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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.


N 1s HXPES spectra. (a) FePc/PhRs and (b) HB-FePc catalysts, each spectrum fitted with Voigt functions followed by background subtraction by the Shirley method (dashed line). Orange, green, red, and purple solid lines are pyridine-like or FePc (NP1), pyrrole- or cyanide-like (NP2), graphite-like (NP3), and oxide (NP4) nitrogen components, respectively. (c) Structural formulae of four nitrogen components in graphite. (d) Plot of calculated nitrogen content as a function of pyrolysis temperature.
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Fig7: N 1s HXPES spectra. (a) FePc/PhRs and (b) HB-FePc catalysts, each spectrum fitted with Voigt functions followed by background subtraction by the Shirley method (dashed line). Orange, green, red, and purple solid lines are pyridine-like or FePc (NP1), pyrrole- or cyanide-like (NP2), graphite-like (NP3), and oxide (NP4) nitrogen components, respectively. (c) Structural formulae of four nitrogen components in graphite. (d) Plot of calculated nitrogen content as a function of pyrolysis temperature.

Mentions: Figure 7a,b shows N 1s HXPES spectra of the FePc/PhRs and HB-FePc catalysts, respectively. To reveal the chemical states of nitrogen, each spectrum was fitted with a Voigt function and decomposed into different chemical species (Figure 7c). The Gaussian width was used as a common parameter among chemical species to fit the experimental data. The Lorentzian width and asymmetric parameter α were fixed to 0.25 eV and 0.10, respectively, where α is adjusted to the mean value for the C 1s HXPES spectra. By applying the above conditions, we were able to decompose the N 1s HXPES spectra into four peaks, NP1 to NP4 [7,30,33-37]. The resultant fitting parameters and relative composition ratios of the four nitrogen components of the FePc/PhRs and HB-FePc catalysts are summarized in Tables 2 and 3, respectively. The nitrogen content of FePc/PhRs was not calculated because of contamination with indium. Peak NP1 was observed at 398.2 to 398.6 eV except for FePc, where it appeared at 399.1 eV, and can be assigned to pyridine-like nitrogen (nitrogen atoms with two carbon neighbors in an aromatic ring) [7,33,35] or nitrogen of FePc [7,30]. Considering that the majority of the Fe-N4 structure of FePc is decomposed below 600°C as revealed by Fe 2p XPS [33], NP1 is assigned to pyridine-like nitrogen. Peak NP2 (399.8 to 400.2 eV) may be a mixture of pyrrole-like nitrogen (nitrogen atoms with two carbon neighbors in a five-membered aromatic ring) [7,33] and cyanide-like nitrogen (one neighboring carbon atom with a triple bond) [34]. Peak NP3 (400.6 to 401.1 eV) can be attributed to graphite-like nitrogen, which is nitrogen with three carbon bonds incorporated into an aromatic ring [7,35-37]. The binding energy of peak NP4 varies randomly for each catalyst (403.0 to 403.6 eV). Based on the previous reports [7,33], NP4 is assigned to N-oxide groups.Figure 7


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)

N 1s HXPES spectra. (a) FePc/PhRs and (b) HB-FePc catalysts, each spectrum fitted with Voigt functions followed by background subtraction by the Shirley method (dashed line). Orange, green, red, and purple solid lines are pyridine-like or FePc (NP1), pyrrole- or cyanide-like (NP2), graphite-like (NP3), and oxide (NP4) nitrogen components, respectively. (c) Structural formulae of four nitrogen components in graphite. (d) Plot of calculated nitrogen content as a function of pyrolysis temperature.
© Copyright Policy - open-access
Related In: Results  -  Collection

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Fig7: N 1s HXPES spectra. (a) FePc/PhRs and (b) HB-FePc catalysts, each spectrum fitted with Voigt functions followed by background subtraction by the Shirley method (dashed line). Orange, green, red, and purple solid lines are pyridine-like or FePc (NP1), pyrrole- or cyanide-like (NP2), graphite-like (NP3), and oxide (NP4) nitrogen components, respectively. (c) Structural formulae of four nitrogen components in graphite. (d) Plot of calculated nitrogen content as a function of pyrolysis temperature.
Mentions: Figure 7a,b shows N 1s HXPES spectra of the FePc/PhRs and HB-FePc catalysts, respectively. To reveal the chemical states of nitrogen, each spectrum was fitted with a Voigt function and decomposed into different chemical species (Figure 7c). The Gaussian width was used as a common parameter among chemical species to fit the experimental data. The Lorentzian width and asymmetric parameter α were fixed to 0.25 eV and 0.10, respectively, where α is adjusted to the mean value for the C 1s HXPES spectra. By applying the above conditions, we were able to decompose the N 1s HXPES spectra into four peaks, NP1 to NP4 [7,30,33-37]. The resultant fitting parameters and relative composition ratios of the four nitrogen components of the FePc/PhRs and HB-FePc catalysts are summarized in Tables 2 and 3, respectively. The nitrogen content of FePc/PhRs was not calculated because of contamination with indium. Peak NP1 was observed at 398.2 to 398.6 eV except for FePc, where it appeared at 399.1 eV, and can be assigned to pyridine-like nitrogen (nitrogen atoms with two carbon neighbors in an aromatic ring) [7,33,35] or nitrogen of FePc [7,30]. Considering that the majority of the Fe-N4 structure of FePc is decomposed below 600°C as revealed by Fe 2p XPS [33], NP1 is assigned to pyridine-like nitrogen. Peak NP2 (399.8 to 400.2 eV) may be a mixture of pyrrole-like nitrogen (nitrogen atoms with two carbon neighbors in a five-membered aromatic ring) [7,33] and cyanide-like nitrogen (one neighboring carbon atom with a triple bond) [34]. Peak NP3 (400.6 to 401.1 eV) can be attributed to graphite-like nitrogen, which is nitrogen with three carbon bonds incorporated into an aromatic ring [7,35-37]. The binding energy of peak NP4 varies randomly for each catalyst (403.0 to 403.6 eV). Based on the previous reports [7,33], NP4 is assigned to N-oxide groups.Figure 7

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.