**Author details**

*Science, Technology and Advanced Application of Supercapacitors*

these dissolved into the electrolyte.

**6. Conclusion**

and barium. The electrolyte commonly used for EDLCs is tetraethylammonium tetrafluoroborate [(C2H5)4NBF4/C4H6O3], which does not contain the elements above. However, activated carbon, which is the polarizable electrode material, is over 90% carbon, and part of carbon consists of oxygen and hydrogen compounds. Carbon also contains, as components characteristic to the raw material, sodium, silicon, potassium, calcium, iron, etc., as ash content. Therefore, it is possible that

Furthermore, the possibility that these components are included as additives to improve the EDLC performance can also be considered. It is clear from the analytical results that whereas almost none of the dissolved element concentrations showed any variance between before and after the test at any holding voltage, only the silicon concentration decreased in relation to an increase in the holding voltage. A decreased concentration in the electrolyte signifies deposits onto the electrode surface. It is surmised that tetravalent silicon, similar to carbon (which is the main component of activated carbon), deposits onto the electrode surface and is a primary factor in the deterioration that increases the internal resistance of the EDLC.

For this chapter, we performed a voltage hold test, which is an accelerated deterioration test, on a commercial cylindrical electric double-layer capacitor (EDLC) by applying overvoltage. We confirmed behaviors of deterioration induced by the application of overvoltage. Furthermore, we disassembled the EDLCs after testing and conducted chemical analyses on the polarizable electrodes and the electrolyte, which are the main components of the EDLCs. This resulted in the three findings:

1.It was found that the internal resistance tended to increase with a rise in the holding voltage applied in the voltage hold test. On the other hand, the capacitance was not affected by the holding voltage and did not change significantly. This also revealed that some factors leading to an increase in internal resistance

2.It was found for polarizable electrodes that as the holding voltage rises, the polarizable electrodes react at the solid-fluid interface with the fluorine in the electrolyte, and fluorine compounds are generated up to a depth of 10–13 nm from the electrode surface in the depth direction. It is surmised that this fluorine compound layer closes the micropores, reduces the surface area, reduces the EDLC capacitance, and is a deteriorating factor that increases the internal resistance.

3.It was found that as the holding voltage increases, the silicon concentration in the electrolyte decreases. It is believed that as the electrochemical reaction progresses, deposited silicon becomes a deteriorating factor and, by hampering the charge transfer, leads to an increase in the internal resistance of the EDLC.

The author thanks Mr. Mikio Taguchi and Dr. Hiromu Yoshitama, the graduate students from the Department of Electrical and Electronic Engineering at the Graduate School of Engineering of the University of Miyazaki, for their help with

also increase with a rise in the holding voltage.

**68**

**Acknowledgements**

conducting this study.

Daisuke Tashima Fukuoka Institute of Technology, Fukuoka, Japan

\*Address all correspondence to: tashima@fit.ac.jp

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
