Synthesis of Activated Carbon as Electrode Materials for Supercapacitor Application

Abstract

Abundance of agricultural waste has caused an environmental problem due to release of methane gas to the environment. These agricultural waste usually exist in porous structure has huge potential to be utilized in energy storage device. These porous materials could yield better efficiency and performance of the energy storage system. Empty fruit bunch (EFB) being one of the huge agricultural waste in Malaysia has potential to be used as one of the raw materials to produce AC for electrode materials in an energy storage device due to its low cost, high availability and porosity. However, EFB requires processing to convert it into AC via a two-step activation process to maximize its value and to enhance its electrochemical performance. In this research, a two-step activation was employed, in which EFB undergoes pyrolysis at 500 °C before being activated via chemical and physical activation methods under various conditions to produce AC. In addition, surface modification treatment was conducted using 65% nitric acid at room condition to improve the wettability and hydrophilicity of the AC. The produced AC samples were then characterized using weight loss analysis, Field Emission Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (FESEM-EDX), Raman Spectroscopy, Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Photoelectron Spectroscopy (XPS) to evaluate their physical characteristics. Meanwhile, cyclic voltammetry (CV), galvanostatic charge discharge (GCD) and electrochemical impedance spectroscopy (EIS) were performed to evaluate their electrochemical properties. From the weight loss analysis, it is shown that samples from physical activation showed higher weight loss during activation compared to samples from chemical activation. From FTIR and XPS analysis, it was shown that the surface modification treatment was feasible to add and increase the amount of the surface functional group on the AC sample. Raman spectroscopy indicated that samples from physical activation had higher defect structures compared to samples from chemical activation which is in line with the weight loss analysis. Meanwhile, the surface modification treatment does not give significance impact of the structural defect of the AC samples. The specific capacitance measured from CV analysis shows better performance for samples from physical activation which were in the range of 24 to 140 Fg-1 compared to samples from chemical activation that resulted in only 36 to 90 Fg-1. In comparison with the surface treated AC sample, the sample from physical activation method shows specific capacitance ranging from 42 to 183 Fg-1. Meanwhile the sample from chemical activation shows specific capacitance ranging from 30 to 64 Fg-1. It can be inferred that, the addition of surface functional group on the AC give an improvement on the specific capacitance of AC that were prepared with physical activation method. The addition of surface functional group needs to be done in a careful manner so that it will not cause an excessive presence which may lead to collapse of the AC structure and increase in the internal resistance of AC. In brief, AC derived from EFB has shown promising characteristic to be electrode materials for energy storage application particularly due its physical characteristic which contributed to its excellent electrochemical performance.