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Active site formation mechanism of carbon-based oxygen reduction catalysts derived from a hyperbranched iron phthalocyanine polymer.

Hiraike Y, Saito M, Niwa H, Kobayashi M, Harada Y, Oshima M, Kim J, Nabae Y, Kakimoto MA - Nanoscale Res Lett (2015)

Bottom Line: The properties of the HB-FePc catalyst are compared with those of a catalyst with high oxygen reduction reaction (ORR) activity synthesized from a mixture of iron phthalocyanine and phenolic resin (FePc/PhRs).Electrochemical measurements demonstrate that the HB-FePc catalyst does not lose its ORR activity up to 900°C, whereas that of the FePc/PhRs catalyst decreases above 700°C.Consequently, effective doping of active nitrogen species into the sp (2) carbon network of the HB-FePc catalysts may occur up to 900°C.

View Article: PubMed Central - PubMed

Affiliation: Department of Applied Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656 Japan ; Current address: Toray Industries, Incorporated, Nihonbashi-Muromachi 2-chome, Tokyo, Japan.

ABSTRACT
Carbon-based cathode catalysts derived from a hyperbranched iron phthalocyanine polymer (HB-FePc) were characterized, and their active-site formation mechanism was studied by synchrotron-based spectroscopy. The properties of the HB-FePc catalyst are compared with those of a catalyst with high oxygen reduction reaction (ORR) activity synthesized from a mixture of iron phthalocyanine and phenolic resin (FePc/PhRs). Electrochemical measurements demonstrate that the HB-FePc catalyst does not lose its ORR activity up to 900°C, whereas that of the FePc/PhRs catalyst decreases above 700°C. Hard X-ray photoemission spectra reveal that the HB-FePc catalysts retain more nitrogen components than the FePc/PhRs catalysts between pyrolysis temperatures of 600°C and 800°C. This is because the linked structure of the HB-FePc precursor has high thermostability against nitrogen desorption. Consequently, effective doping of active nitrogen species into the sp (2) carbon network of the HB-FePc catalysts may occur up to 900°C.

No MeSH data available.


XRD patterns. (a) FePc/PhRs and (b) HB-FePc catalysts.
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Fig4: XRD patterns. (a) FePc/PhRs and (b) HB-FePc catalysts.

Mentions: In the XRD patterns shown in Figure 4, quite strong diffraction peaks are observed for the pristine FePc precursor as a result of its crystalline nature [27], whereas the HB-FePc precursor does not show any diffraction peaks because it has a large, randomly linked structure that does not readily crystallize. HB550 and HB600 have amorphous signatures as a broad structure around 26° and multiple iron oxide (Fe2O3) peaks, while above HB650 graphitized carbon network around 26° as well as iron metal (Fe) and/or iron carbides (Fe3C) around 45° appeared. Thus the formation of the graphitic structure and the iron metal and/or iron carbides are strongly correlated. This can be understood by considering the Yarmulke mechanism; i.e., formation of a graphitic carbon shell structure around reduced metal nanoparticles [20]. In the case of the FePc/PhRs catalysts, the intensity of the diffraction peaks at both 26° and 45° gradually increases from 600°C. These peaks are absent in the XRD pattern of HB600, indicating that the HB-FePc precursor has higher thermostability than the FePc/PhRs precursor. Instead, these peaks suddenly emerge at 650°C, and then their intensities remain almost unchanged up to 900°C. Therefore, the formation of reduced iron components and their subsequent graphitization may proceed in a different manner in the FePc/PhRs and HB-FePc catalysts against pyrolysis temperature, which is consistent with the TG-DTA/DTG results. The sudden appearance of the metallic iron components at 650°C in the XRD patterns of the HB-FePc catalysts can be explained by reduction of the iron moieties, mostly iron oxide (Fe2O3). The presence of the iron oxide in HB550 and HB600 is quite in contrast to the FePc/PhRs catalysts. As already seen in the TG-DTA/DTG results, HB-FePc precursors are very reactive around 600°C. We believe that the biphenyl linker in the HB-FePc catalyst may connect phthalonitrile fragments decomposed from FePc, while the residual iron atoms dissociate and then generate clusters, which are easily oxidized in air to form iron oxides.Figure 4


Active site formation mechanism of carbon-based oxygen reduction catalysts derived from a hyperbranched iron phthalocyanine polymer.

Hiraike Y, Saito M, Niwa H, Kobayashi M, Harada Y, Oshima M, Kim J, Nabae Y, Kakimoto MA - Nanoscale Res Lett (2015)

XRD patterns. (a) FePc/PhRs and (b) HB-FePc catalysts.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4401482&req=5

Fig4: XRD patterns. (a) FePc/PhRs and (b) HB-FePc catalysts.
Mentions: In the XRD patterns shown in Figure 4, quite strong diffraction peaks are observed for the pristine FePc precursor as a result of its crystalline nature [27], whereas the HB-FePc precursor does not show any diffraction peaks because it has a large, randomly linked structure that does not readily crystallize. HB550 and HB600 have amorphous signatures as a broad structure around 26° and multiple iron oxide (Fe2O3) peaks, while above HB650 graphitized carbon network around 26° as well as iron metal (Fe) and/or iron carbides (Fe3C) around 45° appeared. Thus the formation of the graphitic structure and the iron metal and/or iron carbides are strongly correlated. This can be understood by considering the Yarmulke mechanism; i.e., formation of a graphitic carbon shell structure around reduced metal nanoparticles [20]. In the case of the FePc/PhRs catalysts, the intensity of the diffraction peaks at both 26° and 45° gradually increases from 600°C. These peaks are absent in the XRD pattern of HB600, indicating that the HB-FePc precursor has higher thermostability than the FePc/PhRs precursor. Instead, these peaks suddenly emerge at 650°C, and then their intensities remain almost unchanged up to 900°C. Therefore, the formation of reduced iron components and their subsequent graphitization may proceed in a different manner in the FePc/PhRs and HB-FePc catalysts against pyrolysis temperature, which is consistent with the TG-DTA/DTG results. The sudden appearance of the metallic iron components at 650°C in the XRD patterns of the HB-FePc catalysts can be explained by reduction of the iron moieties, mostly iron oxide (Fe2O3). The presence of the iron oxide in HB550 and HB600 is quite in contrast to the FePc/PhRs catalysts. As already seen in the TG-DTA/DTG results, HB-FePc precursors are very reactive around 600°C. We believe that the biphenyl linker in the HB-FePc catalyst may connect phthalonitrile fragments decomposed from FePc, while the residual iron atoms dissociate and then generate clusters, which are easily oxidized in air to form iron oxides.Figure 4

Bottom Line: The properties of the HB-FePc catalyst are compared with those of a catalyst with high oxygen reduction reaction (ORR) activity synthesized from a mixture of iron phthalocyanine and phenolic resin (FePc/PhRs).Electrochemical measurements demonstrate that the HB-FePc catalyst does not lose its ORR activity up to 900°C, whereas that of the FePc/PhRs catalyst decreases above 700°C.Consequently, effective doping of active nitrogen species into the sp (2) carbon network of the HB-FePc catalysts may occur up to 900°C.

View Article: PubMed Central - PubMed

Affiliation: Department of Applied Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656 Japan ; Current address: Toray Industries, Incorporated, Nihonbashi-Muromachi 2-chome, Tokyo, Japan.

ABSTRACT
Carbon-based cathode catalysts derived from a hyperbranched iron phthalocyanine polymer (HB-FePc) were characterized, and their active-site formation mechanism was studied by synchrotron-based spectroscopy. The properties of the HB-FePc catalyst are compared with those of a catalyst with high oxygen reduction reaction (ORR) activity synthesized from a mixture of iron phthalocyanine and phenolic resin (FePc/PhRs). Electrochemical measurements demonstrate that the HB-FePc catalyst does not lose its ORR activity up to 900°C, whereas that of the FePc/PhRs catalyst decreases above 700°C. Hard X-ray photoemission spectra reveal that the HB-FePc catalysts retain more nitrogen components than the FePc/PhRs catalysts between pyrolysis temperatures of 600°C and 800°C. This is because the linked structure of the HB-FePc precursor has high thermostability against nitrogen desorption. Consequently, effective doping of active nitrogen species into the sp (2) carbon network of the HB-FePc catalysts may occur up to 900°C.

No MeSH data available.