Development of Novel Carbon Electrode for Electrochemical Energy Storage. Nano-sized Carbon and Classic Carbon Electrodes for Capacitors

Abstract

Electrochemical capacitors are energy storage devices characterized by high power and long cycle life for charging/discharging. Carbon materials play a very key role as electrode active materials for the electrochemical capacitors. This comprehensive paper mainly covers the author's research regarding the carbon materials as electrodes for the electric double layer capacitor used as an electrochemical capacitor. The importance of focusing on the pore structure, the surface chemistry, and the three-dimensional structure of the electrodes is stated from the viewpoint of improving the energy density and reliability, and the characteristic carbon materials that have a high capacitance even with a low microporosity are also reported. These research results indicate the potential of nano-sized carbons and classic carbons, such as activated carbon and graphite-related materials, to further develop the technologies of the capacitor electrodes.

Extraordinary Capacitance and Stability of Carbon Electrode for Electrochemical Capacitors

“…However, the degradation of the binder or the deposition of decomposition products between the active material particles under aggressive operating conditions (e.g., overcharging) destroys the electric network in the electrode and increases the internal resistance, resulting in an actual loss of the capacitor performance. 1,40 Such an electric network degradation, as well as the problem of micropore blockage caused by decomposition products, hinders the improvement of the withstand voltage of the EDLCs and makes it difficult to improve the energy density and the further reliability of the capacitor.…”

“…Based on this problem, the group of the author (S.S.), in collaboration with AION Co. Ltd., have developed an activated carbon monolith without particle boundaries, i.e., a "seamless activated carbon electrode". 1,2,40,41 A schematic illustration of the seamless activated carbon electrode is shown in Fig. 3b.…”

“…Nanoporous carbons, such as activated carbon, are key materials for use in electrochemical capacitors, especially in electric double layer capacitors (EDLCs) and lithium ion capacitors (LICs). 1,2 The energy (E) stored in these carbon-based electrochemical capacitors is proportional to the capacitance (C) of the capacitor and the square of the maximum charging voltage (V ) as follows. [1][2][3]…”

“…1,2 The energy (E) stored in these carbon-based electrochemical capacitors is proportional to the capacitance (C) of the capacitor and the square of the maximum charging voltage (V ) as follows. [1][2][3]…”

“…Therefore, the material design of the porous carbon electrode should also be aimed at improving the maximum voltage charging voltages for the capacitors. 1,8,9 Charging with a voltage higher than the maximum charging voltage causes the Coulombic efficiency to decrease and electrochemical decomposition (Faradaic process) to occur, resulting in the destruction of the electrodes and the degradation of the capacitor. The degradation mechanism of the capacitor by high-voltage charging is complicated and difficult to understand, so it has not yet been well clarified despite of a lot of efforts.…”