Ionics https://doi.org/10.1007/s11581-018-2766-1
SHORT COMMUNICATION
Hydrothermal synthesis and electrochemical properties of ZnCo2O4 microspheres B. Saravanakumar 1 & G. Ravi 1 & R. Yuvakkumar 1 & V. Ganesh 2 & S. Ravichandran 3 & M. Thambidurai 4 & A. Sakunthala 5 Received: 23 April 2018 / Revised: 8 October 2018 / Accepted: 10 October 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2018
Abstract Zinc cobalt oxide (ZnCo2O4) microspheres are prepared at three different hydrothermal process temperatures (100 °C, 130 °C, and 160 °C) assisted with urea. XRD studies reveal the spinel face-centered cubic (Fd3m) structure of ZnCo2O4 microspheres. The optical and vibrational properties of the product are characterized by photoluminescence and FTIR studies. The strong nearband edge emission peak observed at 392 nm corresponds to the direct recombination of the exciton-exciton collision process for all three synthesized products; SEM analysis reveals the complete growth stage of spherical ZnCo2O4 microspheres at three different temperatures. The electrochemical properties of synthesized ZnCo2O4 microspheres are analyzed by cyclic voltammetry, electroimpedance spectroscopy, and galvanostatic charging and discharging studies. ZnCo2O4 microspheres (SH3–160 °C) exhibit the superior specific capacitance of 500 F/g at 0.75 A/g current density and retain their specific capacitance of 80% at current density 2 A/g. ZnCo2O4 microspheres (SH3–160 °C) may be considered as a good candidate as electrode in supercapacitor applications. Keywords Hydrothermal . ZnCo2O4 microspheres . Supercapacitor applications
Introduction The demand for clean and green renewable energy sources to power devices turns out to be a very significant impasse for our modern society due to environmental toxic waste, greenhouse gas effect, exhaustion of fossil fuels, etc. [1–3]. In
* R. Yuvakkumar
[email protected] 1
Nanomaterials Laboratory, Department of Physics, Alagappa University, Karaikudi, Tamil Nadu 630 003, India
2
Electrodics and Electrocatalysis (EEC) Division, CSIR–Central Electrochemical Research Institute (CSIR–CECRI), Karaikudi, Tamil Nadu 630003, India
3
Electro Inorganic Division, CSIR–Central Electrochemical Research Institute (CSIR–CECRI), Karaikudi, Tamil Nadu 630003, India
4
Luminous! Centre of Excellence for Semiconductor Lighting and Displays, School of Electrical & Electronic Engineering, The Photonics Institute (TPI), Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
5
Department of Physics, School of Science and Humanities, Karunya Institute of Technology and Sciences, Karunya Nagar, Coimbatore, Tamil Nadu 641114, India
recent time’s researches, more focus was laid down for the invention of a new kind of energy sources and storage devices; especially, a large number of works were focused on Li-ion battery (LIB) and supercapacitor applications [4–6]. Recently, plentiful endeavors have been executed to recognize the synthesis of transition metal oxides (MnO2, γ-Mn2O3 Co3O4, NiO, ZnO, Fe2O4, RuO2, etc.) as electrodes used in batteries and supercapacitor applications. Moreover, binary metal oxides (NiCo2O4, ZnCo2O4, ZnFe2O4, LiMnPO4, etc.) have also been extensively studied as an electrode material for batteries, supercapacitors, and electrochemical applications [7–23]. For example, Qiu and his co-workers studied a novel nanostructured spinel ZnCo2O4 electrode material in lithium ion batteries [24]. The synthesis of metal oxides [25–27] and metal cobaltite [24, 28–30] (ZnCo2O4 and NicO2O4) nanomaterials has been investigated, and especially ZnCo2O4 nanomaterials for symmetric supercapacitor applications by mixing an equal proportion of carbon nanofoam (CNF) were discussed and showed that a ZnCo2O4-based supercapacitor cell showed good cyclic stability and high columbic efficiency [31, 32]. Liu and his co-workers have reported the hierarchical threedimensional ZnCo2O4 nanowire arrays/carbon cloth anodes for a novel class of high-performance flexible lithium-ion application [33]. ZnCo2O4 NWCAs on Ni foam were directly
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Fig. 1 XRD pattern of ZnCo2O4: a 100 °C, b 130 °C, and c 160 °C
used as integrated electrodes for supercapacitors and exhibited a high specific capacitance [34]. Several reports have also been presented in order to understand the electrochemical behavior of ZnCo2O4-based nanomaterials on Ni foam [35–37] and its composite by toting up with NiCo2O4 as core-sheath nanowires [38], ZnCo2O4·2H2O whiskers to ZnO nanoparticles [39], and a 3D hierarchically assembled porous wrinkledpaper-like structure of ZnCo2O4 and Co-ZnO@C [40]. Huang and his co-workers discussed a facile synthesis of a porous ZnCo2O4 rod-like nanostructure for high-rate supercapacitors [41]. Xie has developed double-shelled ZnCo2O4 hollow microspheres which manifest a large reversible capacity, superior cycling stability, and good rate capability [42]. The zinc–cobalt-layered double hydroxide has also been employed for solar cell applications, photocatalytic activities [43, 44], and oxygen evolution reaction (OER) performance [45], and its properties were studied in detail to understand the mechanism. However, in the present work, the effects of hydrothermal processing temperature on ZnCo2O4 microsphere synthesis were studied and reported. The optimization of the hydrothermal processing temperature for configuration of complete microspheres is to be conceded, and their electrochemical study Table 1
Fig. 2 PL spectra of ZnCo2O4: a 100 °C, b 130 °C, and c 160 °C
was studied on all the obtained products. The result has been compared, and a suitable ZnCo2O4 microsphere electrode has been identified and considered as a good candidate for super capacitor applications.
Materials and methods The solvothermal synthesis method to prepare ZnCo2O4 microspheres with the same molar concentration of the precursor maintained at different hydrothermal temperatures (100 °C, 130 °C, 160 °C) was adopted; 0.297 g (0.05 M) zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and 0.036 g (0.05 M) cobalt nitrate hexahydrate (Co(NO3)2·6H2O) were dissolved separately in a 40-mL mixture of double-distilled water and ethanol in the ratio of 50:50. The suspended precursor salts are transferred into a beaker and stirrer for 30 min to achieve homogeneity of solution. Then, 0.045 g (0.15 M) of urea was also added drop by drop in the above solution. The homogenous precursor solution is transferred to a 100-mL Teflon-coated autoclave and maintained at 100 °C for 14 h. Then, the autoclave is allowed to cool down to room temperature naturally. The obtained product is cleaned and washed
The crystalline parameters for synthesized products SH1, SH2, and SH3
Sample
Crystal structure
Unit cell lattice parameter Ba^ (Å)
Unit cell volume (Å)3
Crystallite size (nm)
Dislocation density lines/m2
Microstrain
SHI SH2 SH3
FCC (Fd3m) FCC (Fd3m) FCC (Fd3m)
8.217 8.225 8.238
554.80 556.42 559.06
13.93 56.87 42.482
0.0051 0.0003 0.0005
0.1680 0.1923 0.1894
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ethanol and placed in a hot air oven for 12 h at 80 °C. The active material mass amount was 4 mg loaded on the nickel foam by making a slurry of composition of the active material. Activated carbon and PVDF (polyvinylidene fluoride) in 80:10:10 ratios with NMP (N-methyl-2-pyrrolidone) were used as dissolving solution. The slurry was coated onto 1 cm × 1 cm by a drop cast method and once again dried in hot air for 12 h at 80 °C. The active material coated on Ni-foam acted as a working electrode; platinum wire and the Ag/AgCl electrode acted as counter and reference electrodes with 2 M of KOH solution used as electrolyte for all electrochemical studies.
Results and discussion
Fig. 3 IR spectra of ZnCo2O4: a 100 °C, b 130 °C, and c 160 °C
by distilled water and ethanol for four times to remove the unwanted salt precursors and impurities. Then, the washed products were dried in a hot air oven at 80 °C for overnight and named as SH1, and finally, the sample was annealed at 400 °C for 3 h. Subsequently, the same procedure is followed for the other two products by only varying the hydrothermal temperature maintained at 130 °C and 160 °C and named as SH2 and SH3. The characterization method of synthesized products was given in detail in our previous reported literatures [45–49]. ZnCo2O4 as a working electrode was prepared by the following procedure, where nickel foam was cleaned in 3 M aqueous HCl solution for 30 min under ultrasonication. Then, nickel foam was further cleaned in deionized water and Fig. 4 SEM micrograph of ZnCo2O4: a 100 °C, b 130 °C, and c 160 °C
The powder X-ray diffraction pattern of the calcinated products named as SH1, SH2, and SH3 is shown in Fig. 1a–c. Figure 1a–c exhibits the diffraction peaks at 2θ of 31.2°, 36.8°, 38.5°, 44.7°, 55.5°, and 65.1°. These peaks can be attributed to the corresponding planes (220), (311), (400), (331), (422), and (531), which confirms the formation of ZnCo2O4 nanoparticles with a spinel structure. It is well matched with the JCPDS file no. 23-1390. The additional small peaks also found in the SH2 and SH3 sample at 47.84° and 62.77° can be attributed to (102) and (103) planes which correspond to the formation of zinc oxide (ZnO). Figure 1a shows the low-intensity primary peak of ZnCo2O4 attained at hydrothermal processing temperature of 100 °C. After increasing the hydrothermal temperature to 130 °C and 160 °C, the intensity and sharpness of the peaks have been increased and indicated the good crystalline formation of ZnCo2O4 nanoparticles at high processing temperatures (Fig.
Ionics Fig. 5 Cyclic voltammetry curves: a SH1 (100 °C), b SH2 (130 °C), c SH3 (160 °C), and d EIS spectra for SH1, SH2, and SH3
1b, c). The average calculated lattice parameter, a = 8.216 Å, is in good accord with the reported value for the ZnCo2O4 spinel structure (a = 8.09 A°), and other parameters are shown in Table 1. The crystallite size was calculated by using the Debye-Scherer equation Dp¼ β0:94*λ Cos θ, where Dp is the crystallite size, λ is the wavelength of the X-ray used, β1 is the wavelength of full-width half maximum, and θ is the diffraction angle. Figure 2a–c shows the optical properties of the synthesized products SH1, SH2, and SH3 studied by employing photoluminescence analysis which are excited at a wavelength of 320 nm. The strong near-band edge emission peak observed at 392 nm corresponds to the direct recombination of the exciton-exciton collision process for all three synthesized products [50]. Figure 3a–c illustrates the FTIR spectra for synthesized products SH1, SH2, and SH3. The IR spectra of ZnCo2O4 nanoparticles represent the peaks at 1630, 1097, 661, and 569 cm −1 which have been matched well with already
reported literature [49]. The two strong absorption bands at 661 and 569 cm−1 can be attributed to the M–O (Zn and Co) vibration modes of the metal presented in the tetrahedral and octahedral sites of the spinel structure. The small peak observed at 1097 cm−1 corresponds to the C–O bond vibration which may contribute to the carboxyl group of urea [51]. The broad band at 1630 cm−1 corresponds to the bending modes of the OH groups of water [24]. The morphological configuration of the synthesized products SH1, SH2, and SH3 has been characterized by SEM. The SEM images in Figure 4a–c clearly described the formation of ZnCo2O4 microspheres at different hydrothermal temperatures (100 °C, 130 °C, and 160 °C). The first SEM image portraits the agglomerated formation of ZnCo2O4 at 100 °C due to insufficient physico-chemical conditions for the formation mechanism. Figure 4b witnesses the incomplete formation of spheres at 130 °C owing to the inadequate condition for formation of spheres. The suitable condition for configuration of ZnCo2O4 spheres occurred at optimized temperature of
Ionics Fig. 6 GCD curves: a SH1 (100 °C), b SH2 (130 °C), c SH3 (160 °C), d specific capacitance vs current density curves for SH1, SH2, and SH3
160 °C. The physico-chemical condition essential for complete formation of spheres is attained at 160 °C, and it can be evidenced from Fig. 4c. The result revealed the relatively microsized particles owing to the complete spherical particles of ZnCo2O4 which is a significant parameter in charge storage mechanisms. The electrochemical properties of synthesized ZnCo2O4 microspheres were analyzed by following techniques, namely, cyclic voltammetry, electroimpedance spectroscopy, and galvanostatic charging and discharging studies. Figure 5a–c shows the cyclic voltammetry curves for SH1, SH2, and SH3 products for different scan rates as 10, 20, 30, 50, 80, and 100 mV in 1 M KOH electrolyte solution over the potential window of − 0.1–0.6 V. Before estimating cyclic voltammetry properties of active material, bare nickel foam has been tested and their results are compared and seem very feeble for bare nickel foam to active material. From CV curves, we can observe that the oxidation peak of Co2+ ⇄
Co3+ in anodic and reduction peaks in the cathodic region between − 0.1 and 0.6 V and a shift in redox peaks occur with the increase in scan rate from 10 to 100 mV. In addition, the current of redox peaks increases linearly with increase in scan rate, signifying the mechanism of interfacial Faradic redox effects and rapid rise in rate of electronic and ionic transport [52–54]. This phenomenon indicated that the obtained products possess a pseudocapacitive nature due to a faradic interaction between ions in the electrolyte and charges in the surface of active material of working materials. The pseudocapacitive nature of obtained product SH3 shows a superior capacitive nature which might be due to the formation of uniform and complete microsphere formation revealed from SEM images, and it enhances the intercalation of charge carrier ions in electrolyte to the working electrode. It is also interesting to note that these high pseudocapacitance behaviors for SH3 may be due to the exposure of nanosized ZnCo2O4 microspheres. Figure 5d shows the Nyquist plots
Ionics Table 2 Calculated specific capacitance of obtained products at different current density
S. no.
Potential window (V)
Current density (A/g)
Specific capacitance (F/g) SH1
SH2
SH3
1
0.45
0.5
392
431.6
506
2
0.45
0.75
383
392.5
500
3
0.45
1
356
336
460
4 5
0.45 0.45
2 5
273 131
277 162
407 252
6
0.45
10
26
50
93
of the EIS spectra of synthesized ZnCo2O4 microspheres. The juncture of EIS curves with the real axis specifies the electrochemical system resistance which comprises the internal resistance of the active material, ionic resistance of electrolyte, and contact resistance at the boundary connecting electrolyte and electrode. In addition, at the low frequency range, all the three products show the parallel curves with the same slope indicating that they have similar ion diffusion properties. The capacitive behavior of synthesized products galvanostatic charging and discharging studies was performed at different current densities of 0.5, 1, 2, 5, and 10 A/g in potential window of 0–0.45 V. Figure 6a–c displays charging and discharging curves of all products, SH1, SH2, and SH3, at different current densities, and it indicates the asymmetric pattern of charging and discharging curves. The nonlinearity and presence of the plateau region occur at the beginning of the discharging cycle which arises from faradic electron transfer and internal drop resistance (IR) of the active material [55]. The evaluation of specific capacitance can be done by
adopting the following expression, Cs ¼ mI ∇∇ Vt , where, I stands for the current applied to the cathode, ∇ t is the discharging time, m is the mass of the active material, and ∇ V is the potential window applied. The calculated specific capacitance of obtained products at different current densities is given in Table 2. It is clear that the specific capacitance of SH3 has superior capacitance than SH2 and SH3 primarily due to the fact that the incomplete formation of spheres inherent to the charge carrier to reach the active sites of SH1 and SH2 products (Table 1). Another main reason for superior capacitance of SH3 might be nanosized spheres which enables the more participation of active site interaction with charge carrier in the electrolyte. From Table 2, one can infer that the specific capacitance of SH3 remains constant at 0.5 A/g and 0.75 A/g current densities. The synthesized products SH1 and SH2 posses an identical discharge pattern and specific capacitance from 0.75 to 5 A/g which can be evidenced from Fig. 7d. The estimated specific capacitance of SH3 exhibits distinctly differently from those
Fig. 7 GCD curves for SH3 (160 °C) for 1000 cycles at current density 5 A/g
Fig. 8 Specific capacitance vs. no. of cycle curve for SH3 (160 °C) at current density 5 A/g
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of SH1 and SH2 with superior capacitive properties of 500 F/g at 0.75 A/g current density, and they retain its specific capacitance (460 F/g) of 80% at current density 2 A/g. The important study to investigate the stability of materials can be executed by adopting the stability measurement. The cyclic stability of SH3–ZnCo2O4 for 1000 cycles was examined in a three-electrode system and displayed in Fig. 7. Figure 7 explains the GCD curves for the first and last few cycles of 1000 GCD cyclic stability curves. The time for discharge extensively increased as the number of cycles increases and can be inferred from the GCD curves in Fig. 7. The pattern of specific capacitance of SH3 with increase in number cycle is displayed in Fig. 8 which enlightens the material stability of SH3–ZnCo2O4. A steady rise in specific capacitance from 252.5 to 416.4 F/g can be revealed, and the increment reaches up to 165% of initial capacitance for 1000 cycles at a current density of 5 A/g [56]. The important aspect in the supercapacitor electrode is the stability factor of the material and the product SH3–ZnCo2O4 could be considered as the potential and efficient electrode for pseudocapacitive applications.
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Conclusions 10.
Zinc cobalt spheres were successfully synthesized at different hydrothermal temperatures, and an optimum temperature was found to be at 160 °C. The synthesized zinc cobalt sphere was confirmed and characterized by XRD, PL FTIR, and SEM observations. The identified morphology from SEM images was situated as attestation for optimized temperature 160 °C for the complete formation of zinc cobalt spheres. The CV, EIS, and GCD studies explain the pseudocapacitive properties of all synthesized zinc cobalt spheres. The obtained product zinc cobalt (ZnCo2O4) microspheres (SH3) exhibit the superior specific capacitance of 0.75 A/g at current density, and they retain its specific capacitance of 80% at current density 2 A/g. Zinc cobalt (ZnCo2O4) microspheres (SH3) could be considered as a good candidate as electrode in supercapacitor applications. Further, cyclic stability studies show the steady rise in specific capacitance from 252.5 to 416.4 F/g, and the increment reaches up to 165% of the initial capacitance for 1000 cycles at a current density of 5 A/g for the obtained product zinc cobalt (ZnCo2O4) microspheres (SH3). Funding information This work was supported by UGC Start-Up Research Grant No. F.30-326/2016 (BSR).
Compliance with ethical standards Conflict of interest interest.
The authors declare that there is no conflict of
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