Chapter 3. Investigation of supercapacitor effect of SWBs
3.2 Results and discussion
3.2.2 Measurement of specific capacitance of each SWB
In previous studies, it was revealed that the cathode current collector which has a high surface area can stabilize the voltage and improve the overall efficiency of batteries2,22,25,33. This phenomenon is called as EDL capacitance and it occurred through these steps.
i) The anions in the electrolyte are accumulated on the charged cathode current collector.
When the charging process, the cathode electrode has a positive charge at surfaces. The anions are stacked on the surfaces by electrostatic interaction. So the electrical layer is formed on the electrode surfaces.
ii) The cations are stacked on the anion layer.
After that, the cations in the electrolyte are stacked on the anion layer by electrostatic interaction.
Therefore, the electrical double layer (EDL) is formed through these steps. Accordingly, we can realize
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that amount of EDL formation is affected the surface area of the cathode electrode. When the surface area was larger, the anions are stacked easily, and the charge amount is increased.
EDL behaves like a capacitor due to its charge stacked property. When the voltage is dropped unexpected problems, the EDL stabilized the energy supply by deteriorating the charge layer. On the other hand, the voltage is increased suddenly, the charge of the cathode current collector becomes more positive, and the overall ions in the electrolyte are stacked. Through these steps, the voltage profile of liquid electrolyte batteries using a high surface area can be stabilized22.
The amount of EDL can be measured by cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) test. The area of the CV graph and the discharge time of GCD were indicated that the quantity of electric charge. Therefore, we investigated the EDL formation of each SWBs through the CV and GCD tests.
First, we investigated the potential range of EDL formation. In the previous study, it was confirmed that the SWBs can form the EDL from 0 to 0.3 V (versus Ag/AgCl). We also observed that the CV curves at the same voltage range to check the tendency. Figure 3.3 was shown the CV curves of each cathode current collector at a scan rate of 0.1 mA s-1 in the voltage range from -0.57 to 0.97 V (versus Ag/AgCl). We can notice that the voltage ranges of EDL formation occurred from 0 to 0.3 V which was used at previous work.
Figure 3.3 The electrochemical cyclic voltammetry (CV) curves of each cathode current collector at a scan rate of 0.1 mA s-1. (a) ACP-DPA642, (b) ACC and (c) ACP-PvdF, respectively.
Also, we measured the CV curves from 0 to 0.3 V at a scan rate of 0.5 mA s-1 and the specific surface area of SWBs was calculated using the equation (1). The measured CV curves were shown at Figure 3.4 and specific surface areas were recorded at Table 3.1. We can notice that the specific capacitance of ACP-DPA642 (58.32 F g-1) was larger than ACC (53.94 F g-1) and ACP-PvdF (40.5 F g-1).
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Figure 3.4 The electrochemical cyclic voltammetry (CV) curves of each cathode current collector at a scan rate of 0.5 mV s-1. (a) ACP-DPA642, (b) ACC and (c) ACP-PvdF, respectively.
Table 3.1 The calculated specific capacitance of each electrode after CV test
ACP-DPA642 ACC ACP-PvdF
Specific capacitance(F/g) 58.32 53.94 40.5
We also investigated the specific capacitance using the GCD test in range from 0 to 0.3 V. The voltage-time profile and the results of calculated specific capacitance is exhibited at figure 3.5 and table 3.2
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Figure 3.5 The voltage-time profile of GCD test at various current of 0.2 mA (blue), 0.4 mA (yellow) and 0.8 mA (green), respectively. (a) ACP-DPA642, (b) ACC and (c) ACP-PvdF. (d) The line graph of specific capacitance according to carbon electrodes.
Table 3.2 The calculated specific capacitance of each electrode after GCD test
Applying current (mA) ACP-DPA642 ACC ACP-PvdF
0.2 94.3 70.6 55.3
0.4 61.4 60.2 51.1
0.8 54.4 49 41.6
The overall specific capacitances were decreased according to the increase of applying current. ACP- DPA64 showed the larger specific capacitance (94.3 F g-1) than ACC (70.6 F g-1) and ACP-PvdF (55.3 F g-1) at the current of 0.2 mA. Also, the overall results were outstanding in the studied current range.
To know the overall capacity of EDLC and pseudocapacitance effect, galvanodynamic charge- discharge test was carried out in a range of 2 mA capacity at a various current from 0.2 to 2 mA. When the EDL formation was finished, the voltage plateau was occurred at voltage profiles and large voltage gap was observed due to sluggish kinetics of OER/ORR process. However, The ACP-DPA642 voltage profiles (Figure 3.6a) did not show the voltage plateau despite of the high current of 2 mA. On the other hand, the other SWBs showed voltage plateau at 2 mA current at figure 3.6b and c. Therefore, we can confirm that the capacitance of ACP-DPA642 was bigger than other SWBs.
Figure 3.6 Galvanodynamic charge-discharge profiles of SWB. (a) ACP-DPA642, (b) ACC, and (c) ACP-PvdF at a various current from 0.2 mA to 2 mA.
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In conclusion, we observed that ACP-DPA642 had larger specific capacitance than ACC and ACP- PvdF under various applying current. Therefore, we can notice that even if the specific surface area of ACC was larger than ACP-DPA642, the ACP-DPA642 SWBs electrochemical performance and cycle retention were better than ACC. It seems that DOPA has the redox-active property and antioxidant moiety. In the previous studies, DOPA oxidation reaction occurs at 0.2 V versus SHE which is lower than OER/ORR theoretical onset potential62,10. Therefore, the ACP-DPA642 can improve energy efficiency and prevent carbon degradation at charging. Figure 3.7 shows the summary of the DOPA effect in this study.
Figure 3.7 The summary of the DOPA effects at this study.
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