Green synthesis of Spinel ZnCo2O4 nanomaterial via Moringa Oleifera as high electrochemical electrode for supercapacitors

Research Article

Green synthesis of Spinel ZnCo2O4 nanomaterial via Moringa Oleifera as high electrochemical electrode for supercapacitors

Corresponding author: Dr. Nolubabalo Matinise, iThemba Laboratory for Accelerator Based Science, Old Faure road, P.O. Box 722, Sommerset West 7129, Western Cape-South Africa, Tel: 021 843 1165, Email:


Mixed transition metal oxides with a normal spinel structure are considered as the most promising electrodes for high-perfor- mance 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 ZnCo2O4 nanomaterials via a green synthetic way us- ing 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-capac- itive characteristics of the Ni/ZnCo3O4 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/ZnCo2O4 nanomaterial is a promising electrode material for super- capacitors.

Keywords: Biomaterial, Supercapacitors, Nanostructure, Moringa Oleifera, Green synthesis, Spinel ZnCo2O4, electrochemical


Supercapacitors or electrochemical capacitors are considered as a most promising electrical energy storage devices due to their superior performance characteristics, including long cy- cle life time, high power density, energy density, short charging time, excellent cycling stability, fast charge – discharge, excel- lent 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 dou- ble 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 mate- rials for EDLCs owing to their excellent stability, low cost, high specific area and good electrical conductivity. Pseudo-capaci- tors have received much attention as they can provide much

higher specific capacitance and energy densities than those of electrochemical double layer capacitors (EDLCs). Pseudo-ca- pacitors, 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 con- ducting polymers such as, RuO2, Mn3O4, NiO, Co3O4, 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

Preparation of Moringa Oleifera extract: 30 g of cleaned Moringa Oleifera dried leaves were immersed in 300 ml of boiled deionized water (DI-H2O) under magnetic stirring for 1h45 at 50 ⁰C. Then the mixture was cooled to room tempera- ture and filtered through a nylon mesh, followed by a Millipore filter. The filtered Moringa Olefeira extract was stored in a re- frigerator at 4 ⁰C for further studies.

diffusion rates and poor electron conductivities and slow ion

Synthesis of spinel Co O

nanoparticles: 50 ml of Moringa

diffusion rates transition metal oxides would limit their elec- 3 4

trochemical performances with high rate capabilities [12]. Therefore, it is required to develop an advanced supercapac- itor device which would improve the electrochemical per- formance, provide high capacity performance and excellent rate capability [17-19]. To date, binary metal oxides (such as NiCo2O4 and ZnCo2O4) with a normal spinel structure have fascinated greatly consideration due to their stronger electro-

Oleifera extract was used to dissolve 5 g of Zn(NO3)2 6H2O 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 pho- to-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 ZnCo O complex chemical activities, improve electronic conductivity and richer 2 4 redox reactions compared with the single-component metal oxides [14]. Among binary metal oxides, the compound Zn- Co2O4 has been recognized as promising electrode materials for supercapacitors due to its better electro-activity and supe- rior supercapacitive properties than those of single cobalt ox- ides (Co3O4) [19-22]. ZnCo2O4 nanomaterials have high redox and outstanding electric conductivity which make the material suitable for supercapacitor. The crystal structure of spinel Zn- Co2O4 nanomaterials have shown great potential in various field including lithium ion batteries, catalyst sensors and su- percapacitors due to its environmental friendly, low cost, easi- ly synthesized, high theoretical capacity, high surface area and better electro-activity material [17-22].

Recently, various methods to fabricate ZnCo2O4 nanomateri- als with several nanostructures i.e. nanoparticles, nanowires, nanorods, microspheres and nanotubes have been reported including numerous chemical, physical and biosynthetic meth- ods [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 meth- od [29-33].

In this work, we report a facile biosynthesis of spinel ZnCo2O4 via a green synthetic method using the Moringa oleifera nat- ural extract which act as both capping and reducing agents. Their optical, morphological, structural activities and high electrochemical performance as electrode materials for super- capacitors are presented.

was in the form of a suspension. The solution was dried in a standard oven at 100 ⁰C and was then washed several times with DI-H2O to remove the redundant materials of the extract.

Finally, the sample was annealed at different temperature (500

and 700 ⁰C) 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 distri- butions of the samples, High Resolution Transmission Elec- tron Microscopy (HRTEM) (Philips Technai TEM) operated at an accelerating voltage of 120 kV was used. The Energy Dis- persive 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 ad- vance) with radiation Cuk =1.5406 Å was performed to obtain the crystalline structure of the nanomaterials. Fourier trans- form-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 wave- length of 300 nm. Differential Scanning Calorimetry/Thermo Gravimetric technique (DSC/TGA) simultaneously over a tem- perature range of 50-600 C in ambient air at a heating of 10 C/ min was used for thermal analysis.

Electrochemical properties: The electrochemical analysis of

the ZnCo O electrode was performed by cyclic voltammetry,

2 4

Material and Methods

Zinc nitrate hexahydrate (Zn(NO3)2 6H2O) and Cobalt chloride hexahydrate were purchased from Sigma Aldrich, and Morin- ga oleifera leaves were from Burkina Faso (West Africa).

galvanostatic charge-discharge test and electrochemical im- pedance spectroscopy (EIS) conducted on Autolab Potentio- stat (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 ZnCo2O4 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/ZnCo2O4 for further studies. The potential win- dow for cyclic voltammetry (CV) measurements varied from 0 to 0.8 V with the various scan rates of 10 – 80 mV/s. All exper- imental 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.


Structure analysis:

Figure 1 presents the schematically biosynthetic method of the Spinel cubic ZnCo2O4 nanomaterial. The syn- thetic method includes the following steps: the raw materials react under the biosynthetic method using Morin- ga Olefeira extract which acted as both reducing and capping agent to foam Spinel cubic ZnCo2O4 nanomaterial. Then, the nanomaterial was calcined in air (500 and 700 ⁰C for 2 h) to obtain the pure final products.

Figure 2 presents a proposed mechanism towards the formation of Spinel cubic ZnCo2O4 in view of understanding the inter- action of both Zn(NO3)2 and CoCl2 metal precursors through biological compounds of the Moringa oleifera i.e. L-ascorbic acid, Chlorogenic acid and Myricetin based biological compounds. An altered chemical behavior of L- ascorbic acid, Zinc nitrate and Cobalt chloride, probable oxidation of biological compound i.e. L-ascorbic acid to L-dehydro ascorbic acid via free radical, followed by electrostatic attraction between free radical and cation the of precursors.

Then the calcined at different temperature (500 and 700 ⁰C for 2 h) to form pure Spinel cubic ZnCo2O4 nanomaterial.

Figure 3 reveals differential Scanning Calorimetry/Thermo Gravimetric technique (DSC/TGA) simultaneously spinel ZnCo2O4 nanomaterials. TGA profile reveals a continuously weight loss with 3 quasi sharp changes occurring at 110, 160, and 393 ⁰C followed by nearly a constant plateau. Hence, annealing above 400 C seems to guarantee the formation of stable ZnCo2O4. The weight loss (5%), observed between 50 ⁰C and 110 ⁰C is essentially correspond to the discharge of adsorbed water and alcohol. The weight loss of 10.3%, observed between 110 and 160 ⁰C is due to the elimination of the hydrated water. The last major weight loss (35 %), observed between 160 and 393⁰C, is the decomposition of organic materials [34, 35].

The DSC reveals 2 endothermic peaks centered at about 160 and 393 C. Based on similar TGA/DSC results reported in the literature the peak around 160 C is attributed to the loss of volatile surfactant molecules adsorbed on the surface of Zinc Cobalt oxide based complexes during synthesis [34]. The peak at about 392 C can be assigned to the formation of Zinc Cobalt oxide nanoparticles and decomposition of organic materials [34].

The XRD patterns of the highly ordered cubic ZnCo2O4 obtained after the calcination (500 and 700 ⁰C) process are shown in Figure 4.

It proves the purity in crystalline structure and phase formation of the biosynthesized product. XRD patterns display well-re- solved Bragg diffractions, which are in good agreement with pure planes of cubic ZnCo2O4 spinel structure (JCPDF: nº00 -023- 1390, with a lattice parameter of a = 8.09460 Å). There were no contaminants or residues peaks were detected, indicating that the structure is pure crystalline phase of Spinel cubic ZnCo2O4. The diffraction values at 2 Θ are corresponding to (220), (311), (222), (400), (422), (511) and (440) planes of cubic ZnCo2O4. Lattice parameters aexp of both samples (500 and 700 ⁰C) were calculated to be 8.0983 and 8.0856 respectively by using reticular plane distance’s relation (Eq. 1)


Where d = d-spacing, a = lattice constant and hkl are miller indices. The average crystalline size of the samples were estimated by Debye-Scherrer approximation (Eq. 2)


Where d is the crystalline size, θ – Bragg diffraction angle, К- Plank’s constant, λ- wavelength (1.54), and β- width of the XRD peak at half maximum height and found to be 21 and 26 nm respectively. The XRD analysis of the products (annealed at 500 ⁰C and 700 ⁰C) are listed in table 1.

The photoluminescence spectra of the ZnCo2O4 (annealed at 500 and 700 ⁰C) in the range of 400-550 nm is shown in Figure 5. ZnCo2O4 annealed at 500 ⁰C reveals a luminescence signal peaks at about 433 nm, 492 nm and 543 nm emission band. The band gap energies were calculated to be 2.86 eV, 2.52 and 2.28 eV respectively. ZnCo2O4 annealed at 700 ⁰C exhibits four emission peaks at about 418, 440, 450 and 490 nm, the band gap en- ergies gave 2.97 eV, 2.19 eV, 2.76 eV and 2.53 eV, respectively. In the two cases, the pure ZnC2O4 nanomaterials exhibit emissions which are due to ultraviolet near-band edge emission peak, blue and green emissions and the band gap energies (2.86 eV, 2.52 eV, etc) reveal their semi-conductive behavior [36].

Figure 5. Photoluminescence spectra of the ZnCo2O4 (annealed at 500 and 700 ⁰C)

Figure 6(a) represent HRTEM images of the biosynthesized ZnCo2O4 obtained at different temperatures (500 and 700 ⁰C). The biosynthesized ZnCo2O4 nanomaterials reveal certain degree of cubic shape with a particle size ranging from 25 to 50 nm. Noticeably, with the increase in temperature the particle sizes of nanomaterials are getting larger. Moreover, these nanomaterials exhibit good crys- tallinity and the inter-planar distance has been calculated to be 2.46 and 2.84 nm, which corresponded to the (111) and (220) planes of

spinel ZnCo2O4, respectively.

Figure 6. (a) HRTEM images (b) SAED (c) EDS patterns of ZnCo2O4 at 500 and 700 ⁰C

The corresponding selected- area electron diffraction (SAED) pattern of Spinel cubic ZnCo2O4 (annealed at 500 & 700 ⁰C) shown in Figure 6 (b) reveals a clear bright sport diffraction rings, suggesting the polycrystalline nature of the spinel cubic ZnCo2O4 phase. The rings correspond well with the XRD patterns and demonstrating that the crystal was a spinel phase. EDX analysis was used to exam- ine the composition of ZnCo2O4 nanomaterial and results display in Figure 6 (c). The final product of the biosynthesized ZnCo2O4 contains only the elements of Zn, Co and O, indicating the synthetic products have a moderately uniform distribution all over the sam- ples and elements Cu and C comes from carbon cupper grid (HRTM substrate). No unknown elements are visible; indicating the pure phase of Spinel ZnCo2O4 nanomateril has been successfully prepared using Moringa Olefeira extract. This was in agreement with the XRD results that we obtained pure phase ZnCo2O4 nanomaterial.

To identify the nature of the functional groups on their surfaces and chemical composition, FT-IR spectroscopy was employed. FTIR

spectra in Figure 7 represent Moringa extract and biosynthesized nanomaterials (R.T, annealed at 500 and 700 ⁰C). The Moringa Oleifera spectrum reveals IR absorption brands of the various bioactive compounds in different ranges of wavenumbers. A broad IR absorption bands located at 2909-36098 cm-1, can be allocated to O-H, NH2, H3CO, HO-C=O and C–H aromatic [37-40]. The

absorption band at 2063-2182 cm-1 corresponds to N=C=S stretching vibrations of thiamine [37-40]. The stretching band at around

942-1714 cm-1 owing to C=O, C=N, NH, C=C aromatic stretching vibrations. Stretching vibrations located at around 473-703 cm-1 represent C-H, C=C, N-H [41, 42]. In generally, metal oxides are characterized by intrinsic absorption bands below 1000 cm-1 (fin- gerprint region) arising from inter-atomic vibrations [41, 42]. The biosynthesized ZnCo2O4 nanomaterials spectra (R.T. and annealed

{500 and 700 ⁰C}) exhibits around 518- 321 cm-1 region, which is attributed to the stretching vibration mode of M-O-M for the tetra- hedral coordinated metal ions. As seen the non- anneal spectrum (R.T.) shows the stretching vibration mode around higher mid and lower wavelength O-H, C=O, C-H groups. These vibration modes are from the bioactive compound of Moringa which confirms the

binding nature of the sample to the bioactive compound to form a complex (Zn-Co2-O4/ bioactive compound).

As the temperature increases from 500 and 700 ⁰C, there is a decrease/ partial disappeared to the vibration modes of the extract. Therefore, is indicating that the annealed products of Spinel ZnCo2O4 nanomaterials are pure, relative to the non-annealed product.

Figure 7. FTIR spectra of Moringa Olefeira extract and ZnCo2O4 (R.T and annealed at 500 and 700 ⁰C)

Figure 8. (a) CV curves at 50 mV/s (b) CV of Ni/ZnCo2O4 nanomaterial electrode measured at various scan rates ranging from 10 – 60 mV/s in 3 M KOH

Electrochemical performance:

To test their potential utility in supercapacitor applications, we studied electrochemical performance of ZnCo2O4 nanomaterials. The electrochemical performance and specific capacitance of the nickel loaded ZnCo2O4 (Ni/ZnCo2O4) 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/ZnCo2O4 exhibits higher current response when compared with bare Nickel, which shows that ZnCo2O4 effectively promotes electron transfer between the electrolyte and the electrode surface [38, 40, 43, 44]. The CV curves of Ni/ ZnCo2O4 reveals two distinct redox couples, corresponding to the oxidation and reduction process of ZnCo2O4 nanomaterials, respec- tively [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 reac- tion 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 Co3O4 [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/ZnCo2O4 nanomaterial is a promising candidate for high-performance supercapacitors.

Figure 8b presents the cyclic voltammetry (CV) curves of Ni/ZnCo2O4 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 spe- cific capacitances of the nickel foam supported ZnCo2O4 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

Table 2: Specific capacitances of Ni/ZnCo2O4 electrode from CV and galvanostatic charge–discharge

Further investigate the electrochemical performance of a capacitive electrode can be observed by electrochemical impedance spec- troscopy (EIS) analyzed using a Nyquist plot. EIS measurement is a useful tool for evaluating the kinetics Ni/ZnCo2O4 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 ZnCo2O4 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 ex- change 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.

Figure 9. (a) Nyquist plot and (b) Galvanostatic charge–discharge curves at different current densities (1 – 3 A/g) in 3 M KOH

Figure 10. Biological compounds in Moringa Olefeira extract

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/ZnCo2O4 electrode possesses outstanding electrochemical reversibility [37, 38, 40, 44-46]. Furthermore charge-dis- charge 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 Zn- Co2O4 nanomaterial [37, 38, 40, 44-46]. The electrochemical results exhibit that the Spinel cubic ZnCo2O4 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):


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.


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 elec- trochemical 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 electrochem- ical 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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