**1. Introduction**

In contrast to chemical batteries, electric double-layer capacitors (EDLCs) store and release electrical energy by utilizing the physical adsorption and desorption of an electric charge, but without utilizing any chemical reactions. Compared to chemical batteries, EDLCs have superior responsiveness, and since they allow for rapid high-voltage charging and discharging, they are high-performance device, last through a high number of charging/discharging cycles, and theoretically do not deteriorate. Owing to these advantages, EDLCs have recently been utilized as auxiliary power supplies in hybrid electric vehicles (HEVs), which combine an internal combustion engine and an electric motor with the aim of being maintenance free. However, performance degradation has recently been confirmed in commercially used EDLCs owing to overvoltage and high temperatures, including a decrease in capacitance and an increase in internal resistance. To confirm these phenomena and to predict degradation mechanism, chemical and electrochemical aging test were performed using original electrode under acetonitrile (AN)-based electrolyte, propylene carbonate (PC)-based electrolyte [1–7], and ionic liquid as electrolyte [8]. It was also reported that aging model was calculated and aging diagnosis was built up using manufactured EDLCs under overvoltage and high-temperature condition [9–20]. For this chapter, we performed a voltage hold test, which is a type of accelerated degradation test for EDLCs using the application of overvoltage, on

cylindrical EDLCs, which are used in actual applications [21]. We confirmed the degradation behaviors caused by the application of overvoltage by measuring the capacitance and internal resistance before and after the tests. Moreover, we disassembled the EDLCs once testing was complete and ran a variety of analyses on the polarizable electrodes and the electrolytes that form its constituent parts. By comparing and studying the analytical results and the deterioration behaviors, we studied the deterioration mechanism of EDLCs.
