Capacitors are energy storage devices—consisting of two electrodes and an electrolyte—that are capable of rapid charging and discharging because of charge adsorption and desorption properties at the electrode-electrolyte interface. Because capacitors’ energy storage does not involve chemical reactions, their storage capacity is lower than that of lithium-ion batteries, but they are useful for power leveling for renewable energy that requires repeated charging at high currents, regenerative braking energy for trains and electric or hybrid cars, as well as instantaneous voltage drop compensation devices that prevent equipment failure due to lightning strikes. They are also expected to be used to store energy for wearable devices in the near future.
Most capacitors use a liquid electrolyte with a low boiling point, which can only be used at temperatures below 80℃. Ceramic capacitors that use solid inorganic materials as a dielectric can be used at temperatures above 80℃, but their storage capacity is much lower than liquid electrolyte capacitors, which limits their use to electronic circuits.
To increase the energy storage of capacitors, it is necessary to have a large contact area at the interface between the electrode and the electrolyte. Making a large contact area is difficult using solid electrolytes; so, the creation of a capacitor with high storage capacity that can also operate at high temperatures has been desired for a long time.
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