Particle size effect on supercapacitor performance made by coconut shell activated carbon

Abstract

Over the years, biomass-based activated carbon (AC) supercapacitor electrodes have gained interest among researchers because there are wide ranges of abundant biomass such as coconut shell that can easily convert into AC. Fabrication of the electrode using AC fine particle and small portion of coarse particle has been recommended in the past. It is found that the particle size could potentially affect the electrode’s physical and electrochemical properties. Nevertheless, information on the relationship between particle size and supercapacitor performance is very limited. The main objective of the research was to characterize a coconut shell-based AC as supercapacitor’s electrode in term of its physical and electrochemical properties at different particle size distributions. Particle size distributions of sieved AC powders that came from 75, 150, 180 and 300 µm mesh size were measured using laser diffraction method. Each AC electrode was fabricated with 88%wt AC powder, 6%wt carbon black and 6%wt polyvinyl difluoride. The electrodes were denoted as 75AC/+0AC, 75AC/+150AC, 75AC/+180AC and 75AC/+300AC. The 75AC/ was fabricated based on 90% AC powder with 75 µm particle size while the ‘+’ sign indicates the mixture of 10% coarse AC particle powder which comprise of either 150, 180 or 300 µm. Both AC powder and fabricated electrodes were characterized their physical properties in terms of surface area, pore size, micropore volume and morphology. Surface area and micropore volume were calculated using the Brunauer-Emmett-Teller (BET) and Barret-Joyner-Helenda (BJH) models’ respectively. The electrochemical properties of AC electrodes were analysed using cyclic voltammetry, galvanostatic charge discharge and electrochemical impedance spectroscopy. It was found that some of the AC pore structures were blocked by carbon black after the fabricating process which eventually led to major reduction in surface area. The addition of coarse particles causes microcrack on the electrode surface in all samples. However, it increases the surface area and specific capacitance of the electrode where both increments increase the energy density of the supercapacitor. As a comparison between 75AC/+0AC and 75AC/+150AC, the electrode BET surface area, BJH micropore volume and specific capacitance increased up to 20.00%, 20.12% and 22.74%, respectively. On the other hand, the addition of higher coarse particle size than +150AC has demonstrated a major drawback on the electrolyte decomposition. It can be concluded that the performance of supercapacitor can be improved further with the mixture of fine and coarse particles as opposed to single composition of fine powder alone. However, the coarse particle size should not be too big as it will affect on the electrolyte decomposition properties.