Green Synthesis of Metal Oxide Nanoparticles Using Mandarin Orange Peels (Citrus reticulata) for Phase and Electrochemical Study of Supercapacitor.

Nanoparticle-based materials are the class of significantly emerging materials in all aspects of the field for research in biomedical, pharmaceutical, sensors, and electrical applications which incorporate various branches of science. These nanoparticles can be synthesized using a variety of methods like laser ablation, spark discharge, sputtering, sol-gel, and many more but the products obtained using these methods cause an adverse effect on the environment due to the presence of hazardous, non-biodegradable, and toxic products in nanoparticles. However, green synthesis is one of the solutions to the above-mentioned issues. Therefore, many researchers are pursuing green synthesis methods nowadays where plant parts such as leaves, stems, roots, flowers, and fruit peels are currently utilized for the fabrication of nanoparticles (NPs) by their extracts. Hence it is considered an inexpensive and eco-friendly method of synthesis where bio-based NPs shows versatile characteristics. These bio-based NPs are widely used in energy storage applications. There are various devices that can store energy like fuel cells, batteries, and supercapacitors (SCs). Among all the energy storage devices (ESDs) SCs show extremely vibrant properties such as fast charge-discharge, high specific capacitance, and cyclic stability. This work includes a facile synthesis route of Cobalt Oxide nanoparticles (CONPs) using Mandarin orange peels. An aqueous extract of orange peel was considered as a precursor which performs a biological reduction mechanism for the synthesis of CONPs from Cobalt nitrate hexahydrate Co(NO₃)₂.6H₂O. Additionally, CONPs are also synthesized by using sodium hydroxide (NaOH) as a precursor for the comparative phase study which has been characterized by X-ray diffraction (XRD) and Fourier Transform infrared spectroscopy (FTIR). Furthermore, the obtained NPs are sulfurized and phosphorized to study the effect of morphology and crystalline structure on electrochemical activity. The pseudo-capacitor traits were analyzed by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), electrochemical impedance spectroscopy(EIS) in 3M KOH, and cyclic stability for over 5000 cycles. The noticeable specific capacitance showed by phosphorized cobalt oxide (P-Co₃O₄) is 244 F/g at a discharge current density of 1 A/g. Furthermore, the prepared CONPs retain a capacity of more than 82% after 5000 cycles.