2. I MPACT OF T EXTURAL P ROPERTIES OF M ESOPOROUS P ORPHYRINIC C ARBON
3.3. R ESULTS AND D ISCUSSION
3.3.2. Effect of Carbon Support Types
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compounds like FeTMPPCl, but also for combinations of Fe and N precursors. Hereafter, the impact of carbon supports, Fe loadings, and pyrolysis temperatures on the electrocatalytic properties of Fe–
N/C catalysts are explored with the most active Fe-Phen/CNT-based catalysts.
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Figure 3.5. XRD patterns of the Fe−Phen/C and Fe−Phen/C_w/o SiO2 catalysts prepared with different carbon supports. Standard diffraction patterns for Fe (JCPDS 34-0529) and Fe3C (JCPDS 06- 0686) are shown as dark blue and gray bar graphs, respectively.
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Figure 3.6. Deconvoluted N 1s XPS spectra from the Fe−Phen/C and Fe−Phen/C_w/o SiO2 catalysts prepared with different carbon supports: (a) CNT, (b) KB, and (c) VC. (d) Bar graph comparing the relative peak areas for Fe−Nx species (corresponding to the red shaded peaks in a−c).
Fe K-edge XANES and EXAFS spectra for the Fe-Phen/C and Fe-Phen/C_w/o SiO2 catalysts are shown in Figure 3.7 along with the reference spectra. Similar to Fe-Phen/CNT, the XANES spectra of Fe-Phen/KB and Fe-Phen/VC (Figure 3.7a) featured peak A in the pre-edge region (7114 eV), as well as a relatively higher intensity of peak C (7132 eV) compared to peak D (7140 eV). The XANES data suggest that all Fe-Phen/C catalysts mainly consist of distorted Fe–Nx sites. On the other hand, Fe-Phen/KB_w/o SiO2 and Fe-Phen/VC_w/o SiO2 catalysts exhibited significant absorption intensity near 7116 eV, which was also observed in Fe-Phen/CNT_w/o SiO2, indicating the generation of metallic Fe species in these catalysts. Further quantitative investigation of XANES spectra were carried out by LCF analyses using Fe foil, FePc, and FeTMPPCl as references (Figure 3.8). The LCF analyses reveal that Fe-Phen/CNT and Fe-Phen/KB are entirely composed of Fe–Nx sites, whereas Fe- Phen/VC contains a small amount of Fe phase (15%) with a dominant presence of Fe–Nx sites. On the
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contrary, three Fe-Phen/C_w/o SiO2 catalysts were found to possess a relatively high portion of Fe phase (40–50%). Therefore, the LCF analyses again confirmed that the silica coating strategy is an effective method for producing Fe–N/C catalysts with relatively higher active site density. RDFs of the EXAFS spectra for Fe-Phen/CNT and Fe-Phen/KB (Figure 3.7b) exhibited only a peak near 1.4 Å , indicating the exclusive existence of Fe–N/O bonds. Fe-Phen/VC showed another small peak at 2.1 Å corresponding to Fe–Fe bonds in Fe particles, which is consistent with the XANES results. All Fe- Phen/C_w/o SiO2 catalysts showed a pronounced Fe–Fe RDF peak at 2.1 Å with a smaller Fe–N/O peak, suggesting the formation of a significant amount of less active Fe particles.
Figure 3.7. Fe K-edge (a) XANES spectra and (b) Fourier-transformed EXAFS spectra from the Fe−Phen/C and Fe−Phen/C_w/o SiO2 catalysts prepared with different carbon supports, along with those from the Fe foil (gray), FePc (black), and FeTMPPCl (purple) references.
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Figure 3.8. XANES raw data and LCF spectra of the Fe-Phen/C and Fe-Phen/C_w/o SiO2 catalysts.
The LCF spectra were generated from the spectra of Fe foil, FePc, and FeTMPPCl.
XRD, TEM, and XAS analyses consistently verify the effectiveness of the silica coating for preparing Fe–N/C catalysts consisting primarily of active Fe–Nx sites, regardless of the carbon supports investigated. However, it was less efficient for the VC support. We suppose that the textural properties of the carbon support are also crucial for the formation of active sites with high density.
Several groups reported the importance of the structural properties of sacrificial metal-organic framework precursors for Fe–N/C catalysts.36,80,81 For instance, Jaouen et al. recently demonstrated that the ORR activity of zeolitic imidazolate framework (ZIF)-derived Fe–N/C catalysts was linearly correlated with pore volume of the parental ZIF.81 The textural properties of the three carbon supports were investigated by N2 physisorption analyses. The physisorption data revealed that CNT, KB, and VC have Brunauer–Emmett–Teller (BET) surface areas of 310 m2 g−1, 727 m2 g−1, and 222 m2 g−1 and pore volumes of 0.61 cm3 g−1, 0.69 cm3 g−1, and 0.27 cm3 g−1, respectively, suggesting a relatively lower BET surface area and pore volume of VC. Therefore, the marginal silica coating effect in the VC-based catalyst may originate from insufficient surface sites available to host atomically dispersed active sites. This indicates that Fe-based particles are generated even the silica coating step is applied during the Fe–N/C catalyst synthesis above the critical point where a maximum number of atomically dispersed Fe–Nx site can be generated.
The ORR activities of Fe-Phen/C and Fe-Phen/C_w/o SiO2 catalysts were evaluated using the
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RRDE method in a 0.1 M KOH solution. As explained in the preceding section, Fe-Phen/CNT showed enhanced ORR activity compared to Fe-Phen/CNT_w/o SiO2 in terms of half-wave potential, kinetic current density, Tafel slope, and 4-electron selectivity (Figure 3.9). The other two sets of Fe- Phen/C and Fe-Phen/C_w/o SiO2 catalysts showed similar trends. For the KB-based catalysts, Fe- Phen/KB exhibited a half-wave potential of 0.89 V and a kinetic current density of 4.1 mA cm−2, which are 20 mV and 2.5 times higher than the values from Fe-Phen/KB_w/o SiO2 (Figure 3.9a). Fe- Phen/VC showed an 80 mV higher half-wave potential and 9 times higher kinetic current density than Fe-Phen/VC_w/o SiO2 (Figure 3.9a). Tafel slopes of Fe-Phen/KB (49 mV dec−1) and Fe-Phen/VC (45 mV dec−1) are smaller than those of Fe-Phen/KB_w/o SiO2 (57 mV dec−1) and Fe-Phen/VC_w/o SiO2
(79 mV dec−1), respectively, indicating the silica coating process improved ORR kinetics (Figure 3.9b). The activity parameters of the catalysts are presented in Figures 3.9c,d and Table 3.2, which clearly demonstrate the promotion effect of the silica coating in the ORR.
Figure 3.9. (a) ORR polarization curves and (b) Tafel plots from the Fe−Phen/C and Fe−Phen/C_w/o SiO2 catalysts. Bar graphs comparing the catalyst (c) half-wave potentials and (d) kinetic current densities at 0.9 V (vs. RHE).
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