**5. Chemical analysis of electrolyte**

#### **5.1 Measuring moisture content of electrolyte**

It was shown that in samples subject to high voltage, the electrolyte reacts with the polarizable electrode at the solid-fluid interface, altering the atomic composition ratio of the polarizable electrode. Moreover, since the reaction at the solid-fluid interface accelerates with an increase in the applied voltage, it can be inferred that some sort of change is also occurring in the electrolyte. We, therefore, measured the

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these measurements.

**Figure 5.**

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

moisture content in the electrolyte taken from the EDLCs. A Karl Fischer Moisture Titrator (MKC-610) by Kyoto Electronics Manufacturing Co., Ltd., was used for

**Figure 9** shows the moisture content in the electrolyte taken from the EDLCs before the tests and after the tests at each applied voltage. A comparison of moisture content in the electrolyte before and after the test when 2.8 V is applied found an increase of approximately 14.4%. However, it was found that compared to the value before the test, the moisture content decreased after tests that applied 2.9, 3.0, 3.2, and 3.5 V. The lowest moisture content was seen in the electrolyte after the 2.9-V

**5.2 Moisture content of electrolyte: measurement results**

*Polarizable electrode after test with holding voltage of 2.9 V.*

*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 5.**

*Science, Technology and Advanced Application of Supercapacitors*

to increase. Furthermore, it was shown that no major difference was seen in the composition changes in the depth direction of the positive and negative electrodes.

It was shown that in samples subject to high voltage, the electrolyte reacts with the polarizable electrode at the solid-fluid interface, altering the atomic composition ratio of the polarizable electrode. Moreover, since the reaction at the solid-fluid interface accelerates with an increase in the applied voltage, it can be inferred that some sort of change is also occurring in the electrolyte. We, therefore, measured the

**5. Chemical analysis of electrolyte**

**5.1 Measuring moisture content of electrolyte**

*Polarizable electrode after test with holding voltage of 2.8 V.*

**60**

**Figure 4.**

*Polarizable electrode after test with holding voltage of 2.9 V.*

moisture content in the electrolyte taken from the EDLCs. A Karl Fischer Moisture Titrator (MKC-610) by Kyoto Electronics Manufacturing Co., Ltd., was used for these measurements.

#### **5.2 Moisture content of electrolyte: measurement results**

**Figure 9** shows the moisture content in the electrolyte taken from the EDLCs before the tests and after the tests at each applied voltage. A comparison of moisture content in the electrolyte before and after the test when 2.8 V is applied found an increase of approximately 14.4%. However, it was found that compared to the value before the test, the moisture content decreased after tests that applied 2.9, 3.0, 3.2, and 3.5 V. The lowest moisture content was seen in the electrolyte after the 2.9-V

**Figure 6.** *Polarizable electrode after test with holding voltage of 3.0 V.*

test, with a 16.9% decrease compared to that before the test. Based on these results, it is believed that there is no correlation between increasing the applied voltage and decreasing the moisture content, and that the moisture content included when the EDLCs were built is retained.

## **5.3 Measuring electrolyte element concentrations**

It was revealed that there was no correlation between an increase in the holding voltage in the voltage hold test and an increase or decrease in the moisture content shown in **Figure 9**. Accordingly, any chemical changes in the electrolyte owing to an increased holding voltage can be considered to be other than moisture related. The elution of electrode material into the electrolyte sometimes occurs during

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**Figure 7.**

process is worth noting.

*Polarizable electrode after test with holding voltage of 3.2 V.*

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

electrolysis and is also applied in electrolytic refining. Chemical reactions of the electrodes are not desirable in EDLCs, and a mere action through physical adsorption and desorption are considered ideal [14]. However, the behavior of components contained in ash that cannot be eliminated in the activated carbon generation

Owing to the difference in viscosity between the harvested electrolyte and the oilbased 23 element standard solution by Seishin Trading Co., Ltd., used in the calibration curve method, the interference must be suppressed. Moreover, since the volume required for analysis (15 ml) was not reached, this needed to be adjusted. To that end, microwave-assisted decomposition was performed using a microwave digestion device (SpeedWave MS3) by Actac Project Services Corporation, adding 8 ml of nitric acid to 0.1 ml of electrolyte. Since this decomposition treatment was performed in a closed

*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 7.**

*Science, Technology and Advanced Application of Supercapacitors*

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**Figure 6.**

*Polarizable electrode after test with holding voltage of 3.0 V.*

**5.3 Measuring electrolyte element concentrations**

EDLCs were built is retained.

test, with a 16.9% decrease compared to that before the test. Based on these results, it is believed that there is no correlation between increasing the applied voltage and decreasing the moisture content, and that the moisture content included when the

It was revealed that there was no correlation between an increase in the holding voltage in the voltage hold test and an increase or decrease in the moisture content shown in **Figure 9**. Accordingly, any chemical changes in the electrolyte owing to an increased holding voltage can be considered to be other than moisture related. The elution of electrode material into the electrolyte sometimes occurs during

*Polarizable electrode after test with holding voltage of 3.2 V.*

electrolysis and is also applied in electrolytic refining. Chemical reactions of the electrodes are not desirable in EDLCs, and a mere action through physical adsorption and desorption are considered ideal [14]. However, the behavior of components contained in ash that cannot be eliminated in the activated carbon generation process is worth noting.

Owing to the difference in viscosity between the harvested electrolyte and the oilbased 23 element standard solution by Seishin Trading Co., Ltd., used in the calibration curve method, the interference must be suppressed. Moreover, since the volume required for analysis (15 ml) was not reached, this needed to be adjusted. To that end, microwave-assisted decomposition was performed using a microwave digestion device (SpeedWave MS3) by Actac Project Services Corporation, adding 8 ml of nitric acid to 0.1 ml of electrolyte. Since this decomposition treatment was performed in a closed

**Figure 8.** *Polarizable electrode after test with holding voltage of 3.5 V.*

**65**

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

**5.4 Element concentrations in electrolyte: measurement results**

system, we considered external element contamination to be almost nil. Subsequently, the solution was adjusted to 15 ml by diluting it with ultrapure water. After adjusting the volume as described above, the element concentration in the electrolyte was measured by introducing it into the inductively coupled plasma emission spectrometer

**Figure 10** shows the element concentrations in the electrolyte taken from the EDLCs before and after the tests for each applied voltage. An analysis showed that the electrolyte contained boron, sodium, aluminum, silicon, potassium, calcium,

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

(ICPS-8100) by Shimadzu Corporation.

**Figure 9.** *Moisture content in electrolyte before and after test for each applied voltage.*

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

system, we considered external element contamination to be almost nil. Subsequently, the solution was adjusted to 15 ml by diluting it with ultrapure water. After adjusting the volume as described above, the element concentration in the electrolyte was measured by introducing it into the inductively coupled plasma emission spectrometer (ICPS-8100) by Shimadzu Corporation.

#### **5.4 Element concentrations in electrolyte: measurement results**

**Figure 10** shows the element concentrations in the electrolyte taken from the EDLCs before and after the tests for each applied voltage. An analysis showed that the electrolyte contained boron, sodium, aluminum, silicon, potassium, calcium,

*Science, Technology and Advanced Application of Supercapacitors*

**64**

**Figure 9.**

**Figure 8.**

*Moisture content in electrolyte before and after test for each applied voltage.*

*Polarizable electrode after test with holding voltage of 3.5 V.*

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

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

**Figure 10.**

*Element concentrations in electrolyte before and after test for each applied voltage.*

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

**Figure 10.** *Element concentrations in electrolyte before and after test for each applied voltage.*

*Science, Technology and Advanced Application of Supercapacitors*

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 these dissolved into the electrolyte.

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.
