Green synthesis of cobalt (II, III) oxide nanoparticles using Moringa Oleifera natural extract as high electrochemical electrode for supercapacitors N. Matinise, N. Mayedwa, X. G. Fuku, N. Mongwaketsi, and M. Maaza
Citation: AIP Conference Proceedings 1962, 040005 (2018); doi: 10.1063/1.5035543 View online: https://doi.org/10.1063/1.5035543 View Table of Contents: http://aip.scitation.org/toc/apc/1962/1 Published by the American Institute of Physics
Green Synthesis of Cobalt (II, III) Oxide Nanoparticles using Moringa Oleifera Natural Extract as High Electrochemical Electrode for Supercapacitors N. Matinise 1,2, a), N. Mayedwa 1,2, X.G. Fuku 1,2, N. Mongwaketsi1, M. Maaza 1,2 1 2
Nanosciences African Network (NANOAFNET), iThemba LABS-National Research Foundation, 1 Old Faure road, Somerset West 7129, PO
Box 722, Western Cape, South Africa UNESCO-UNISA Africa Chair in Nanosciences-Nanotechnology, College of Graduate Studies, University of South Africa, Muckleneuk Ridge, PO Box 392, Pretoria, South Africa a)
Corresponding author:
[email protected]
Abstract. The research work involved the development of a better, inexpensive, reliable, easily and accurate way for the fabrication of Cobalt (II, III) oxide (Co3O4) nanoparticles through a green synthetic method using Moringa Oleifera extract. The electrochemical activity, crystalline structure, morphology, isothermal behaviour and optical properties of Co3O4 nanoparticles were studied using various characterization techniques. The X-ray diffraction (XRD) and Energy Dispersive X-ray Spectroscopy (EDS) analysis confirmed the formation of Co3O4 nanoparticles. The pseudo-capacitor behaviour of spinel Co3O4 nanoparticles on Nickel foam electrode was investigated by cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS) in 3M KOH solution. The CV curve revealed a pairs of redox peaks, indicating the pseudo-capacitive characteristics of the Ni/Co3O4 electrode. EIS results showed a small semicircle and Warburg impedance, indicating that the electrochemical process on the surface electrode is kinetically and diffusion controlled. The charge-discharge results indicating that the specific capacitance Ni/Co3O4 electrode is approximately 1060 F/g at a discharge current density of at 2 A/g.
INTRODUCTION Electrochemical capacitors (supercapacitors) as an innovative class of electric energy storage device are attracting great attention because of their superior performance characteristics, including excellent environmental safety, high power density, short charging time, long cycle life time, high energy density, superior reversibility and fast charge – discharge [1-9]. Up to now, transition metal oxides, conducting polymers and carbon materials have been recognized as most promising materials for supercapacitors due to their excellent redox reversibility, high electrochemical performances and high capacity/capacitance. Among the different types of metal oxides, CoxOy, NiO, MnO2 and RuO2, have been extensively studied due to variable oxidation states of metal ions which facilitate redox transitions and higher charge storage [10-13]. However RuO2 exhibits a high capacitance and the unaffordable cost delays its commercial application. Therefore, considerable energy has been paid on designing alternative electrode materials for supercapacitors. Cobalt oxides receive more intense attention due to their environmentally friendly nature, low cost, good corrosion stability and high theoretical specific capacitance [14-19]. Recently, several physical and chemical based methodologies have been adopted to synthesize the various nanostructures of Co3O4 and thus enhance its electrochemical properties. This includes precipitation, sonochemical methods, sol-gel
Proceedings of the 28th World Conference of the International Nuclear Target Development Society (INTDS2016) AIP Conf. Proc. 1962, 040005-1–040005-8; https://doi.org/10.1063/1.5035543 Published by AIP Publishing. 978-0-7354-1667-3/$30.00
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combustion, thermal decomposition, chemical vapor deposition, wet polymerization, laser ablation, solvothermal methods, micro-emulsion and hydrothermal methods [2, 14-21]. In addition to these standard synthesis methodologies, green chemistry routes are on the rise due to their various advantages including cost effectiveness, no requirement of additional chemicals, reliability and the fact that is a very easy, environmental friendly method with a minimum of waste generation [22-26]. Herein we report a facile biosynthesis of spinel Co 3O4 nanoparticles as an electrode material for supercapacitors. The Co 3O4 nanoparticles were synthesized via green chemistry using the Moringa Oleifera’s natural extract as an effective chelating agent. The mechanism of Co 3O4 nanoparticles formation via the reaction of the cobalt chloride and phyotochemical bioactive compounds of the natural extract is presented. To the best of our knowledge, a green chemistry route to synthesize an asymmetric supercapacitor device using Co3O4 nanoparticles has not been published before. Their optical, morphological, structural activities and high electrochemical performance as electrode materials for supercapacitors are presented
Experimental details Cobalt chloride hexahydrate was purchased from Sigma Aldrich and Moringa oleifera leaves were from Burkina Faso (West Africa). Moringa Olefeira contains several phytochemical compounds in its natural extract. 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 temperature and filtered through a nylon mesh, followed by a Millipore filter. The filtered Moringa Olefeira extract was stored in a refrigerator at 4 ⁰C for further studies. Synthesis of spinel Co3O4 nanoparticles: 50 ml of Moringa Oleifera extract was used to dissolve 5 g of Cobalt (II) chloride hexahydrate under magnetic stirring for 1 h. The solution was covered with an opaque foil to avoid any photo-induced phenomenon and kept at room temperature for 18 h. 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 500 ⁰C for 2 h. Characterization of Co3O4 nanoparticles: 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 X-Max solid state silicon drift detector (Oxford instrument) operating at 20 keV. X-ray powder diffraction (X-Ray diffraction Model Bruker AXS D8 advance) with radiation Cuk =1.5406 Å was performed to obtain the crystalline structure. Fourier transforminfrared (FT-IR) absorption spectrometer (Shimadzu 8400s spectrophotometer) was utilized in the range of 3004000 cm-1 to verify the chemical bonding. Electrochemical properties: The electrochemical analysis of the Co3O4 nanoparticles (NPs) electrode was performed by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) 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, in which nickel foam was applied as working electrode, platinum wire as counter electrode and Ag/AgCl as reference electrodes with a 3 M NaCl salt bridge solution. Preparation of Ni/ Co2O4 electrode: The amount of Co3O4 nanoparticles were 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. The potential window for cyclic voltammetry (CV) measurements varied from - 0.6 to 0.8 V at a scan rate of 50 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.
RESULTS AND DISCUSSION Fig. 1 (A) represents HRTEM images of the biosynthesized Co 3O4 NPs as prepared (upper picture) and annealed at 500 ⁰C for 2 h (lower picture)). For as prepared sample image, one can observe a series of small particles with a size of approximately2 nm. The annealed sample consists of highly crystalline cubic Co 3O4 nanoparticles with a particle size ranging from 20 to 50 nm. The corresponding selected- area electron diffraction (SAED) patterns of the spinal cubic Co3O4 (as prepared and 500 ⁰C) shown in Fig. 1 (B) reveal an amorphous and crystalline structure, respectively. More accurately, the annealed sample exhibits bright spot diffraction rings demonstrating the polycrystalline nature of the spinel cubic Co3O4 phase. EDX analysis was used to examine the composition of Co 3O4
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(R.T. and 500 ⁰C) nanoparticles and is display in Fig. 1 (C).The products were identified as the elements C, O, Co and Cu and in addition K and Cl peaks are visible at mid energy channels for the as prepared sample. The K was likely to originate from the bio-compounds whilst the Cl was sourced from cobalt salt precursor. It is worth to note that these contaminants have disappeared after annealing at 500 ⁰C, confirming the formation of pure Co3O4. Likewise, the C and O could partially originate from the natural extract too. The other contaminant peaks originate from the C coated Cu grid. No unknown elements are visible indicating that the pure phase of spinal Co3O4 nanoparticles has been successfully prepared using Moringa Olefeira extract. The results were in agreement with the XRD analysis that we obtained pure phase Co3O4 nanoparticles.
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FIGURE 1. (A) HRTEM images, (B) SAED and (C) EDS patterns of the spinel cubic Co3O4 nanoparticles as prepared and annealed samples.
The XRD patterns of the highly ordered cubic Co3O4 NPs obtained before and after the calcination (500 ⁰C) process are shown in Fig. 2. It proves the purity in crystal structure and phase formation of the biosynthesized product. The XRD pattern of the annealed sample (500 ⁰C) displays well-resolved Bragg peaks, which are in good agreement with the cubic Co3O4 spinel structure. No contaminants peaks were detected, indicating that the structure is pure crystalline phase of spinel cubic Co3O4. The diffraction values at 2 Θ are corresponding to (220), (311), (222), (400), (422), (511) and (440) planes of cubic Co3O4. The (311) diffraction peak has the highest intensity, indicating the oriented growth of the sample in the (311) direction. The XRD pattern of the non-annealed Co3O4 sample exhibits only three known diffraction peaks which correspond to 220, 222 and 422 and also evident some impurity diffraction peaks, indicating the structure is impure and amorphous at low temperature. The lattice parameters a of the annealed sample (500 ⁰C) was calculated to be 8.0883 nm by using reticular plane distance’s relation (Eq. 1)
1 h2 k 2 l 2 d 2 a2
(1)
Where d = crystalline size, a = lattice constant and hkl are Miller indices. The average crystalline size of the samples were estimated by Debye-Scherrer approximation (Eq. 2)
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d
K cos
(2)
Where d is the crystalline size, 𝜃 - Bragg diffraction angle, 𝐾 - Plank’s constant, 𝜆- wavelength (1.54 nm), and 𝛽 - width of the XRD peak at half maximum height and found to be 38 nm. The average crystalline size and lattice constant (a) were found to be 38.3514 nm and 8.0883 nm respectively.
Figure 2. XRD patterns of the spinel cubic Co3O4 nanoparticles.
The electrochemical performance and specific capacitance of the nickel loaded Co 3O4 (Ni/Co3O4) nanoparticles electrode were investigated by cyclic voltammetry (CV), electrochemical impedance (EIS) and galvanostatic chargedischarge (GCD) measurements using a three electrode system at room temperature. Fig 3(A) presents the cyclic voltammetry (CV) curves of the pure Nickel foam and Ni/Co 3O4 nanoparticles electrode conducted in the potential region of 0 – 0.8 V at a scan rate of 50 mV/s. The CV curves of Ni/Co 3O4 reveals two distinct redox couples, corresponding to the oxidation and reduction process of Co 3O4 nanoparticles and clearly revealing the pseudocapacitive characteristics derived from Faradaic reactions. Fig 3(B) shows Nyquist plot of the capacitive electrode of the Ni/Co3O4 nanoparticles at an applied potential of 0.32V (vs. Ag/AgCl). The Nyquist plot shows a small semicircle in the high frequency range followed by a straight line in the low frequency range, indicating that the electrochemical process on the surface electrode is kinetically and diffusion controlled. The electrochemical results indicate that the spinel cubic Ni/Co3O4 nanoparticles electrode shows a good electrochemical performance making the nanoparticles a promising candidate for high-performance supercapacitors. To further highlight the electrochemical behaviour of Ni/Co3O4 nanocomposites electrode, galvanostatic charge-discharge tests were performed at different current densities from 2 – 10 A/g, shown in Fig. 3c. The galvanostatic charge-discharge curves reveal a distinctive profile nearly triangular and extremely symmetric in shape, which indicates the Ni/Co3O4 electrode possesses outstanding electrochemical reversibility. Furthermore the charge-discharge curves reveal that with increasing the current densities, the specific capacitance gradually decreases without a voltage drop during the changing of polarity, indicating the perfect capacitive behaviour and good rate capability of the Co 3O4 nanoparticles. The specific capacitance of the nanoparticles can be calculated by Eq. 3 and tabulated in table1.
C
It mV
(3)
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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.
TABLE 1. Specific capacitances of Ni/Co3O4 electrode. Current density (A/g)
Specific capacitance (F/g)
2 3 4 5 10
1060 885 770 600 550
FIGURE 3. (a) Cyclic voltammetry curves; (b) Nyquist plot; (c) Galvanostatic charge–discharge curves at 50 mV/s (B) CV of Ni/Co3O4 electrode in 3 M KOH.
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Conclusion In summary, spinel Co3O4 nanoparticles have been successfully synthesized through a simple green method using Moringa oleifera extract. The electrochemical performances were evaluated by cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge discharge. The loaded spinel Co 3O4 nanoparticles on nickel foam electrode exhibited an 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, this green method may hold abundant potential for assembly of other nanostructured electrode materials for high-performance hybrid supercapacitors.
ACKNOWLEDGMENTS 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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