40
Table 3-4. The amount of desorption gas from TPD measurement by comparing the sample with pretreatment with the sample without pretreatment.
Sample Desorption gas (CO, CO2 or hydrocarbon) (mol/g)
CB400 2×10-4
CB600 2.2×10-4
CB800 2.1×10-4
CB1000 2×10-4
CB2300 2×10-4
41
Figure 3-13. a) TEM images of CBs heated at 1000℃ without ALD process. b) TEM images of CBs heated at 1000℃ with 1cycles of ALD process. c) TEM images of CBs heated at 1000℃ with 5cycles of ALD process. d) TEM images of CBs heated at 1000℃ with 30cycles of ALD process.
42
Figure 3-14. TEM-EDS (Energy Dispersive X-Ray Spectroscopy) mapping of CBs with ALD process and without ALD process. The red dots indicate EDS C-K map, green dots indicate O-K map and emerald dots indicate Al-K map. a) EDS mapping of CB1000 without ALD process, b) EDS mapping of CB1000 with 1cycles of ALD process, c) EDS mapping of CB1000 with 5cycles of ALD process, d) EDS mapping of CB1000 with 30cycles of ALD process.
43
Electrochemical performance of CBs with ALD process as shown in figure 3-15, have been conducted. ORR activity of CBs with ALD process was definitely reduced in comparison with that of CBs without ALD process because the defects determining electroactivities were blocked by Al2O3. In figure 3-15a, CBs with 1 cycle of ALD process and CBs with 5 cycles of ALD process performed almost same onset potential of ORR while the onset potential of CBs with 30 cycles of ALD process were shifted to more negative potential. In other words, the ORR activities were not dependent on the amount of Al2O3 deposited on CBs between CB(1ALD) and CB(5ALD). That is because the limited edge-defects generated from removal of surface functional groups through heat-treatment were already blocked even at the first cycle of ALD process. The reason why the ORR activities of CB(30ALD) was reduced more is even the basal planes which could affected ORR activities less than edge planes were totally blocked by forming shell of Al2O3. In addition, the shell of Al2O3 prevented O2 adsorption from active sites of CBs surface for ORR. When it comes to reaction slope (-0.2V ~ -0.5V) indicating ORR kinetics, as deposited Al2O3 became thicker by increasing the number of ALD cycles, the reaction slope became more gentle. That is because electron-pathway became longer as deposited Al2O3 became thicker. The ORR activities were not dependent on the amount of Al2O3 deposited on CBs between CB(1ALD) and CB(5ALD) because the limited edge-defects on surface of CBs have already been blocked. However, the ORR activities of CB(30ALD) was reduced more, because even basal planes which affected ORR a little were totally blocked.
Electron transfer number of defect-blocked CBs was remarkably reduced compared to CBs without ALD process. The electron transfer number of CBs without ALD, CB(1ALD), CB(5ALD), and CB(30ALD) are 3.65, 2.97, 2.7, and 2.4 respectively. That is also explained by the amount of active sites participating into ORR and the electron-pathway. As area of deposited Al2O3 which covered CBs surface enlarged more, the amount of active sites for ORR consisting of basal planes and edge planes decreased. In addition, rate of transferred electron decreased because the electron-pathway became longer and the shell of Al2O3 prevented direct contact between active sites on CBs and approaching O2
for ORR.
44
Figure 3-15. a) ORR polarization curves of each CBs heated at 1000℃ without ALD process, with 1cycles of ALD process, with 5cycles of ALD process, with 30cycles of ALD process. The amount of each CBs on rotating disk electrode was 0.48mg/cm2. b) Electron transfer number of each CB1000, CB(1ALD), CB(5ALD), CB(30ALD).
45 4. Conclusion
In this study, we would like to reveal that oxygen reduction reaction (ORR) activity is directly related with defects of carbon. To demonstrate this hypothesis, we firstly synthesized carbon balls via hydrothermal process and additional heat-treatment in the range from 400℃ to 1000℃. By hydrothermal process amorphous carbon materials were obtained and defects would be generated by the heat-treatment through decomposition of functional groups on carbon balls surface. The degree of defects or defects density varied with the heat-treatment temperature. More functional groups were removed at higher temperature of heat-treatment resulting in higher defects density. Temperature programmed desorption (TPD), Fourier transform infrared spectroscopy (FT-IR), and thermogravimetric analysis (TGA) were conducted to identify remaining functional groups on carbon balls. The degree of defects or defects density were confirmed by Raman spectroscopy and x-ray photoelectron spectroscopy. Electrochemical performances of each CBs heated at 400℃, 600℃, 800℃, and 1000℃ including ORR activity and electron transfer number was getting enhanced as ID/IG ratio and full width at half maximum of sp2 C1s peak increased except for CBs heated at 2300℃. The ORR activity of CB heated at 2300℃ was as similar as that of CB heated at 600℃ although ID/IG ratio of CB heated at 2300℃ was much higher than that of CB heated at 600℃. The shrank and compacted structure confirmed by TEM images would lead to make defects inaccessible. Hence, we can conclude that the amount of accessible defects considered as active sites is critical factor, not the total amount of defects. For further investigation of defect effect on electrochemical performances, atomic layer deposition (ALD) process was conducted. ALD of Al2O3 process blocked the defects on CBs in priority.
Defect-blocked CBs obtained by ALD treatment showed worse electrochemical performances compared with bare CBs. From these study, we could expect that the accessible defects of carbon is a factor determining electroactivities of oxygen reduction. The hypothesis that is the density of defects determined electroactivities of oxygen reduction regarding to onset-potential and electron transfer number for ORR was proved from experimental results. However, the mechanism study about how the defects practically militate on ORR is absolutely required to be investigated for designing enhanced metal-free carbon catalysts.
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