As shown in Figure 1.1 B, the oxygen reduction reaction in an aqueous solution mainly takes place via two types of routes: the two-step two-electron reduction route where H2O2 (in acidic medium) or HO2- (in alkaline medium) is formed from O2 or the direct the reduction route with four electrons that form H2O (in acidic medium) or OH- (in alkaline medium) from O2.8,9 The one-electron reduction pathway that forms O2- from O2 can also take place in non-aqueous aprotic medium and in alkaline medium.9 In the practical systems using ORR, the ORR sequences are more complicated because many intermediates, byproducts and electron transfer number, and chemical reactions can be changed depending on the nature of the catalysts and the electrolyte.7 The reaction potentials are arranged according to a series ORR responses in Table 1.1. The direct four-electron reduction pathway is preferred in fuel cell systems, while the two-electron reduction pathway is preferred in H2O2 production. During the O2 reduction reaction, the product of the 2-electron reduction pathway, H2O2 (peroxide), is detected at the ring electrode and further oxidized to H2O by applying +0.4 V to the ring electrode. By using RRDE, the electron transfer number (n) can be calculated to identify whether 2-electron reduction is predominant or whether direct 4-electron reduction is dominant.
Therefore, if the calculated 𝑛𝑒− is close to 2, then the 2-electron reduction transfer is dominant and not the 4-electron reduction transfer. On the other hand, if the calculated 𝑛𝑒− is close to 4, then the 4-electron reduction transfer is dominant rather than the 2-electron reduction transfer.
ORR catalysts
Pt is located at the top of the volcano plot indicating the highest oxygen reduction activity.25. Metal-free catalysts for ORR that do not consist of Pt or any transition metal have been studied since 1990 in terms of the efforts to replace Pt. The adsorption of reactants on catalysts is essential for the catalytic reaction.17 In fact, oxygen adsorption is also considered to be one of the most important steps that determine ORR activity, so that one of the reasons why the ORR activity of N doped carbon increases can be explained by oxygen adsorption capacity.
Defects of carbon materials
Typically, point gaps are the most common defect types in incomplete bond defects.49 Gap defects are the most common ones to be found that result in triple bonds that are highly reactive and react further. with the surrounding molecules.49 For example, what we Call vacancy defects 5-1DB consist of one dangling bond connected to form a strained 5-membered pentagonal ring, leaving a single dangling 1DB bond. 51 Since the dangling bonds are highly reactive and unstable, the carbon surfaces are eager to interact with the absorbed gas, surrounding molecules and moisture to saturate the unstable dangling bonds.49. A single vacancy tends to combine with the adjacent carbon to saturate the rearrangement-dependent bond, as shown in Figure 1-9. The two-vacancy defects also tend to reconstruct into the pentagon, octagon, and pentagon (5-8-5) structure, which is none of the dangling bonds.52 In addition, the two-vacancies in single-walled nanotubes (SWNTs ) have lower formation energies than a vacancy.
Consequently, these incomplete bond defects have the property of reacting with the adjacent carbon, surrounding molecules, to saturate the reactive dangling bonds. In ORR catalysts, incomplete binding defects certainly contribute to the enhancement of electroactivity for ORR because oxygen molecules are easily adsorbed on the catalyst surface.
Carbon ball synthesis
Materials characterization
Electrochemical characterization
Al2O3 was grown by ALD on carbon beads heated at 1000℃ using a customized rotary ALD reactor.
Temperature programmed desorption
Concept of the research
The CBs were prepared from glucose solution via a hydrothermal process followed by heat treatment in argon.56 Hydrothermal synthesis is well known for crystallization of substances from high temperature aqueous solutions at high vapor pressure. During the heat treatment, chemical substances containing oxygen, hydrogen from functional groups were removed, so that the places where the functional groups existed were transformed into the defects. After followed heat treatment process, through transmission electron microscopy as shown in Figure 3-1b, it has been found that all CBs have spherical shape and amorphous structure regardless of heat treatment temperature.
Defects with surface functional groups
As a result, more defects were generated as the heat treatment temperature increased due to thermal decomposition of functional groups. Similar to a change in the degree of graphitization with the heat treatment temperature, the surface area of CBs also showed a change as the heat treatment temperature increased.10 Table 3-1 and Figure 3-3 show detailed values and the tendency of each CBs heated to and 1000℃ by BET measurement. BET surface areas increase as the heat treatment temperature was increased in the range of 400℃ to 1000℃.
The surface areas of each CB with different heat treatment temperatures were also identified by electrochemical analysis. In Figure 3-5, as the heat treatment temperature of CBs increases, the ID / IG ratio also increases in the range from 400℃ to 1000℃, which indicates an increase in the defect rate. The degree of graphitization obviously increased, although the IG did not appear to increase dramatically due to the mild temperature conditions (400℃ to 1000℃).
The degree of defects consisting of edge planes will be identified by comparison with the sp2 peak of C1s spectra. It was also found that as the temperature of the heat treatment increased in the range of 400 ℃. These results showed that CBs heated to 1000℃ had the widest FWHM among other CBs, indicating that CBs heated to 1000℃ had the highest degree of defects because the defects are correlated with the FWHM of the sp2 peak.
From the above data of Raman spectrum and XPS spectrum, we can conclude that the degree of defects increased with the increase of heat treatment temperature.
Electrochemical characterization
The ORR activities of the CBs increase as the heat treatment temperature increases in the range of 400℃ to 1000℃, which means that the onset potential of ORR is shifted to positive potential region. The electron transfer number of CBs obtained by rotating ring disk electrode (RRDE) shows a trend similar to the ORR activities of CBs. In particular, the electron transfer number of CBs heated to 800oC and 1000℃ (CB800, CB1000) is higher than 3.5, which is a significantly high electron transfer number as non-doped carbon-only catalysts compared to other carbon catalysts such as Vulcan XC-72 as known. since the electron transfer number is about 3.61. In other words, the direct 4 electron pathway is dominant in this system.
Correlation between degree of defects and electroactivities of ORR
Carbon balls annealed at extremely high temperature
Since graphene layers were found from CBs heated at 2300℃, X-ray diffraction (XRD) measurement was performed to investigate the graphitization degree of each CB heated at different temperatures as shown in Figure 3-9.10 Spectra XRD of CBs heated at mild temperature showed the broad (002) peak usually found in amorphous carbon materials, while that of CBs heated at 2300 showed sharp peak intensity indicating (002) reflection and small peak indicating reflection (100). Moreover, the phase separation was also confirmed by the (002) reflection of the CBs heated at 2300℃, showing both the broad peak indicating the amorphous phase and the sharp peak indicating the crystalline phase. It is found that the crystallite parameters accompanying the domain boundary size decreased by calculating La and Lc as the annealing temperature of CBs increased, indicating the increase in defect density.
In order to investigate the defect rate of CBs heated at 2300℃, Raman spectroscopy and XPS spectroscopy were absolutely performed as for other CBs.57,63 Figure 3-10a showed the increased sharpness of the ID and IG peaks. It is noticeable that the ID/IG ratio of CBs heated to 2300℃ is much higher than other CBs. From the XPS data, as shown in Figure 3-10b, the FWHM of the sp2 peak in CBs heated to 2300℃ increased than that of CBs heated to 1000℃, indicating an increase in the degree of defects and graphitization.
Finally, electrochemical analyzes were performed to compare the ORR electroactivities of CBs heated at 2300℃ and other CBs. The ORR activities of CBs increase with increasing heat treatment temperature in the range from 400℃ to 1000℃, except for the ORR activities of CB2300 as shown in Figure 3-11a. The ORR activity of CB2300 was reduced almost the same as that of CBs heated at 600℃.
Since the structure of CB2300 is shrunken and compacted with a high density of closed defects, the ORR activity of CB2300 is lower than that of CB1000 which has more accessible defects.
Oxygen adsorption ability
If the results followed the hypothesis explained above, the desorption gas of each CB would increase with the heating temperature. The reason why the results were contrary to our expectations is that the amount of hydrocarbon gas from CB was too much more than the amount of carbon oxide from oxygen adsorption. The most important factor for measuring the oxygen adsorption capacity is the carbon monoxide gas comparison of each CB.
However, since the amount of carbon monoxide gas was much smaller than the amount of hydrocarbon gas, the amount of hydrocarbon gas mainly affected the total amount of gas for desorption. Although the attempt to measure oxygen adsorption capacity with TPD was not effective, it was worth a try. If certain modes were determined, TPD would be a useful measurement of oxygen adsorption capacity.
The amount of desorption gas from TPD measurement by comparing the sample with pretreatment to the sample without pretreatment.
Blocking defects by atomic layer deposition
In this study, we aim to reveal that the oxygen reduction reaction (ORR) activity is directly related to carbon defects. From this study we might expect that the accessible defects of carbon are a factor determining the electroactivity of oxygen reduction. The hypothesis that defect density is certain electroactivities of oxygen reduction with respect to the onset potential and electron transfer number for ORR was proven based on experimental results.
Ab initio molecular dynamics simulations of the oxygen reduction reaction on a Pt(111) surface in the presence of hydrated hydronium (H3O)(+)(H2O)(2): Direct or series connection. Fundamental mechanistic understanding of electrocatalysis of oxygen reduction on Pt and non-Pt surfaces: acidic versus alkaline media. Recent developments in the catalysis of non-noble metals for oxygen reduction reactions in polymer electrolyte fuel cells.
Oxygen reduction on low-index platinum single-crystal surfaces in sulfuric acid solution: spinning ring-Pt (hkl) disk studies. Low-platinum and platinum-free catalysts for the oxygen reduction reaction at fuel cell cathodes. Facile, scalable synthesis of edge-halogenated graphene nanosheets as efficient metal-free electrocatalysts for the oxygen reduction reaction.
Investigations of Oxygen Reduction Reaction Active Sites and Stability of Nitrogen-Modified Carbon Composite Catalysts for PEM Fuel Cells.
