(Invited) Development of Hybrid Capacitor with Mesopores-Rich Activated Carbon Positive Electrode and Aluminum Metal Negative Electrode
Authors: Chiku Masanobu , Uchida Shota , Higuchi Eiji , Inoue Hiroshi 
Published: December 2024

A hybrid capacitor (HC) is an energy storage device using a battery-type electrode and an activated carbon electrode to realize high energy density, high power density and long cycle life. The current commercialized HC is the lithium-ion capacitor, but it has safety and cost issues. Therefore, we focused on aluminum electrode, which has a large theoretical capacity, high safety, and low cost, and aimed to develop energy storage devices with high energy density, high power density, and long cycle life, as well as high safety and low cost.
In this study, We fabricated electrical double-layer capacitors (EDLCs) using two types of activated carbon (AC) electrodes and various electrolytes to determine the best combination of aluminum salt, organic solvents, and AC electrodes suitable for HCs. Based on the electrochemical performances of EDLCs, an HC consisting of an AC electrode, an aluminum metal electrode and an optimal electrolyte was fabricated and its electrochemical properties were evaluated.
Mesopore-rich and micropore-rich AC electrodes were prepared, and EDLCs using these ACs and six types of non-aqueous electrolytes (aluminum chloride (AlCl3)-dimethylsulfone (DMSO2), AlCl3-dipropylsulfone (DPSO2)-toluene (TOL), and aluminum bis(trifluoromethanesulfonyl)imide (Al(TFSI)3)-glyme, Al(TFSI)3-diglyme, Al(TFSI)3-urea-N-methylacetamide (NMA), Al(TFSI)3-1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMIMTFSI)) were fabricated. Then the charge/discharge performances of EDLCs were evaluated. After that, with an optimal electrolyte, an AC positive electrode and an aluminum metal plate negative electrode were used for an HC and charge/discharge measurements were performed.
The electrolyte containing AlCl3 exhibited higher double-layer capacitance than the Al(TFSI)3 because Cl- has a smaller ion size than TFSI-, which increases the mobility in the pores of ACs. AlCl3-DMSO2 exhibited higher double-layer capacitance than AlCl3-DPSO2-TOL, suggesting that smaller solvent molecules are suitable for increasing double-layer capacitance because of their high mobility in the pores. Based on these results, AlCl3-DMSO2 was detemined to be the best electrolyte in this study.
The effect of the pore size of AC on double-layer capacitance was also investigated. Mesopores-rich AC showed higher capacitance than micropores-rich AC. This may be due to the large solvated ion size in the electrolyte solution, which requires a large pore size for internal diffusion.
HC with AlCl3-DMSO2 electrolyte and aluminum metal plate and mesopore-rich AC electrodes was fabricated and used for charge-discharge tests (Fig. 1). As a result, the capacitance of the HC was 61.77 F g-1 in the charge-discharge test at 0.1 A g-1, which was about twice the capacitance of the EDLC with AlCl3-DMSO2 electrolyte and mesopore-rich AC electrode. Furthermore, we performed electrochemical measurements at 100 °C and found the possibility of an increase in charge/discharge capacity.

Figure 1