Ed. 2005, 44, 797. (23) (a) Rouquerol, F.; Rouquerol, J.; Sing, K. Adsorption by Powders and Porous Solids; Academic Press: London, 1999. (b) Rose, M.; B€ohlmann, W.; Sabo, M.; Kaskel, S. Chem. Commun 2008, 2462. 15653

dx.doi.org/10.1021/jp201200e |J. Phys. Chem. C 2011, 115, 15646–15654

The Journal of Physical Chemistry C

ARTICLE

(24) Patzke, G. R.; Zhou, Y.; Kontic, R.; Conrad, F. Angew. Chem., Int. Ed. 2011, 50, 826. (25) Chen, J. S.; Zhu, T.; Li, C. M.; Lou, X. W. Angew. Chem., Int. Ed. 2011, 50, 650. (26) Whitesides, G. M.; Boncheva, M. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 4769. (27) C€olfen, H.; Antonietti, M. Angew. Chem., Int. Ed. 2005, 44, 5576. (28) Lou, X. W.; Deng, D.; Lee, J. Y.; Archer, L. A. J. Mater. Chem. 2008, 18, 4397. (29) Yuan, C.; Zhang, X.; Su, L.; Gao, B.; Shen, L. J. Mater. Chem. 2009, 19, 5772. (30) Keng, P. Y.; Kim, B. Y.; Shim, I. -B.; Sahoo, R.; Veneman, P. E.; Armstrong, N. R.; Yoo, H.; Pemberton, J. E.; Bull, M. M.; Griebel, J. J.; Ratcliff, E. L.; Nebesny, K. G.; Pyun, J. ACS Nano 2009, 3, 3143. (31) Xu, R.; Zeng, H. C. Langmuir 2004, 20, 9780. (32) (a) Barreca, D.; Massignan, C.; Daolio, S.; Fabrizio, M.; Piccirillo, C.; Armelao, L.; Tondello, E. Chem. Mater. 2001, 13, 588. (b) Gulino, A.; Dapporto, P.; Rossi, P.; Fragala, I. Chem. Mater. 2003, 15, 3748. (33) Casella, I. G.; Gatta, M. J. Electroanal. Chem. 2002, 534, 31. (34) Svegl, F.; Orel, B.; Hutchins, M. G.; Kalcher, K. J. Electrochem. Soc. 1996, 143, 1532. (35) (a) Behl, W. K.; Toni, J. E. J. Electroanal. Chem. Interfacial Electrochem. 1971, 31, 63. (b) Castro, E. B.; Gervasi, C. A.; Vilche, J. R. J. Appl. Electrochem. 1998, 28, 835. (c) Palmas, S.; Ferrara, F.; Vacca, A.; Mascia, M.; Polcaro, A. M. Electrochim. Acta 2007, 53, 400. (d) Casella, I. G.; Gatta, M. J. Electroanal. Chem. 2002, 534, 31. (36) (a) Hu, C.-C.; Hsu, T.-Y. Electrochim. Acta 2008, 53, 2386. (b) Wu, M.-S.; Huang, Y.-A.; Yang, C.-H. J. Electrochem. Soc. 2008, 155, A798. (37) Pell, W. G.; Conway, B. E. J. Electroanal. Chem. 2001, 500, 121. (38) Liang, Y.; Schwab, M. G.; Zhi, L.; Mugnaioli, E.; Kolb, U.; Feng, X.; M€ullen, K. J. Am. Chem. Soc. 2010, 132, 15030. (39) (a) Srinivasan, V.; Weidner, J. W. J. Power Sources 2002, 108, 15. (40) Hu, C.-C.; Cheng, C.-Y. J. Power Sources 2002, 111, 137. (41) Yan, J.; Wei, T.; Qiao, W.; Shao, B.; Zhao, Q.; Zhang, L.; Fan, Z. Electrochim. Acta 2010, 55, 6973. (42) Lou, X. W.; Archer, L. A.; Yang, Z. Adv. Mater. 2008, 20, 3987. (43) Wang, X.; Wu, X. -L.; Guo, Y. -G.; Zhong, Y.; Cao, X.; Ma, Y.; Yao, J. Adv. Funct. Mater. 2010, 20, 1680. (44) Frackowiak, E.; Beguin, F. Carbon 2001, 39, 937. (45) Snook, G. A.; Kao, P.; Best, A. S. J. Power Sources 2011, 196, 1.

15654

dx.doi.org/10.1021/jp201200e |J. Phys. Chem. C 2011, 115, 15646–15654