RESUMO
The preparation of Co3O4 nanostructures by a green method has been rapidly increasing owing to its promising aspects, such as facileness, atom economy, low cost, scale-up synthesis, environmental friendliness, and minimal use of hazardous chemicals. In this study, we report on the synthesis of Co3O4 nanostructures using the milky sap of Calotropis procera (CP) by a low-temperature aqueous chemical growth method. The milky sap of CP-mediated Co3O4 nanostructures were investigated for oxygen evolution reactions (OERs) and supercapacitor applications. The structure and shape characterizations were done by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) techniques. The prepared Co3O4 nanostructures showed a heterogeneous morphology consisting of nanoparticles and large micro clusters. A typical cubic phase and a spinel structure of Co3O4 nanostructures were also observed. The OER result was obtained at a low overpotential of 250 mV at 10 mA cm-2 and a low Tafel slope of 53 mV dec-1. In addition, the durability of 45 hours was also found at 20 mA cm-2. The newly prepared Co3O4 nanostructures using the milky sap of CP were also used to demonstrate a high specific capacitance of 700 F g-1 at a current density of 0.8 A g-1 and a power density of 30 W h kg-1. The enhanced electrochemical performance of Co3O4 nanostructures prepared using the milky sap of CP could be attributed to the surface oxygen vacancies, a relatively high amount of Co2+, the reduction in the optical band gap and the fast charge transfer rate. These surface, structural, and optical properties were induced by reducing, capping, and stabilizing agents from the milky sap of CP. The obtained results of OERs and supercapacitor applications strongly recommend the use of the milky sap of CP for the synthesis of diverse efficient nanostructured materials in a specific application, particularly in energy conversion and storage devices.
RESUMO
Recently, the nanostructured nickel-cobalt bimetallic oxide (NiCo2O4) material with high electrochemical activity has received intensive attention. Beside this, the biomass assisted synthesis of NiCo2O4 is gaining popularity due to its advantageous features such as being low cost, simplicity, minimal use of toxic chemicals, and environment-friendly and ecofriendly nature. The electrochemical activity of spinel NiCo2O4 is associated with its mixed metal oxidation states. Therefore, much attention has been paid to the crystal quality, morphology and tunable surface chemistry of NiCo2O4 nanostructures. In this study, we have used citrus lemon juice consisting of a variety of chemical compounds having the properties of a stabilizing agent, capping agent and chelating agent. Moreover, the presence of several acidic chemical compounds in citrus lemon juice changed the pH of the growth solution and consequently we observed surface modified and structural changes that were found to be very effective for the development of energy conversion and energy storage systems. These naturally occurring compounds in citrus lemon juice played a dynamic role in transforming the nanorod morphology of NiCo2O4 into small and well-packed nanoparticles. Hence, the prepared NiCo2O4 nanostructures exhibited a new surface-oriented nanoparticle morphology, high concentration of defects on the surface (especially oxygen vacancies), sufficient ionic diffusion and reaction of electrolytic ions, enhanced electrical conductivity, and favorable reaction kinetics at the interface. The electrocatalytic properties of the NiCo2O4 nanostructures were studied in oxygen evolution reaction (OER) at a low overpotential of 250 mV for 10 mA cm-2, Tafel slope of 98 mV dec-1, and durability of 40 h. Moreover, an asymmetric supercapacitor was produced and the obtained results indicated a high specific capacitance of (Cs) of 1519.19 F g-1, and energy density of 33.08 W h kg-1 at 0.8 A g-1. The enhanced electrochemical performance could be attributed to the favorable structural changes, surface modification, and surface crystal facet exposure due to the use of citrus lemon juice. The proposed method of transformation of nanorod to nanoparticles could be used for the design of a new generation of efficient electrocatalyst materials for energy storage and conversion uses.
