international journal of hydrogen energy 39 (2014) 934e941

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Effects of structures of pyrolyzed , and on oxygen reduction reaction

Sun-Tang Chang a, Hsin-Chih Huang a, Hsiao-Chien Wang a, Hsin-Cheng Hsu a, Jyh-Fu Lee b, Chen-Hao Wang a,* a Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan b National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan article info abstract

Article history: The effects of the structures of various pyrolyzed cobalt-based macrocyclic compounds, Received 9 July 2013 which are pyrolyzed cobalt-corrin compounds (py-Co-Corrin/C), pyrolyzed cobalt-corrole Received in revised form compounds (py-Co-Corrole/C) and pyrolyzed cobalt-porphyrin compounds (py-Co- 7 October 2013 Porphyrin/C), on their activities in the oxygen reduction reaction (ORR) are studied. Accepted 14 October 2013 Following the pyrolysis, py-Co-Corrin/C has a higher ORR activity than py-Co-Corrole/C Available online 3 December 2013 and py-Co-Porphyrin/C, which their electron-transfer numbers are 3.90, 3.87 and 3.37 at 0.3 V, respectively. According to the analysis of near edge X-ray absorption fine structure in e Keywords: N K-edge, only py-Co-Corrin/C has the Co N4 chelate and poly-aromatic hydrocarbons Corrin when pyrolyzed at high temperature. The X-ray absorption spectra reveal the oxidation Corrole states of central cobalt in cobalt-based macrocyclic compounds which are changed from Porphyrin high to low. Also, the atomic distances of cobalt to its neighbors vary among all of the Oxygen reduction reaction samples. The experimental results suggest that the structure and the coordination of cobalt-based macrocyclic compounds strongly affect their ORR activities. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

has published the first paper to use the macrocyclic com- 1. Introduction pound (i.e. Co phthalocyanine) for the ORR [4]. Chen et al. have reviewed the literature over the past 40 years, and they The oxygen reduction reaction (ORR) is a kinetically slow considered that pyrolyzed transition metal nitrogen- process because of its multi-step reaction [1]. The ORR is an containing macrocyclic compounds supported on carbon essential reaction occurs in the cathode of fuel cell. For the e materials (M Nx/C) were the most promising candidates to proton exchange membrane fuel cell (PEMFC), the high replace Pt-based catalysis [5]. Other review papers made by loading of Pt-based catalysis is employed to accelerate the Jaouen et al. [6] and Othman et al. [7] have also pointed out sluggish ORR [2]. However, the high cost of Pt is one of the e that M Nx/C catalysis may provide the performance and the main problems hindering the commercialization of PEMFC. stability close to those of Pt-based catalysis. Developing non-precious metal catalysis to replace Pt catal- The ORR has two main reaction pathways; one is the direct ysis could be feasible to the cost reduction [3]. In 1964, Jasinski pathway, by which oxygen is directly reduced to water with

* Corresponding author. Tel.: þ886 2 2730 3715; fax: þ886 2 2737 6544. E-mail addresses: [email protected], [email protected] (C.-H. Wang). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.10.082 international journal of hydrogen energy 39 (2014) 934e941 935

the transfer of four electrons; the other is the H2O2 pathway, To obtain the Co-corrole that contained cobalt and the by which oxygen is reduced to H2O2 by the transfer of two corrole ring molecule, cobalt(III) (triphenylphosphine)(5,10,15- electrons. These reactions are as follows. triphenylcorrolato) was synthesized as described in the liter- ature [16]. Then, the fabrication of pyrolyzed Co-corrole sup- þ k1 0 Reaction 1 : O2 þ 4H þ 4e /2H2O E ¼ 1:229 V (R1) ported by carbon black (py-Co-corrole/C) was followed by the process conditions that were performed by the fabrication of

þ k2 0 py-Co-Porphyrin/C. Reaction 2 : O2 þ 2H þ 2e /H2O2 E ¼ 0:695 V (R2) To obtain py-Co-Corrin, py-Co-Porphyrin and py-Co- The H2O2, the product of Reaction 2, may undergo the Corrole without the background associated with carbon following reaction. black, all precursors without carbon black were loaded into

þ fused aluminum oxide boats for the pyrolysis. Then, the above Reaction 3 : H O þ 2H þ 2e/2H O E0 ¼ 1:763 V (R3) 2 2 2 processes were carried out.

The H2O2 may also chemically decompose into O2 and H2O. Since the direct pathway has a higher thermodynamic po- 2.2. Transmission electron microscopy (TEM) tential than the H2O2 pathway, the cathode catalyst is ex- pected to catalyze the former reaction in a PEMFC. Numerous Transmission electron microscopy (TEM) was conducted studies have shown that the pyrolysis at a temperature in the using a JEOL-2000FX TEM system that was equipped with a range 600e1000 C substantially enhances the ORR activities high-speed slow-scan CCD camera with a LaB6 filament that of Fe- and Co-based macrocyclic compounds [8e13]. Although operated at 200 kV. these studies analyzed the transformation of these transition metal macrocyclic compounds in quaternary-type nitrogen or 2.3. X-ray absorption spectra of Co K-edge and N K-edge in two-nitrogen chelates such as FeeNx chelates and CoeNx chelates (x ¼ 2 or 4), they did not determine how the pyrolyzed The X-ray absorption spectrum (XAS) includes two regions structure affects ORR activity. which are X-ray absorption near-edge structure (XANES) and Corrinoid is formed by the direct connection of a tetra- extended X-ray absorption fine structure (EXAFS). In this pyrrolic ring with two pyrroles, which is a macrocyclic system study, the Co K-edge of XAS was used to understand the that contains four pyrrole rings similar to but with change in structure (site ligation) and oxidation state. More- a low symmetric nitrogen-containing chelates [14]. Rovria over, near edge X-ray absorption fine structure (NEXAFS) was et al. have discussed the structural and electronic properties analyzed the connection type of nitrogen and the ratio of p/s. of Corrin, Corrole, and Porphyrin, which the spin state and p These data were recorded by using the beam line 17C1 and aromatic vary among these N4-based macrocyclic compounds 20A1 at the National Synchrotron Radiation Research Center, [15]. The spin states of Co-Corrole and Co-Corrin are S ¼ 0, 1, 2, Hsinchu, Taiwan. The electron storage ring was operated at and those of Co-Porphyrin are S ¼ 1/2, 3/2, 5/2. In a p aromatic energy of 1.5 GeV with a beam current of 120e200 mA. The D 4 system, Co-Corrole exhibits C2v symmetry, Co-Porphyrin ex- resolution ( E/E) of beam line was better than 2 10 with e hibits D4h symmetry, and Co-Corrin exhibits C2 symmetry [15]. the energy range of 5 15 keV. The study used fluorescence Three Co-based macrocyclic compounds, (Co- mode because the ratio of cobalt in catalyst was less than 1%. Corrin), CoTMPP (Co-Porphyrin) and Co-Corrole are investigated The powdered catalyst was pressed into a hole of the herein. Some of their characteristics have been examined in the stainless-steel holder and then placed in situ in a cell under authors’ previous investigations [16e19], but the present study room temperature. The standard sample of Co foil was provides new relationship between their pyrolyzed structures measured simultaneously in the third ionization chamber to and the ORR activities. After the pyrolysis, the morphology, calibrate the energy of Co (0) at 7709 eV during every sample oxidation state number, coordination number and neighboring scan. The data pre-treatment was included by background atomic distances in CoeNx chelates are studied to elucidate the subtraction, normalization with respect to the edge jump, effect of the pyrolyzed structures on ORR activities. Fourier transformation, and curve fitting, which was done by ATHENA. For N K-edge measurements, data were collected at the beam line of a 6-m high-energy spherical grating mono- 2. Experiment chromator (HSGM) with 40 mm opening slits, corresponding to 0.3 eV energy resolutions for the N K-edge energy range. All 2.1. Preparation of py-Co-corrin/C, py-Co-porphyrin/C samples were briquetted, attached to conducting tape, and and py-Co-corrole/C then subjected to an ultrahigh vacuum chamber (10 9 Torr). The spectra were recorded in total electron yield mode with a Vitamin B12, which includes cobalt and a corrin ring molecule, microchannel plate as detector. Each spectrum was calibrated was used to fabricate pyrolyzed Co-Corrin supported by car- by using the known K-edge absorption peak at 401.1 eV of BN. bon black (py-Co-Corrin/C). The preparing steps have been reported by our previous work [17,19]. CoTMPP (cobalt (II) 2.4. Electrochemical measurement tetramethoxyphenylporphyrin), comprising cobalt and a porphyrin ring molecule, was utilized to fabricate pyrolyzed The three-cell electrode was used in this study. The rotating- Co-Porphyrin that was supported by carbon black (py-Co- ring disk electrode (RRDE, PINE AFE7R9GCPT) with a glassy Porphyrin/C). The preparing steps have been reported by our carbon (GC) disk was used as the working electrode, which the previous works [18]. GC area was 0.2472 cm 2 and a platinum ring was used herein. Download English Version: https://daneshyari.com/en/article/1276540

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