Biomass Based Carbon Composites As Negative Electrodes for Asymmetric Supercapacitors

Abstract

Nowadays, the role of electrochemical storage devices such as supercapacitors and batteries are very prominent in our modern society since they are used in various applications including for the hybrid electric vehicles. Compared to batteries, supercapacitors have some advantages such as longer cycle life,higher charge and discharge rate and higher power density. However, most of the supercapacitor technology still relies on the symmetric electric double layer capacitors (EDLCs) which are known as symmetric supercapacitors. Based on the formation of electric double layer layer and high surface electrodes,this type of capacitors demonstrates a high power density but it possesses a low energy density due to the low specific capacitance and voltage window. Hence, it is quite urgent to develop new type of supercapacitors which possess also a high energy density as well as high power density and stable cycle profiles. Asymmetric supercapacitors, consisting of two different electrodes (electric double layer and redox reaction based electrodes), can be regarded as alternative system for the next generation of supercapacitors. For the positive electrodes of asymmetric supercapacitors, most of the metal oxides and hydroxides are commonly investigated. such as NiO. Various carbon materials are commonly used as negative electrodes for supercapacitors such as activated carbons, graphenes, carbon nanotubes (CNT), carbon nanosheets, electrospun carbon , reduced graphene oxides and CNT composites. Among all of them, activated carbons are categorized as preferable negative electrodes for supercapacitors due to high surface area and their availability. Activated carbons used as negative electrodes in supercapacitors can be made from various sources of biomass such as salacca peel. In this present work, nano carbon composites were prepared by two sequential methods, Initially, the activated carbons were synthesized by KOH activation method. It was then followed by the deposition of nano carbons on the surface of activated carbon using nebulized spray pyrolysis. The waste cooking oil and ferrocene were used as carbon precursor and metal catalyst, respectively. Nebulized spray pyrolysis has been applied for the synthesis of carbon nanospheres derived from Kerosene and Turpentine oil. The carbon composite products were then employed as negative electrodes coupled with NiO as positive electrodes and KOH as electrolytes. The nano carbon composites were then characterized by scanning electron microscope (SEM), nitrogen adsorption-desorption, x-ray diffraction (XRD) and Raman spectroscopy. SEM observations showed that mixtures of nano carbons consist of carbon nanotubes and carbon nanospheres were formed on the surface of activated carbons. The carbon composites were then tested as negative electrodes for asymmetric supercapacitors coupled with nickel oxides (NiO) as positive electrodes and KOH as electrolyte. The electrochemical characteristics were studied by cyclic voltammetry (CV), galvanostatic charge discharge and electrochemical impedance spectroscopy. The electrochemical performance of nano carbon composites was dependent on the preparation conditions in the nebulized spray pyrolysis method such as catalyst concentration and deposition temperature. The specific capacitance of nano carbon composites can achieve 31.02 F/g from the CV measurement at scan rate of 2 mV/s.