Electrochemical performance of plate-like zinc ...

Journal of Physics and Chemistry of Solids 121 (2018) 93–101

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Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs

Electrochemical performance of plate-like zinc cobaltite electrode material for supercapacitor applications

T

Tholkappiyan Ramachandran, Fathalla Hamed∗ Department of Physics, College of Science, United Arab Emirates University, Al Ain, P.O. Box 15551, United Arab Emirates

A R T I C LE I N FO

A B S T R A C T

Keywords: Supercapacitor Plate-like zinc cobaltite Hydrothermal synthesis Specific capacitance Electrochemical performance

The electrochemical performance of zinc cobaltite–based nanomaterial depends on its shape and morphology. Here we report on the electrochemical performance of plate-like zinc cobaltite nanocrystalline material synthesized via a facile hydrothermal method. The synthesized material was characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and Brunauer-Emmett-Teller analysis. It was found to be a single-phase zinc cobaltite nanocrystalline material with a cubic spinel crystal structure. The electrochemical performance of the synthesized plate-like zinc cobaltite nanocrystalline material was evaluated by cyclic voltammetry, cyclic chronopotentiometry and electrochemical impedance spectroscopy. The plate-like zinc cobaltite nanocrystalline material displayed a maximum coulombic efficiency of 78% and a maximum specific capacitance of 812 F/g, and it retained 88% of its capacitance after 5100 cycles. Such electrochemical performance may qualify the plate-like zinc cobaltite nanocrystalline material as a potential electroactive material in supercapacitors.

1. Introduction Energy storage materials have recently attracted a great deal of attention. They could be used in many day-to-day applications such as wireless communication devices, hybrid power sources, power tools, fuel cells, and microchips [1,2,3]. Among the various energy storage devices, supercapacitors show great promise in modern electronic devices. This is due to their flexibility, high energy density, high energy conversion rate, long life cycle, simple maintenance, and environmental friendliness [4,5]. On the basis of their charge storage mechanism, supercapacitors are classified as either electric double layer capacitors or pseudocapacitors [6]. Electric double layer capacitors depend on electrostatic charge separation between the electrode-electrolyte interface, whereas pseudocapacitors relay on fast reversible faradaic redox processes [7,8]. Therefore, the physical and electrochemical properties of the electrode material play an important role in the performance of the supercapacitor. Early on, transition metal oxides such as MnO2, NiO, and Co3O4 and conducting polymers such as polyaniline and polypyrrole were investigated as promising electrode materials for supercapacitor applications [9,10]. They showed promising performance; however, ternary transition metal oxides such as MnCo2O4, ZnCo2O4, CoMn2O4, and NiCo2O4 have attracted more attention because of their higher specific capacitance and richer redox chemistry [11,12,13].

∗

The cobalt-based ternary oxide ZnCo2O4 has recently attracted attention because of its high conductivity, electroactivity, and excellent electrochemical properties [14,15,16,17,18]. It is also environmentally benign, cost-effective, and abundant [19]. ZnCo2O4 has been shown to be an effective anode material in lithium ion batteries with a high capacity of 900 mA h/g [20]. It has shown potential for supercapacitor applications [21], and it has demonstrated photocatalytic activity [22]. Zinc cobaltite, ZnCo2O4, belongs to the spinel crystal structure with the general formula AB2O4, where Zn occupies the tetrahedral A sites and Co occupies the octahedral B sites. ZnCo2O4 is isomorphic to the Co3O4 spinel crystal structure with the replacement of Co2+ ions (high spin) by Zn2+ ions [23]. An electrode material can exhibit efficient mass transfer (i.e., electrolyte penetration and ion transport) if it has a large surface area. This enhances the electrochemical processes and increases the power density and cyclic stability of the supercapacitor [24]. A number of researchers have investigated the electrochemical performance of zinc cobaltite–based material with different morphologies, such as nanoparticles [25], nanowires [26], nanowire arrays [27,28], nanorods [29,30], nanoflakes [20,31], nanotubes [32], hexagonal-like nanostructures [33], urchinlike microspheres [34], core-shell microspheres [35], and porousstructure microspheres [36]. However, it is still a great challenge to develop a simple, economical, environmentally benign, large-scale synthesis method for zinc cobaltite–based electrode material.

Corresponding author. E-mail address: [email protected] (F. Hamed).

https://doi.org/10.1016/j.jpcs.2018.04.044 Received 24 December 2017; Received in revised form 26 April 2018; Accepted 27 April 2018 0022-3697/ © 2018 Elsevier Ltd. All rights reserved.


T. Ramachandran, F. Hamed

electron microscope equipped with an energy-dispersive X-ray detector. X-ray photoelectron spectroscopy (XPS) measurements were conducted with a Kratos Analytical Axis Ultra DLD instrument with a monochromatized Al Kα1 source. X-ray photons of energy 1.486 keV with pass energies of 160 and 40 eV were used for the survey spectrum and the narrow scans, respectively. CasaXPS was used for peak fitting, and the procedure was kept consistent for all the peaks. The surface area was studied by Barrett-Joyner-Halenda algorithm (ASAP 2420 version 2.09) volumetric nitrogen adsorption-desorption experiments.

The objective of this work was to study the electrochemical properties and faradaic behavior of plate-like zinc cobaltite as an electrode active material in supercapacitors. We attempted to synthesize platelike zinc cobaltite nanocrystalline material via a facile hydrothermal method. As far as we know, no such attempt had been undertaken before this work. One of the advantages of the hydrothermal method is the control of the reaction temperature and pressure. The reaction occurs between metallic salts in sodium hydroxide (NaOH) solution without the need for an organic capping agent or a template. The electrochemical properties of the synthesized material were investigated to evaluate the performance and suitability of the plate-like zinc cobaltite material as electrode material in supercapacitors.

2.4. Electrochemical measurements and working electrode fabrication Electrochemical properties of the synthesized sample were obtained by cyclic voltammetry (CV), cyclic chronopotentiometry (CP), and electrochemical impedance spectroscopy. The electrochemical measurements were performed at room temperature with a CHI 7081C electrochemical workstation. CV measurements were collected over a potential range between 0.0 and 0.5 V at various scan rates from 5 to 100 mV/s. The specific capacitance was evaluated from the CP and CV charge-discharge curves. CP measurements were performed at various constant current densities over the potential range from 0 to 0.5 V. Electrochemical impedance spectroscopy measurements were conducted in the frequency region from 1 Hz to 1 MHz. The electrochemical measurements were conducted with a three-compartment cell with a working electrode, platinum wire as the counter electrode, and Ag/AgCl as the reference electrode. The electrodes were immersed in 2 M KOH electrolyte solution during the electrochemical measurements. The working electrode was fabricated in a fashion similar to that outlined in Ref. [37]. The synthesized sample as an electroactive material was mixed with polyvinylidene fluoride and activated carbon in a weight ratio of 8:1:1. A few drops of 1-methyl-2-pyrrolidinone were then added to the mixture to turn it into a paste. A 0.5 mm thick square nickel plate (1 × 1 cm2) was coated with the paste. The nickel plate was left to dry in air, and then it was annealed at 80 °C for 4 h. The mass of active electrode material was in the range 0.3–0.5 mg. The experimental conditions for the two-electrode system are provided in the supplementary information.

2. Materials and methods 2.1. Chemicals and materials The starting chemical precursors were zinc nitrate hexahydrate (Zn (NO3)2.6H2O; 98%, Sigma-Aldrich), cobalt(II) nitrate hexahydrate (CoN2O6.6H2O; ≥98%, Sigma-Aldrich), and NaOH pellets (97.5%, SDFCL, Mumbai). The starting chemicals were used as purchased without any further purification. Distilled water was used throughout the hydrothermal synthesis. 2.2. Synthesis of plate-like zinc cobaltite material The starting metal nitrates were weighed in accordance with a molar ratio of Zn to Co of 1:2 and dissolved in 20 ml of distilled water. NaOH was weighed such that the molar ratio of Zn to Na was 1:1 and was dissolved in 10 ml of distilled water. The nitrate solution was stirred until a homogeneous solution was obtained, and then the 10 ml NaOH solution was added dropwise. The resulting solution was subsequently transferred to a Teflon-lined stainless steel autoclave with a capacity of 50 ml. The autoclave was then transferred to a box furnace operating at 120 °C for 12 h. The final solution was allowed to cool to room temperature. The resulting products were removed from the final solution by centrifugation. The products were washed with distilled water seven times and then with acetone three times. The washed products were dried at 60 °C for 8 h. The dried sample was collected and stored for further characterization. A schematic diagram of the hydrothermal synthesis is presented in Fig. 1.

3. Results and discussion 3.1. Physiochemical properties of the synthesized plate-like zinc cobaltite material

2.3. Material characterization The crystal structure of the synthesized sample was investigated with a Shimadzu LabX XRD-6100 powder X-ray diffractometer. The Xray diffractometer was operated at 30 kV with a current intensity of 30 mA. The X-ray diffraction (XRD) profile was recorded over the 2θ range from 20° to 80° with a step size of 0.02°/min with use of Cu Kα radiation (1.541 Å) at room temperature. The XRD profile was screened against the International Centre for Diffraction Data (ICDD) PDF-2 database. The topology, morphology, and elemental composition of the synthesized sample were examined with a JEOL JSM-6010LA scanning

Fig. 2 presents the XRD profile collected from the synthesized material. The diffraction peaks in the XRD profile match quite well with those of the spinel ZnCo2O4 in accordance with ICDD PDF-2 card no. 00-023-1390. The observed diffraction peaks correspond to the (220), (311), (222), (400), (422), (511), (440), (620), (533), (642), (731), and (751) reflection planes of ZnCo2O4 with a face-centered cubic spinel crystal structure. No extra diffraction peaks were observed within the limits of the diffractometer, which indicates that the synthesized

Fig. 1. The facile hydrothermal synthesis method for plate-like zinc cobaltite material. 94



limits of the EDS detector. The inset in Fig. 4e shows the quantitative EDS analysis of the synthesized material. Theoretically, the atomic percentages should be 14.28% for Zn, 28.57% for Co, and 57.14% for O within the stoichiometric composition of ZnCo2O4. In the present study, the atomic ratio of Co/Zn is 1.2, which is far from the stoichiometric ratio. This indicates that the as-synthesized material has more Zn, which could result in some defects. The molecular formula may be best described as ZnxCo3−xO4; in the present case it is Zn1.36Co1.64O4. We expect that these deviations from stoichiometry will have some effects. It was shown that varying the Zn/Co ratio in ZnxCo3−xO4 had a noticeable effect on the morphology and the electrochemical performance in synthesized ZnxCo3−xO4 nanomaterials [38,39]. To analyze the specific surface area and pore size of the synthesized zinc cobaltite nanocrystalline material, N2 adsorption-desorption isotherms were measured. Fig. 5a shows the adsorption-desorption isotherm curves for the synthesized plate-like zinc cobaltite nanocrystalline material. According to the International Union of Pure and Applied Chemistry (IUPAC) classification, the recorded N2 adsorption-desorption isotherms can be categorized as type IV with an H3-type hysteresis loop. The observed wide region between P / P0 = 0.48 and P / P0 = 1 in the hysteresis loop indicates a mesoporic nature [40]. The Brunauer-Emmett-Teller specific surface area was determined to be 28.19 m2/g. Fig. 5b depicts the narrow pore size distribution in the synthesized plate-like zinc cobaltite nanocrystalline material. Most of the pores had sizes between 2 and 6 nm. With such pore sizes and large specific surface area, the synthesized plate-like zinc cobaltite nanocrystalline material is considered to be a suitable candidate electrode material for supercapacitor applications [37]. An XPS survey spectrum was collected from the synthesized platelike zinc cobaltite nanocrystalline material as presented in Fig. 6a. The identified C 1s peak from the double-sided carbon tape used to fix the sample was considered as an energy reference. The observed photoelectron peaks are identified as the states of Zn 2p, Co 2p, and O 1s, except for the broad peak at 976.7 eV, which is usually assigned to O Auger (KLL). No other elements were identified, which further supports the findings of the EDS studies. Fig. 6c shows the narrow scan of the Zn 2p peak and the corresponding deconvolution. It consists of two major peaks. They are

Fig. 2. X-ray diffraction profile of the synthesized plate-like zinc cobaltite material.

material is a single-phase material. The average crystallite size of the synthesized zinc cobaltite material was calculated in accordance with the Scherrer equation:

D = 0.9λ /β × cosθ ,

(1)

where D is the average crystallite size of the sample, β is the full width at half maximum, λ is the X-ray wavelength, and θ is the diffraction angle. The average crystallite size of the synthesized zinc cobaltite material was calculated to be around 16 nm, which is in good agreement with the previously reported value of 20.7 nm [19]. Fig. 3 shows scanning electron microscopy images highlighting the morphology and topology of the synthesized zinc cobaltite nanocrystalline material. It is clear that the synthesized material is made up of plate-like particulates. The energy-dispersive X-ray spectroscopy (EDS) elemental mapping in Fig. 4a–d indicates a uniform distribution of the elements Zn (yellow), Co (green), and O (red) throughout the zinc cobaltite sample. The EDS spectrum presented in Fig. 4e shows the presence of Zn, Co, and O only. No other elements were detected within the

Fig. 3. Scanning electron microscopy surface morphology at (a) 100 μm, (b) 50 μm, (c) 10 μm, and (d) 20 μm of the synthesized zinc cobaltite material. 95



Fig. 4. Elemental mapping: (a) the area of mapping, (b) zinc distribution, (c) cobalt distribution, (d) oxygen distribution, and (e) energy-dispersive X-ray spectroscopy spectrum of the synthesized zinc cobaltite material.

Fig. 5. (a) N2 adsorption-desorption isotherms and (b) pore size distribution in the synthesized plate-like zinc cobaltite nanocrystalline material.

deficiency in the present plate-like zinc cobaltite nanocrystalline material. The narrow scan of the O 1s peak is presented in Fig. 6d. The peak can be deconvoluted into two fine peaks at 528.65 and 530.60 eV. These peaks are associated with the binding energies of O2− within the zinc cobaltite lattice and hydroxide ions, OH−, adsorbed on the surface of the zinc cobaltite sample [44]. These XPS observations confirm the chemical states of the elements within the zinc cobaltite lattice. Table lists the quantitative XPS analysis data obtained from the deconvolution of Zn 2p, Co 2p, and O 1s peaks.

identified as corresponding to Zn 2p3/2 at 1020.06 eV and Zn 2p1/2 at 1043.03 eV. This indicates an oxidation state of Zn2+ in the zinc cobaltite sample [41]. Fig. 6b shows the narrow scan of the Co 2p peak. It exhibits two subpeaks due to spin-orbit splitting. The deconvoluted peaks at 779.91 and 781.65 eV are assigned to Co 2p3/2, and those at 794.95 and 796.65 eV correspond to Co 2p1/2. This indicates the presence of Co3+ ions within the zinc cobaltite lattice [42]. Two additional satellite peaks are observed at 785.76 and 803.91 eV, and are identified as S1 and S2, respectively. A binding energy difference of 9.19 eV from satellite peak S1 to the Co 2p1/2 peak indicates a Co3+ oxidation state in the zinc cobaltite lattice [43]. Table 1 displays the fitted peak width of the Co 2p core level. Compared with the Zn 2p peak, the peak width (full width at half maximum) of the Co 2p peak increased slightly because of the incorporation of Zn ions into the octahedral sites. In spinel-type ZnCo2O4, Zn occupies the tetrahedral sites and Co occupies the octahedral sites. Such a small increase in peak width could lead to Co

3.2. Electrochemical studies of plate-like zinc cobaltite electrode material CV curves measured at different scan rates are presented in Fig. 7a. The CV curves show the nearly characteristic rectangular shapes of ideal capacitance. The observed redox peaks (P1 and P2) in the CV curves originate from the reversible transitions between the oxidation 96



Fig. 6. X-ray photoelectron spectroscopy (XPS) wide spectrum (a) and deconvoluted high-resolution XPS spectra for Co 2p (b), Zn 2p (c), and O 1s (d) peaks of the synthesized plate-like zinc cobaltite nanocrystalline material.

where SC is the specific capacitance, m is the mass of electroactive material, and the potential voltage window (Va − Vc ). The specific capacitance was calculated from the integrated area under the I-V curves divided by the scan rate (mV/s). The specific capacitance was determined to be 812, 784, 676, 631, 524, and 401 F/g at scan rates of 5, 10, 25, 50, 75, and 100 mV/s, respectively. Fig. 8a shows the specific capacitance as a function of the scan rate. The specific capacitance decreases as the scan rate is increased; this is due to the surface sites available during the electrochemical reaction. At lower scan rates, the electrolyte ions have sufficient time to diffuse into the electrode material and access the active sites. Diffusion-controlled kinetics of the electrochemical reaction can be inferred from the Randles-Sevcik plot. Fig. 8b shows the Randles-Sevcik plot for the synthesized plate-like zinc cobaltite nanocrystalline electrode material. The linear relationship between peak current and the square root of the scan rate indicates a controlled mass transfer process of oxidation via diffusion. The specific capacitance of the synthesized plate-like zinc cobaltite nanocrystalline electrode material is compared with the specific capacitance obtained in previous studies in Table 2. The electrochemical performance of the present plate-like zinc cobaltite is quite remarkable considering the facile synthesis process.

states. This indicates that the major charge storage in the present synthesized plate-like zinc cobaltite nanocrystalline material is faradaic in nature. This is consistent with earlier reports [45,46]. Generally, the capacitance of an electrode material is influenced by crystallinity, surface area, pore size, chemical composition, and morphology. In the present case, we think that the plate-like morphology increased the interfacial area, which facilitated ion insertion/extraction. The pair of redox peaks were assigned to the faradic redox reactions related to Co (OH)2/Co–OOH. The electrochemical redox reactions in the alkaline electrolyte are based on the following equations [47]:

Co2 O24− + 2H2 O⇌ 2Co–OOH + 2OH−

(2)

2Co–OOH + 2H2 O+ e− ⇌ Co(OH)2 + OH−

(3)

The intensity of the anodic and cathodic peaks increased with scan rate, which signifies intercalation/deintercation or oxidation/reduction processes [48]. The specific capacitance was calculated from the CV curves in accordance with the following equation [45]:

SC =

1 V × m (Va − Vc )

∫V

Vc

(IV ) dV ,

(4)

a

Table 1 Quantitative X-ray photoelectron spectroscopy analysis data obtained from Zn 2p, Co 2p, and O 1s spectra for the zinc cobaltite sample. Core level spectra

Peaks

Deconvoluted peaks (eV)

Area (eV)

FWHM (eV)

Concentration (%)a

Zn 2p

3/2 1/2 3/2 3/2 S1 1/2 1/2 S2

1020.06 1043.03 779.91 781.65 785.76 794.95 796.65 803.91 528.65 530.60

1325.3 2482.7 576.6 695.4 152.6 164.6 384.7 144.4 5989.9 3497.4

2.51 2.53 1.98 4.9 2.65 2.65 2.65 2.65 1.48 3.59

67.14 32.86 27.3 32.92 7.20 7.74 18.08 6.76 63.15 36.85

Co 2p

O 1s

(1) (2) (1) (2)

S1 and S2 are satellite peaks. FWHM, full width at half maximum. a At corresponding peaks. 97



Fig. 7. (a) Cyclic voltammetry curves at different scan rates, (b) charge-discharge curves for different current densities, (c) Nyquist plot (the inset shows a blowup of the high-frequency region), and (d) Randles equivalent circuit of the synthesized platelike zinc cobaltite electrode material. Cdl, capacitive double layer; Rct, charge transfer resistance; Cp, pseudocapacitance; Rs, equivalent series resistance or solution resistance and W, warburg impedance.

nonlinear behavior over the potential range from 0.25 to 0.5 V arises from the redox activity of the chemical species. The specific capacitance can be calculated from galvanostatic charge-discharge curves with the help of the following formula [46]:

Cyclic CP curves of the synthesized plate-like zinc cobaltite nanocrystalline electrode material measured at different current densities in 2 M KOH electrolyte are presented in Fig. 7b. The observed galvanostatic charge-discharge curves deviate from linearity; this indicates that the charges originate from faradic redox reactions, which is consistent with our CV studies. The linear relation over the potential range from 0 to 0.25 V results from the capacitive double layer, whereas the

SC =

I × Δt , m × ΔV

(5)

Fig. 8. (a) Specific capacitance versus scan rate, (b) Randles-Sevcik plot, and (c, d) the results of the cycling stability study performed at a current density of 5 A/g for up to 5100 continuous cycles of the synthesized plate-like zinc cobaltite electrode material. 98



Table 2 Comparison of electrochemical performance of the synthesized plate-like zinc cobaltite materials with the performance reported for ZnCo2O4-based material with different morphologies in the literature. Electrode material

Method of synthesis

SC (F/g)

Reference

ZnCo2O4 nanowire cluster arrays Double hydroxide–treated ZnCo2O4 ZnCo2O4 microspheres Ni foam–supported ZnCo2O4 microspheres Highly porous ZnCo2O4 nanotubes ZnCo2O4 nanorods ZnCo2O4 rod-like nanostructure Hierarchical coral-like ZnCo2O4 nanowires Plate-like zinc cobaltite

Hydrothermal Hydrothermal Solvothermal Hydrothermal Electrospinning Polyol refluxing process Oxalate co-precipitation Hydrothermal Hydrothermal

1180 at 20 A/g 845.7 at 1 A/g 953.2 at 4 A/g 689.4 at 1 A/g 770 at 10 A/g 1015 at 20 A/g 604.52 at 1 A/g 694 at 2 A/g 805.3 at 1 A/g

[28] [33] [47] [49] [53] [54] [55] [56] This work

SC, specific capacitance.

Electrochemical impedance spectra were collected from the platelike zinc cobaltite nanocrystalline electrode material over the frequency range from 1 Hz to 100 kHz, and they are presented as a Nyquist plot in Fig. 7c. The inset in Fig. 7c shows a blowup of the high-frequency region. The Nyquist plot can consist of three regions: the equivalent series resistance (ionic resistance of the electrolyte plus intrinsic resistance of the active material plus contact resistance), which can be obtained from the intercept at the x-axis; the charge transfer resistance due to electron diffusion between the electrode-electrolyte interface, which can be obtained from the diameter of the semicircle; and the Warburg impedance due to the diffusion of counterions between the electrolyte and the electrode material, which can obtained from the slope of the curve at low frequencies [50]. The Randles equivalent circuit is illustrated in Fig. 7d. The equivalent series resistance is on the order of 2 Ω. The absence of a semicircle in the high-frequency region indicates very small charge transfer resistance. This denotes fast electrochemical reactions. Lower internal resistance (high conductivity) is of great importance as less energy will be wasted during the charge-discharge process [51]. The slope of 45° in the linear part over the low-frequency region denotes lower diffusion resistance, which favors ion transport [52]. From the overall electrochemical studies, the plate-like zinc cobaltite nanocrystalline electrode material synthesized by a facile hydrothermal method shows superior performance for supercapacitor application.

where SC is the specific capitance, I is the constant current, Δt is the discharge time, m is the mass of the electroactive material loaded, and ΔV is the potential difference (0.5 V). The calculated specific capitances are 805.3, 706.1, 672.7, 652.3, 629.8, 598.2, 450.9, and 299.4 F/g at current densities of 1, 2, 4, 5, 6, 8, 25, and 75 A/g, respectively. These values are comparable with those obtained from CV. The coulombic efficiency of the synthesized plate-like zinc cobaltite nanocrystalline electrode material was calculated from the following formula [45]:

η=

td × 100%, tc

(6)

where η is the coulombic efficiency, td is the discharge time, and tc is the charging time. The synthesized zinc cobaltite electrode material showed a maximum coulombic efficiency of 78%, which is expected from its nearly symmetric charge-discharge curves. The long cyclic stability of the electrode material was investigated by continuous charge-discharge cycling at a current density of 5 A/g for 5100 cycles. Fig. 8c and d displays the long cycling profiles. It can be seen (Fig. 8c) that the specific capacitance retention is 88% even after 5100 cycles. This proves that the synthesized plate-like zinc cobaltite nanocrystalline electrode material has excellent redox behavior and good cyclic stability. Our results compare well with those from previous studies on zinc cobaltite (ZnCo2O4) film [45], ZnCo2O4 grown on Ni foam [46], and flower-like ZnCo2O4 microspheres [49]. The possible interaction between the electrolyte ions and the plate-like zinc cobaltite electrode material is illustrated in Fig. 9. The large surface area of the plate-like morphology enhances the insertion/extraction electrolyte ions. The present plate-like zinc cobaltite nanocrystalline material could be considered as a promising candidate electrode material because of its good electrochemical performance.

3.3. Symmetric two-electrode cell assembly and its electrochemical characterization The symmetric two-electrode cell-membrane assembly where the plate-like zinc cobaltite nanocrystalline material was sandwiched between two nickel plates is illustrated in Fig 10a.This assembly was tested to reflect the actual performance of the plate-like zinc cobaltite nanocrystalline material under actual capacitor settings. The CV curves collected from the two-electrode cell assembly at different scan rates are presented in Fig. 10b. The shape of these curves is quite different from that of those collected from the three-electrode assembly. The CV curves show nearly rectangular cycles, revealing the pseudocapacitive response of the zinc cobaltite sample. Fig. 10c displays the CP curves for the two-electrode cell assembly measured at different current densities. The charge-discharge characteristics display linear relationships, indicative of the double layer capacitive behavior. The specific capacitance of the two-electrode cell assembly was calculated from the following equation:

SC =

4×K , m

(7)

where SC is the specific capacitance, K is the experimentally calculated capacitance of the cell, and m is the total mass of the electrodes. The calculated specific capacitance was 103.9, 99.4, 95.6, 87.6, 81.9, and 80.8 F/g at scan rates of 5, 10, 25, 50, 75, and 100 mV/s, respectively. These values are much lower than those obtained for the three-

Fig. 9. Charging and discharging in the synthesized plate-like nanocrystalline zinc cobaltite electrode material. 99



Fig. 10. (a) Fragmented view of the two-electrode cell-membrane assembly, (b) cyclic voltammetry curves at different scan rates, (c) cyclic chronopotentiometry curves at different current densities, and (d) complex plane impedance plot (the inset shows the long cyclic stability for the symmetric zinc cobaltite capacitor). PVA, polyvinyl alcohol.

electrode configuration. Fig. 10d shows a Nyquist plot for the twoelectrode cell assembly. The inset shows a blowup of the high-frequency region. The plot shows behavior similar to that in Fig. 6c, which favors ion transport. The electrochemical performance of the present plate-like zinc cobaltite nanocrystalline material is good enough to make it a good electrode choice for supercapacitors.

[5]

[6]

[7]

4. Conclusions Plate-like zinc cobaltite nanocrystalline material was synthesized by an economically facile hydrothermal process. The synthesized material displayed excellent electrochemical performance, which favored electrolyte ion transport. The material has a maximum coulombic efficiency of 78% and a maximum specific capacitance of 812 F/g. The material retained 88% of its capacitance after 5100 cycles. The synthesized plate-like zinc cobaltite nanocrystalline material may prove to be promising electroactive material for applications in supercapacitors.

[8] [9]

[10]

[11]

Acknowledgment

[12]

This research was supported by the UAEU Program for Advanced Research under grant number G00001647, United Arab Emirates University, Al Ain, United Arab Emirates.

[13]

Appendix A. Supplementary data

[14]

Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.jpcs.2018.04.044.

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