Mixed transition metal oxides with a normal spinel structure are considered as the most promising electrodes for high-performance hybrid supercapacitors due to their better electro-activity and superior supercapacitors properties than those of single metal oxides. Here, we demonstrated for the first time the fabrication of ZnCo2 O4 nanomaterials via a green synthetic way using Moringa Olefeira extract for high electrochemical supercapacitors. The supercapacitor based on Ni/ZnCo2O4 nanomaterial generated outstanding electrochemical performance with a high specific capacitance of 745 F/g at 10 mV/s and 929 F/g at a discharge current density of at 1 A/g, respectively. Based on electrochemical impedance spectroscopy (EIS), the pseudo-capacitive characteristics of the Ni/ZnCo3 O4 electrode revealed by a small semicircle and Warburg impedance, indicating that the electrochemical process on the surface electrode is kinetically and diffusion controlled. Based on this high specific capacitance and excellent electrochemical performance suggest that Ni/ZnCo2 O4 nanomaterial is a promising electrode material for supercapacitors.
Supercapacitors or electrochemical capacitors are considered as a most promising electrical energy storage devices due to their superior performance characteristics, including long cycle life time, high power density, energy density, short charging time, excellent cycling stability, fast charge – discharge, excellent environmental safety and superior reversibility [1-5]. They have shown great potential in the variety of applications such as industrial power plant, hybrid electric vehicle, portable system, renewable energies and military devices [6-9]. Generally, supercapacitors can be divided into electrical double layer capacitors (EDLCs) and pseudo-capacitors based to the energy storage mechanism [10-12]. EDLCs, energy storage arises mainly from the ionic charge separation at the electrode electrolyte interface [13, 14]. Activated carbons such as carbon nanotubes, graphene, etc. are the most commonly used materials for EDLCs owing to their excellent stability, low cost, high specific area and good electrical conductivity. Pseudo-capacitors have received much attention as they can provide much higher specific capacitance and energy densities than those of electrochemical double layer capacitors (EDLCs). Pseudo-capacitors, reversible redox and a faradic process occurring at the active electrode surface [6, 13-16]. Electrode materials for pseudo-capacitors, transitional metal oxides (TMOs) and conducting polymers such as, RuO2, Mn3O4, NiO, Co3 O4 , PANI and PPy have been extensively studied due to their high specific capacitance, environmental friendly, low cost and prominent electrochemical properties [10-12, 14]. However, the slow ion diffusion rates and poor electron conductivities and slow ion diffusion rates transition metal oxides would limit their electrochemical performances with high rate capabilities . Therefore, it is required to develop an advanced supercapacitor device which would improve the electrochemical performance, provide high capacity performance and excellent rate capability [17-19]. To date, binary metal oxides (such as NiCo2O4 and ZnCo2 O4 ) with a normal spinel structure have fascinated greatly consideration due to their stronger electrochemical activities, improve electronic conductivity and richer redox reactions compared with the single-component metal oxides . Among binary metal oxides, the compound ZnCo2O4 has been recognized as promising electrode materials for supercapacitors due to its better electro-activity and superior supercapacitive properties than those of single cobalt oxides (Co3 O4 ) [19-22]. ZnCo2 O4 nanomaterials have high redox and outstanding electric conductivity which make the material suitable for supercapacitor. The crystal structure of spinel ZnCo2O4 nanomaterials have shown great potential in various field including lithium ion batteries, catalyst sensors and supercapacitors due to its environmental friendly, low cost, easily synthesized, high theoretical capacity, high surface area and better electro-activity material [17-22]. Recently, various methods to fabricate ZnCo2 O4 nanomaterials with several nanostructures i.e. nanoparticles, nanowires, nanorods, microspheres and nanotubes have been reported including numerous chemical, physical and biosynthetic methods [23-28]. Among methods, biosynthetic method such as green chemistry is one of the more broadly recognized route due to its various advantages including cost effectiveness, no requirement of additional chemicals, very easy, reliability, minimum waste generation and environmental friendly method [29-33]. In this work, we report a facile biosynthesis of spinel ZnCo2 O4 via a green synthetic method using the Moringa oleifera natural extract which act as both capping and reducing agents. Their optical, morphological, structural activities and high electrochemical performance as electrode materials for supercapacitors are presented.
Material and Methods
Zinc nitrate hexahydrate (Zn(NO3 )2 6H2 O) and Cobalt chloride hexahydrate were purchased from Sigma Aldrich, and Moringa oleifera leaves were from Burkina Faso (West Africa).
Preparation of Moringa Oleifera extract:
30 g of cleaned Moringa Oleifera dried leaves were immersed in 300 ml of boiled deionized water (DI-H2 O) under magnetic stirring for 1h45 at 50 ?C. Then the mixture was cooled to room temperature and filtered through a nylon mesh, followed by a Millipore filter. The filtered Moringa Olefeira extract was stored in a refrigerator at 4 ?C for further studies.
Synthesis of spinel Co3O4 nanoparticles:
50 ml of Moringa Oleifera extract was used to dissolve 5 g of Zn(NO3 )2 6H2 O and Cobalt (II) chloride hexahydrate under magnetic stirring for 1 h. The salts were completely dissolved in the Moringa oleifera extract solution without any additional heat treatment. The solution was covered with an opaque foil to avoid any photo-induced phenomenon and kept at room temperature for 18 h. After 18 h, there was no precipitation formed but the color change, suggesting that the formation of a ZnCo2 O4 complex was in the form of a suspension. The solution was dried in a standard oven at 100 ?C and was then washed several times with DI-H2 O to remove the redundant materials of the extract. Finally, the sample was annealed at different temperature (500 and 700 ?C) for 2 h.
Characterization of nanomaterials:
Various techniques were used to characterize the physical, optical and chemical properties of Spinel Zinc Cobaltite nanomaterial. To determine the morphologies, particle sizes and size distributions of the samples, High Resolution Transmission Electron Microscopy (HRTEM) (Philips Technai TEM) operated at an accelerating voltage of 120 kV was used. The Energy Dispersive X-ray Spectroscopy (EDS) for elemental analysis was performed with an EDS Oxford instrument X-Max Solid state silicon drift detector operating at 20 keV. X-ray diffraction (X-Ray diffraction Model Bruker AXS D8 advance) with radiation Cuk =1.5406 Å was performed to obtain the crystalline structure of the nanomaterials. Fourier transform-infrared (FT-IR) absorption spectrometer (Shimadzu 8400s spectrophotometer) was utilized in the range of 300- 4000 cm-1 to verify the chemical bonding. Optical analyses of the nanomaterials were utilized by the photoluminescence (PL) spectrometer (Ocean optics, NL) with excitation wavelength of 300 nm. Differential Scanning Calorimetry/Thermo Gravimetric technique (DSC/TGA) simultaneously over a temperature range of 50-600 C in ambient air at a heating of 10 C/ min was used for thermal analysis.
The electrochemical analysis of the ZnCo2 O4 electrode was performed by cyclic voltammetry, galvanostatic charge-discharge test and electrochemical impedance spectroscopy (EIS) conducted on Autolab Potentiostat (CH Instruments, USA) electrochemical workstation. The measurements were carried out in a three-electrode system at room temperature, in which nickel foam as working electrode, platinum wire as counter electrode and Ag/AgCl as reference electrodes with a 3 M NaCl salt brigde solution.
Preparation of Ni/ ZnCo2O4 electrode:
The amount of ZnCo2 O4 nanomaterial was dissolved in ethanol and 2 µl of 5 % nafion solution was added. The solution was ultra-sonicated in a warm water bath for 15 minutes. The cleaned nickel foam electrode was dipped into the solution and dried in the oven at 35 °C for 1h to make Ni/ZnCo2 O4 for further studies. The potential window for cyclic voltammetry (CV) measurements varied from 0 to 0.8 V with the various scan rates of 10 – 80 mV/s. All experimental solutions were de-oxygenated by bubbling with high purity argon gas for 15 min and blanketed with argon during all measurements. All the electrochemical performances were tested in 3 M KOH.
To test their potential utility in supercapacitor applications, we studied electrochemical performance of ZnCo2 O4 nanomaterials. The electrochemical performance and specific capacitance of the nickel loaded ZnCo2O4 (Ni/ZnCo2 O4 ) nanomaterials electrode were investigated by cyclic voltammetry (CV), electrochemical impedance (EIS) and galvanostatic charge-discharge measurements. A CV is useful technique for determining whether a species is electroactive and the number of electrons transferred in a reaction. Figure 8(a) presents the CV curves of the bare Nickel foam and Nickel foam supported ZnCo2O4 nanomaterial electrode conducted at a scan rate of 50 mV/s. The CV curves of Ni/ZnCo2 O4 exhibits higher current response when compared with bare Nickel, which shows that ZnCo2 O4 effectively promotes electron transfer between the electrolyte and the electrode surface [38, 40, 43, 44]. The CV curves of Ni/ ZnCo2 O4 reveals two distinct redox couples, corresponding to the oxidation and reduction process of ZnCo2 O4 nanomaterials, respectively [38, 40, 43, 44]. A pair of cathodic peaks can be seen at 0.13 V (IV) and 0.49 V (III), this can be allocated to the reduction reaction of ZnCo2O4 with Ni into ZnO and Co0; followed by the anodic peaks at 0.30 (I) and 0.56 V (II) can be assigned to the oxidation of Zn0 and Co0 to ZnO and Co3 O4 [38, 40, 43, 44]. A pairs of redox peaks clearly reveals the pseudo-capacitive characteristics derived from Faradaic reactions [38, 40, 43, 44]. Due to high good voltammetric response, high electro-activity and enhanced electrochemical kinetics, Ni/ZnCo2 O4 nanomaterial is a promising candidate for high-performance supercapacitors. Figure 8b presents the cyclic voltammetry (CV) curves of Ni/ZnCo2 O4 nanomaterial electrode measured at various scan rates ranging from 10 – 60 mV/s. The peak current density increases with increasing the scan rates, suggesting a fast diffusion controlled electrolyte ion transport kinetic at the interface [38, 40, 43, 44]. The redox peaks can be observed clearly within the potential region of 0 – 0.8 V for all scan rates, which indicates the fast and excellent electrochemical reversible redox reaction [38, 40, 43, 44]. The slightly shift of redox peak potentials, may indicating the limitation of the ion diffusion rate to satisfy electronic neutralization during the redox reaction [38, 40, 43, 44]. The specific capacitances from the CV curves were calculated according to the following Eq. 3 and listed in Table 2. Moreover, with increase scan rates the capacitances decreases due to the insufficient faradic redox reaction at higher scan rate [38, 40, 43, 44].
Here i is a sampled current (A), dt is a sampling time span (s), and ?V is a total potential deviation of the voltage window (V). The specific capacitances of the nickel foam supported ZnCo2 O4 nanomaterial electrodes are calculated to be 745, 705, 678, 638, 598 and 454 F/g at the scan rates of 10, 20, 30, 40, 50 and 60 mV/ s, respectively
Further investigate the electrochemical performance of a capacitive electrode can be observed by electrochemical impedance spectroscopy (EIS) analyzed using a Nyquist plot. EIS measurement is a useful tool for evaluating the kinetics Ni/ZnCo2 O4 electrode. The electrochemical performances are highly related to the interfacial charge-transfer process and diffusion [38, 40, 43, 44]. Figure 9 (a) shows Nyquist plot of the capacitive electrode of the Nickel foam modified with ZnCo2 O4 nanomaterial at an applied potential of 0.25V (vs. Ag/AgCl). The Nyquist plot reveals a small semicircle in high frequency range followed by a straight line in low frequency range, indicating that the electrochemical process on the surface electrode is kinetically and diffusion controlled [37, 38, 40, 44-46].The charge transfer resistance is a measure of the resistance associated with the electron transfer process and is inversely related to the exchange current density [37, 38, 40, 44-46]. A low Rct indicates a facile interfacial electron transfer process and hence a higher specific pseudo-Faradaic capacitance [37, 38, 40, 44-46]. The Nyquist analysis can be fitted by an equivalent circuit shown in the upper right inset of Figure 9 (a), where the intercept on the Z real axis in the high-frequency region corresponds to the ohmic resistance (Rs), representing the resistance of the electrolyte and electrode material. The high frequency semicircles can be ascribed to the surface film resistance (Rf). The semicircle indicates the charge-transfer resistance (Rct), relating to charge transfer through the electrode–electrolyte interface. The line in the low-frequency region represents the Warburg impedance (Zw), which reflects the solid-state diffusion of the electrode materials.
To further highlight the electrochemical behavior of Ni/ZnCo2O4 nanomaterial electrode, galvanostatic charge–discharge tests were performed at different current densities from 10 – 50 A/g. Figure 9 (b) represents the plot of voltage (V) vs time (s) at different current density. The galvanostatic charge/discharge curves reveal a distinctive profile near triangular and extremely symmetric shape, which indicates the Ni/ZnCo2 O4 electrode possesses outstanding electrochemical reversibility [37, 38, 40, 44-46]. Furthermore charge-discharge curves reveal that the current densities increase, the specific capacitance gradually decreases but no voltage drop during the changing of polarity [37, 38, 40, 44-46]. This is the indicating of the perfect capacitive behavior and good rate capability of the ZnCo2 O4 nanomaterial [37, 38, 40, 44-46]. The electrochemical results exhibit that the Spinel cubic ZnCo2 O4 nanomaterial on nickel electrode possesses a good electrochemical performance making the material to be highly suitable for electrochemical applications. The specific capacitance of the nanomaterial can be calculated by Eq. (4): (4) Herein, C is the specific capacitance (F/g), I is the current (A), ?t is the discharge time (s), m is the mass of active materials (g) and ?V is the potential window. The capacitance values of the electrode have been tabulated in Table 2. Conclusion In summary, spinel ZnCo2O4 nanomaterial has been successfully synthesized through a simple green method using Moringa oleifera extract. The fabricated electrode material was utilized for electrochemical capacitor. As electrode material for supercapacitors, the electrochemical performances were evaluated by cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic methods. The loaded spinel ZnCo2O4 nanomaterial on nickel foam electrode exhibited a highly specific capacitance and excellent electrochemical performance, and thus could be a promising electrode material candidate for supercapacitors. Owing to its simplicity of synthesis, low cost and excellent electrochemical performance, green method may hold abundant potential for assembly of other nanostructured electrode materials for high-performance hybrid supercapacitors.
This research was generously supported by Grant 98144 of the National Research Foundation of South Africa, iThemba LABS, the UNESCO-UNISA Africa Chair in Nanosciences and Nanotechnology, to whom we are all grateful.
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