2. I MPACT OF T EXTURAL P ROPERTIES OF M ESOPOROUS P ORPHYRINIC C ARBON
3.3. R ESULTS AND D ISCUSSION
3.3.1. Effect of Fe and N Precursors
The choice of appropriate Fe and N precursors is important for preparing efficient Fe–N/C catalysts because the formation of active Fe–Nx sites is affected critically by the interaction between Fe and N precursors.55,70,71 We first explored the general applicability of the silica coating-mediated strategy by preparing catalysts from a variety of precursors: FeTMPPCl, FeAc/1,10-phenanthroline, and FeCl3/polyaniline (Table 3.1). From these precursors, three sets of Fe–N/CNT and Fe–
N/CNT_w/o SiO2 catalysts were prepared in the presence or absence of the silica coating step, respectively. Metallomacrocyclic compounds themselves72,73, or their pyrolyzed forms9,70,77–79, have been investigated extensively as Fe–N/C catalysts, because they can serve as model catalysts for mimicking the structure and function of the enzymatic ORR catalyst cytochrome c oxidase. On the
54
other hand, the combination of respective Fe and N precursors can also produce Fe–N/C catalysts, suggesting the use of various combinations of Fe and N precursors as well as providing a low-cost route to Fe–N/C catalysts.
Fe-Por/CNT catalyst was synthesized using FeTMPPCl as the precursor for both Fe and N.42 The procedure consisted of solid-state mixing of FeTMPPCl and acid-treated CNT, low-temperature heat-treatment at 400 °C, silica coating, high-temperature pyrolysis at 800 °C, and silica etching. The XRD pattern for the Fe-Por/CNT catalyst (Figure 3.1) showed only broad diffraction peaks near 26°, 43°, and 53°, which originate from the CNT support. In contrast, the XRD pattern of Fe- Por/CNT_w/o SiO2 (Figure 3.1) exhibited additional peaks for crystalline Fe and Fe3C phases, which are marked with black circles and gray inverted triangles, respectively. Fe-Phen/CNT was prepared in the same manner as the Fe-Por/CNT catalyst, except for the use of FeAc and 1,10-phenanthroline as the Fe and N precursors, respectively. The XRD pattern for the Fe-Phen/CNT catalyst was similar to those of the CNT support and Fe-Por/CNT catalyst, whereas that of Fe-Phen/CNT_w/o SiO2 exhibited distinct peaks corresponding to the Fe and Fe3C phases, similar to Fe-Por/CNT_w/o SiO2 (Figure 3.1).
TEM images of Fe-Phen/CNT and Fe-Phen/CNT_w/o SiO2 catalysts further revealed that no particulates were formed in the Fe-Phen/CNT catalyst, whereas the large crystalline nanoparticles of 15 nm in diameter were generated in Fe-Phen/CNT_w/o SiO2, corroborating the XRD results. Fe- PANI/CNT and Fe-PANI/CNT_w/o SiO2 catalysts were prepared from FeCl3 and polyaniline. The XRD patterns of these two catalysts showed diffraction peaks for the FeS and Fe3C phases. The formation of FeS species in both catalysts originates from the use of APS, which was used as the initiator for aniline polymerization. We note that Fe-PANI/CNT showed less pronounced XRD peak intensities for crystalline FeS species than Fe-PANI/CNT_w/o SiO2, suggesting the silica layer plays a role in suppressing the formation of the FeS phase. The XRD patterns for the three sets of Fe–N/CNT catalysts indicate that the silica coating strategy is an effective means of preventing or suppressing the undesirable formation of Fe, Fe3C, and FeS particles during the preparation of Fe–N/CNT catalysts.
55
Figure 3.1. XRD patterns of the Fe−N/CNT and Fe−N/CNT_w/o SiO2 catalysts prepared with different combinations of Fe and N precursors. Standard diffraction patterns for Fe (JCPDS 34-0529), Fe3C (JCPDS 06-0686), and FeS (JCPDS 71-4469) are shown as dark blue, gray, and dark red bar graphs, respectively.
The Fe and N contents in the catalysts were estimated by ICP-OES and combustion elemental analyses, respectively. The amounts of Fe were 1.8 wt%, 1.1 wt%, and 3.1 wt% for Fe-Por/CNT, Fe- Phen/CNT, and Fe-PANI/CNT catalysts. The higher Fe content in Fe-PANI/CNT than the other two catalysts may originate from the presence of the FeS phase. Interestingly, all three Fe–N/CNT_w/o SiO2 catalysts contained much larger amounts of Fe compared to the Fe–N/CNT counterparts. This could be attributed to the formation of Fe-based large particles in the Fe–N/CNT_w/o SiO2 catalysts as revealed in the TEM image of Fe-Phen/CNT_w/o SiO2. Conversely, nitrogen contents in the Fe–
N/CNT catalysts were greater than those in the Fe–N/CNT_w/o SiO2 catalysts, indicating that the silica coating suppresses evaporation or removal of nitrogen during catalyst preparation. Considering the relative ratio of Fe–Nx species and nitrogen content, we conclude that the catalysts prepared with the silica coating have a higher density of active sites than the catalysts prepared without the silica coating.
56
The chemical states of surface nitrogen species in the catalysts were analyzed by XPS. Figures 3.2a–c shows the N 1s XPS spectra for three sets of Fe–N/CNT catalysts synthesized from various precursors. The XPS spectra were deconvoluted into four peaks, which could be assigned to pyridinic N (397.8–398.6 eV), N coordinated with Fe (Fe–Nx; 399.3–399.7 eV), pyrrolic N (400.3–400.6 eV), and graphitic N (401.3–401.6 eV) species.77,78 One should note that the peak corresponding to Fe–Nx
species appeared between those for pyridinic N and pyrrolic N species, which is consistent with a recent combined experimental and computational study by Artyushkova et al.77,78 A comparison of the relative area ratio of Fe–Nx species between the silica-coated and uncoated catalysts (Figure 3.2d) indicates that the Fe–N/CNT catalysts in general contained greater densities of Fe–Nx sites than the Fe–N/CNT_w/o SiO2 catalysts.
Figure 3.2. Deconvoluted N 1s XPS spectra of the Fe−N/CNT and Fe−N/CNT_w/o SiO2 catalysts prepared with different combinations of Fe and N precursors: (a) FeTMPPCl, (b) FeAc and 1,10- phenanthroline, and (c) FeCl3 and polyaniline. (d) Bar graph comparing the relative peak areas for the Fe−Nx content in the catalysts (corresponding to the red shaded peaks in a−c).
57
The impact of the silica coating on the local and electronic structure of Fe atoms was investigated by XAS (Figure 3.3). Figure 3.3a shows the Fe K-edge XANES spectra from the Fe–
N/C catalysts with Fe foil, FePc, and FeTMPPCl references. FeTMPPCl and FePc showed peaks at 7114 eV (peak A) and 7118 eV (peak B) in the pre-edge region, respectively, which stem from square pyramidal and square planar symmetries. The Fe-Por/CNT catalyst exhibited a similar XANES pre- edge feature as FeTMPPCl with a peak at 7114 eV. However, a notable difference between FeTMPPCl and Fe-Por/CNT was found in the XANES post-edge region. FeTMPPCl showed a lower intensity for the peak at 7132 eV (peak C) than that at 7140 eV (peak D), whereas Fe-Por/CNT exhibited a higher relative intensity for peak C. Mukerjee et al. showed experimentally and computationally that the greater intensity of peak C compared to peak D originates from translocation of the central Fe atom away from the in-plane Fe–N4 structure, thus creating catalytically active, distorted Fe–N4 sites.46 These XANES features suggest that the Fe-Por/CNT catalyst mainly consists of distorted Fe–Nx sites. Fe-Phen/CNT and Fe-PANI/CNT catalysts also showed similar XANES spectra as the Fe-Por/CNT catalyst, indicating that the three Fe–N/CNT catalysts contain similar local structures around Fe, regardless of the types of Fe and N precursors. Among Fe–N/CNT_w/o SiO2
catalysts, Fe-Phen/CNT_w/o SiO2 exhibited a clearly different XANES feature compared to those of the silica-coated catalysts, with relatively strong absorption signal near 7116 eV, which is characteristic of Fe foil.
Figure 3.3. Fe K-edge (a) XANES spectra and (b) Fourier-transformed EXAFS spectra of Fe−N/CNT and Fe−N/CNT_w/o SiO2 catalysts prepared with different combinations of Fe and N precursors, along with those of Fe foil (gray), FePc (black), and FeTMPPCl (purple) references.
Further structural information was gained with extended X-ray absorption fine structure
58
(EXAFS) spectral analyses (Figure 3.3b). The radial distribution function (RDF) from the Fourier- transformed EXAFS spectrum of Fe-Por/CNT showed a major peak at 1.4 Å and a relatively weak shoulder peak at 1.9 Å , which correspond to the Fe–N/O and Fe–Fe bonds, respectively. In contrast, the Fe-Por/CNT_w/o SiO2 catalyst showed a more pronounced Fe–Fe peak than the Fe-Por/CNT catalyst. A pair of Fe-PANI/CNT and Fe-PANI/CNT_w/o SiO2 catalysts showed similar EXAFS spectral features, with both showing peaks corresponding to Fe–N/O and Fe–S bonds. Fe-Phen/CNT and Fe-Phen/CNT_w/o SiO2 catalysts exhibited clearly distinguishable EXAFS features. While Fe- Phen/CNT only showed a peak for Fe–N/O near 1.4 Å , the Fe-Phen/CNT_w/o SiO2 catalyst showed a main Fe–Fe peak at 2.1 Å and a Fe–N/O peak with lower intensity. These results indicate that Fe- Phen/CNT consists almost entirely of Fe–N/O bonds, whereas Fe-Phen/CNT_w/o SiO2 contains a significant amount of Fe particles. XAS analysis confirmed again that the Fe–Nx sites were preferentially formed when the silica coating was exploited for synthesis of the Fe–N/CNT catalysts.
Finally, we conducted XRD and EXAFS analyses of Fe-Phen/CNT and Fe-Phen/CNT_w/o SiO2
samples before the HF washing step, as HF treatment could remove Fe-based particles as well as the silica coating layer. Larger XRD peaks and RDF intensities corresponding to Fe-based particles are shown for the sample prepared without the silica coating, suggesting the silica coating is effective to suppress the formation of such particles during the high-temperature pyrolysis.
Next, we investigated the electrocatalytic activities of the catalysts for the ORR using RRDE measurements in a 0.1 M KOH solution. LSV curves for the ORR were recorded at a rotation speed of 1600 rpm with a scan rate of 5 mV s−1 (Figures 3.4a–c). The Fe-Por/CNT catalyst exhibited excellent ORR activity with a half-wave potential of 0.88 V (vs. RHE), which is more positive than that of Fe- Por/CNT_w/o SiO2 (0.83 V) (Figure 3.4a). Fe-Phen/CNT showed further enhanced ORR activity with half-wave potential at 0.90 V (Figure 3.4b). The activity difference between the Fe-Phen/CNT and Fe-Phen/CNT_w/o SiO2 catalysts is remarkable; the half-wave potential of Fe-Phen/CNT is 90 mV higher and its kinetic current density at 0.9 V is 8 times higher than those in Fe-Phen/CNT_w/o SiO2. Similarly, Fe-PANI/CNT showed a 60 mV lower overpotential than Fe-PANI/CNT_w/o SiO2
and showed enhanced kinetic current density by a factor of 1.5 (Figure 3.4c). Comparing the kinetic parameters of the three sets of catalysts (Figures 3.4d,e and Table 3.2), it is clear that the Fe–N/CNT catalysts show much improved ORR activity than the Fe–N/CNT_w/o SiO2 catalysts in terms of both half-wave potentials and kinetic current densities. Tafel slopes for the catalysts (Figure 3.4f and Table 3.2) reveal that the Fe–N/CNT catalysts generally have smaller Tafel slopes than the Fe–N/CNT_w/o SiO2 catalysts, suggesting the ORR kinetics in the Fe–N/CNT catalysts are favorable. Electron transfer numbers of the catalysts during the ORR were measured using RRDE technique to investigate the effect of the silica coating on the 4-electron selectivity. Fe–N/CNT catalysts prepared with the silica coating exhibited generally higher electron transfer numbers than the Fe–N/CNT_w/o SiO2
catalysts.
59
Figure 3.4. ORR polarization curves of the Fe−N/CNT and Fe−N/CNT_w/o SiO2 catalysts prepared with different combinations of Fe and N precursors: (a) FeTMPPCl, (b) FeAc and 1,10- phenanthroline, and (c) FeCl3 and polyaniline. Bar graph comparing (d) half-wave potentials and (e) kinetic current densities at 0.9 V (vs. RHE) of the catalysts. (f) Tafel slopes of the catalysts.
Overall, structural characterization and electrocatalytic activity data for the three sets of Fe–
N/CNT and Fe–N/CNT_w/o SiO2 catalysts suggest that the silica coating is a generally applicable method for preferentially generating molecularly dispersed, active Fe–Nx sites, which consequently boosts the ORR activity. Importantly, the silica coating method is effective not only for macrocyclic
60
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.