Abstract
Activated carbon (AC) with a very high specific surface area of >3000 m2 g−1 and a number of course particles (average size: 75 µm) was pulverized by means of planetary ball milling under different conditions to find its greatest performances as the active material of an electric double-layer capacitor (EDLC) using a nonaqueous electrolyte. The variations in textural properties and particle morphology of the AC during the ball milling were investigated. The electrochemical performance (specific capacitance, rate and cyclic stabilities, and Ragone plot, both from gravimetric and volumetric viewpoints) was also evaluated for the ACs milled with different particle size distributions. A trade-off relation between the pulverization and the porosity maintenance of the AC was observed within the limited milling time. However, prolonged milling led to a degeneration of pores within the AC and a saturation of pulverization degree. The appropriate milling time provided the AC a high volumetric specific capacitance, as well as the greatest maintenance of both the gravimetric and volumetric specific capacitance. A high volumetric energy density of 6.6 Wh L−1 was attained at the high-power density of 1 kW L−1, which was a 35% increment compared with the nonmilled AC. The electrode densification (decreased interparticle gap) and the enhanced ion-transportation within the AC pores, which were attributed to the pulverization, were responsible for those excellent performances. It was also shown that excessive milling could degrade the EDLC performances because of the lowered micro- and meso-porosity and the excessive electrode densification to restrict the ion-transportation within the pores.
Highlights
Electric double-layer capacitors (EDLCs) are energy storage devices that have been applied to various energy storage sectors such as automobiles, power leveling for renewable energies, and portable electronic devices [1,2,3]
The ball milling process was optimized for Activated carbon (AC) with a very high surface area and course particles, as the active material of EDLC cells using a nonaqueous electrolyte, so as to attain the greatest electrochemical performance
The EDLC electrodes comprising the ACs milled under different conditions were evaluated in the galvanostatic charge–discharge (GCD) rate and cycle tests, providing their gravimetric and volumetric specific capacitances, Ragone plots, and cyclic charge–discharge stability
Summary
Electric double-layer capacitors (EDLCs) are energy storage devices that have been applied to various energy storage sectors such as automobiles, power leveling for renewable energies, and portable electronic devices [1,2,3]. The charge–discharge processes with high-power density are based on a principle of physical adsorption and desorption of electrolytic ions onto the electrode, so as to allow the formation and release of the electric double-layer. Activated carbons (ACs) are mainly employed as the active materials of EDLC electrodes due to their high specific surface area, excellent cost-performance, and large-scale productivity [4]. The specific surface area of the active materials is Batteries 2020, 6, 22; doi:10.3390/batteries6020022 www.mdpi.com/journal/batteries. Batteries 2020, 6, 22 closely related to the formation area of the electric double-layer. Porous carbons, whose specific surface areas are >3000 m2 g−1 , have been produced from various precursors and via novel chemical activation techniques [5,6,7,8,9]. ACs with a high specific surface area are very effective in increasing the gravimetric energy and power density of EDLC electrodes.
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