**3.2 Measuring results of capacitance and internal resistance**

**Figure 1** shows the variance in the capacitance and internal resistance before and after the voltage hold test for each holding voltage. The internal resistance increased most for a holding voltage of 3.5 V, with a rise of approximately 30% compared to

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the internal resistance [5–7, 12].

surfaces.

**Figure 1.**

*Deterioration Factors of Electric Double-Layer Capacitors Obtained from Voltage Hold Test*

the value prior to the voltage hold test. Regardless of the holding voltage, the capacitance did not vary significantly from that before the test. Based on this, it was found that the rise in the holding voltage during the voltage hold test greatly affected the

For qualitative and state analyses of the polarizable electrodes, we used an X-ray photoelectron spectroscopy (XPS) analysis device [electron spectroscopy for chemical analysis (ESCA-3300)] by Shimadzu Corporation. A scanning electron microscope (S-5500) by Hitachi Ltd. was used to observe the polarizable electrode

**Figure 2(a)** and **(b)** shows scanning electron microscopy (SEM) images for the polarizable electrode surfaces during the pretest. Granular electrically conducting material can be confirmed through the images. Furthermore, it was found that the polarizable electrodes consist of activated carbon and electrically conducting material, and it was confirmed that there is no variance between the positive and negative electrodes. **Figure 2(c)**–**(l)** is SEM images for the posttest polarizable electrode surfaces for each holding voltage. It is clear from **Figure 2(a)** and **(d)** that when comparing the pretest polarizable electrode surfaces with those after testing at a holding voltage of 2.8 V, there is no variance in the activated carbon and electrically conducting materials. However, based on **Figure 2(e)**–**(l)**, for polarizable electrode surfaces after tests at holding voltages of 2.9, 3.0, 3.2, and 3.5 V, both an increase in the holding voltage and a deformation of the surfaces of the positive and negative electrodes can be confirmed. Surface irregularities can be confirmed for the positive electrodes, as well as the generation of a deposit layer on the surface. Moreover, the generation of granules smaller than the electrically conducting material can be confirmed on the negative electrodes. Accordingly, it is believed that deposits produced through a reaction of some sort inside the EDLC cell cause an increase in

factors that cause an increase in the internal resistance.

*Pre/postvoltage hold test properties for each applied voltage.*

**4. Chemical analysis of polarizable electrodes**

**4.1 Polarizable electrode surface observation**

*DOI: http://dx.doi.org/10.5772/intechopen.79260*

*Deterioration Factors of Electric Double-Layer Capacitors Obtained from Voltage Hold Test DOI: http://dx.doi.org/10.5772/intechopen.79260*

#### **Figure 1.**

*Science, Technology and Advanced Application of Supercapacitors*

studied the deterioration mechanism of EDLCs.

**3. Measuring capacitance and internal resistance**

calculated based on the voltage drop.

*Experimental conditions and sample properties.*

**3.1 Measuring method of capacitance and internal resistance**

**3.2 Measuring results of capacitance and internal resistance**

Before and after the voltage hold test, charging and discharging were performed at a constant current of 6 A and a voltage of 2.5 V using a stabilized DC power supply (PAN60-6A) and an electronic load device (PLZ603WH) by Kikusui Electronics Corp. Using the energy conversion method, the capacitance was calculated based on the obtained charge and discharge waveforms, and the internal resistance was

**Figure 1** shows the variance in the capacitance and internal resistance before and after the voltage hold test for each holding voltage. The internal resistance increased most for a holding voltage of 3.5 V, with a rise of approximately 30% compared to

**2. Experimental method**

constant temperature of 25°C.

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

As shown in **Table 1**, voltage hold tests were conducted by applying overvoltages of 2.8, 2.9, 3.0, 3.2, and 3.5 V continuously for 1 week to commercially used cylindrical EDLCs with a rated voltage of 2.5 V. Using a Sanyo Electric Co., Ltd., incubator (MIR-254) and a charging/discharging tester (PS-97010) by PowerSystem Co., Ltd., three accelerated deterioration samples were produced for each voltage at a

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**Table 1.**

*Pre/postvoltage hold test properties for each applied voltage.*

the value prior to the voltage hold test. Regardless of the holding voltage, the capacitance did not vary significantly from that before the test. Based on this, it was found that the rise in the holding voltage during the voltage hold test greatly affected the factors that cause an increase in the internal resistance.
