**4. Chemical analysis of polarizable electrodes**

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 surfaces.

#### **4.1 Polarizable electrode surface observation**

**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 the internal resistance [5–7, 12].

#### **Figure 2.**

*Polarizable electrode surface (a) positive electrode (PE) before test, (b) negative electrode (NE) before test, (c) PE after test at holding voltage of 2.8 V, (d) NE after test at holding voltage of 2.8 V, (e) PE after test at holding voltage of 2.9 V, (f) NE after test at holding voltage of 2.9 V, (g) PE after test at holding voltage of 3.0 V, (h) NE after test at holding voltage of 3.0 V, (i) PE after test at holding voltage of 3.2 V, (j) NE after test at holding voltage of 3.2 V, (k) PE after test at holding voltage of 3.5 V, and (l) NE after test at holding voltage of 3.5 V.*

#### **4.2 Analysis of elements existing on polarizable electrodes**

Qualitative analysis results show that no major spectral differences can be seen based on observations of the pretest polarizable electrodes and posttest polarizable electrodes for each holding voltage, and that the observations chiefly consist of three elements: carbon, fluorine, and oxygen. It follows that the deposits observed on the electrode surface are fluorine, oxygen, and carbon derived. To reveal how the deposits, which consist of the same elements as the polarizable electrodes, cause a reduction in performance of the EDLC, we conducted a more detailed state analysis of the electrode surfaces. It is believed that owing to the holding voltage in the voltage hold test (an accelerated deterioration test), a change of some sort is caused in the binding states of carbon, fluorine, and oxygen. We, therefore, applied an etching process to the electrode surface and measured the changes in spectral intensity in the depth direction for C1s, O1s, and F1s, thus studying to what depth the electrode surface deposits were produced.

**Figures 3**–**8** show the variance in the observed spectral intensity in the depth direction for (a) C1s, (b) F1s, and (c) O1s on positive electrode and (e) C1s, (f) F1s, and (g) O1s on negative electrode. Furthermore, (d) and (h) show the atomic composition ratios on positive electrode and negative electrode, respectively. It is clear from **Figure 3** that for the polarizable electrodes before the voltage hold test, the positive and negative electrodes consist of carbon, oxygen, and fluorine until a depth of 25 nm from the surface. Furthermore, since the composition ratio from approximately 2–3 nm below from the electrode surface is stable, it is thought that a 2–3 nm film exists on the polarizable electrodes. It is believed that since the cell interior was exposed to electrolytes prior to the polarizable electrodes being chosen, the polarizable electrodes reacted with the oxygen and the fluorine in the electrolyte at the solid-fluid interface.

No large difference is seen between the pre- and postvoltage hold-test polarizable electrodes at a holding voltage of 2.8 V. However, as shown in **Figure 5**, after the

**59**

**Figure 3.**

*Pretest polarizable electrode.*

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

voltage hold test at a holding voltage of 2.9 V, the O1s spectral intensity decreased and the F1s spectral intensity increased. Graphs (d) in **Figure 5** show that they almost only consist of carbon and fluorine up to a depth of 15 nm. Accordingly, it is believed that through the application of voltages of 2.9 V and higher, the bond between carbon and oxygen in the polarizable electrodes is broken, and oxygen atoms are released. Moreover, since there is virtually no change in the composition ratio at a depth of approx. 10–13 nm from the surface, it is clear that the fluorine compound layer has increased in thickness compared to that before the test. Since this reactant consists of carbon and fluorine [5], it is believed to be graphite fluoride. It can be inferred from the fact that graphite fluoride is an insulator that the electrode resistance was caused to increase, thus causing the internal resistance

*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 3.** *Pretest polarizable electrode.*

voltage hold test at a holding voltage of 2.9 V, the O1s spectral intensity decreased and the F1s spectral intensity increased. Graphs (d) in **Figure 5** show that they almost only consist of carbon and fluorine up to a depth of 15 nm. Accordingly, it is believed that through the application of voltages of 2.9 V and higher, the bond between carbon and oxygen in the polarizable electrodes is broken, and oxygen atoms are released. Moreover, since there is virtually no change in the composition ratio at a depth of approx. 10–13 nm from the surface, it is clear that the fluorine compound layer has increased in thickness compared to that before the test. Since this reactant consists of carbon and fluorine [5], it is believed to be graphite fluoride. It can be inferred from the fact that graphite fluoride is an insulator that the electrode resistance was caused to increase, thus causing the internal resistance

*Science, Technology and Advanced Application of Supercapacitors*

**4.2 Analysis of elements existing on polarizable electrodes**

*(k) PE after test at holding voltage of 3.5 V, and (l) NE after test at holding voltage of 3.5 V.*

the electrode surface deposits were produced.

Qualitative analysis results show that no major spectral differences can be seen based on observations of the pretest polarizable electrodes and posttest polarizable electrodes for each holding voltage, and that the observations chiefly consist of three elements: carbon, fluorine, and oxygen. It follows that the deposits observed on the electrode surface are fluorine, oxygen, and carbon derived. To reveal how the deposits, which consist of the same elements as the polarizable electrodes, cause a reduction in performance of the EDLC, we conducted a more detailed state analysis of the electrode surfaces. It is believed that owing to the holding voltage in the voltage hold test (an accelerated deterioration test), a change of some sort is caused in the binding states of carbon, fluorine, and oxygen. We, therefore, applied an etching process to the electrode surface and measured the changes in spectral intensity in the depth direction for C1s, O1s, and F1s, thus studying to what depth

*Polarizable electrode surface (a) positive electrode (PE) before test, (b) negative electrode (NE) before test, (c) PE after test at holding voltage of 2.8 V, (d) NE after test at holding voltage of 2.8 V, (e) PE after test at holding voltage of 2.9 V, (f) NE after test at holding voltage of 2.9 V, (g) PE after test at holding voltage of 3.0 V, (h) NE after test at holding voltage of 3.0 V, (i) PE after test at holding voltage of 3.2 V, (j) NE after test at holding voltage of 3.2 V,* 

**Figures 3**–**8** show the variance in the observed spectral intensity in the depth direction for (a) C1s, (b) F1s, and (c) O1s on positive electrode and (e) C1s, (f) F1s, and (g) O1s on negative electrode. Furthermore, (d) and (h) show the atomic composition ratios on positive electrode and negative electrode, respectively. It is clear from **Figure 3** that for the polarizable electrodes before the voltage hold test, the positive and negative electrodes consist of carbon, oxygen, and fluorine until a depth of 25 nm from the surface. Furthermore, since the composition ratio from approximately 2–3 nm below from the electrode surface is stable, it is thought that a 2–3 nm film exists on the polarizable electrodes. It is believed that since the cell interior was exposed to electrolytes prior to the polarizable electrodes being chosen, the polarizable electrodes reacted with the oxygen and the fluorine in the electrolyte

No large difference is seen between the pre- and postvoltage hold-test polarizable electrodes at a holding voltage of 2.8 V. However, as shown in **Figure 5**, after the

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**Figure 2.**

at the solid-fluid interface.

**Figure 4.** *Polarizable electrode after test with holding voltage of 2.8 V.*

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.
