Pyrolysis of IronâVitamin B9 As a Potential Nonprecious Metal

Research Article pubs.acs.org/journal/ascecg

Pyrolysis of Iron−Vitamin B9 As a Potential Nonprecious Metal Electrocatalyst for Oxygen Reduction Reaction Hsin-Chih Huang,† Sun-Tang Chang,‡ Hsin-Cheng Hsu,† He-Yun Du,§ Chen-Hao Wang,*,† Li-Chyong Chen,*,∥ and Kuei-Hsien Chen*,§,∥ †

Department of Materials Science and Engineering, National Taiwan University of Science and Technology, No.43, Keelung Rd., Sec.4, Da’an Dist., Taipei 10607, Taiwan ‡ National Synchrotron Radiation Research Center, 101 Hsin-Ann Road, Hsinchu Science Park, Hsinchu 30076, Taiwan § Institute of Atomic and Molecular Science, Academia Sinica, No. 1, Sec. 4, Roosevelt Rd., Taipei 10617, Taiwan ∥ Center for Condensed Matter Sciences, National Taiwan University, No. 1, Sec. 4, Roosevelt Rd., Taipei 10617, Taiwan S Supporting Information *

ABSTRACT: This study presents the performance of a carbon-black-supported pyrolyzed vitamin B9 (folic acid)-treated cathode catalyst (py-Fe-FA/C) in the oxygen reduction reaction (ORR) and proton exchange membrane fuel cell (PEMFC). Electrochemical ORR measurements revealed that using py-Fe-FA/C resulted in excellent ORR activity through the direct four-electron reduction pathway. The H2−O2 PEMFC with py-Fe-FA/C in the cathodic side produces a maximum power density of 330 mW cm−2 with the 80 °C operation temperature and the 1 atm back pressure. X-ray photoelectron spectroscopy and in situ X-ray adsorption spectroscopy proved that the enhanced ORR activity was caused by the network structure of polyaromatic hydrocarbons, quaternary-type (graphitic) nitrogen, and the coordination structure of the py-Fe-FA/C, as confirmed by the ORR mechanism study using detailed XPS and in situ X-ray adsorption spectroscopy. Particularly, in situ X-ray adsorption spectroscopy elucidated the ORR mechanism of the py-Fe-FA/C. KEYWORDS: Oxygen reduction reaction, Fuel cell, Vitamin B9, Electrocatalyst, In-situ X-ray adsorption spectroscopy

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INTRODUCTION A proton exchange membrane fuel cell (PEMFC) converts the chemical energy into electrical energy via the redox reaction of hydrogen and oxygen. PEMFC is a clean-energy device that can be used for both transport and stationary applications. However, the requirement of using an expensive Pt catalyst in PEMFC is a major hindrance to developing additional applications for this technology. An effective option is to use nonprecious metal catalysts at the cathode for catalyzing the oxygen reduction reaction (ORR). The ORR is a crucial cathode electrochemical reaction that generates energy in a fuel cell. The ORR is a slow-rate reaction step in the overall reaction of a fuel cell, and the electrode material also affects the kinetic pathway of the ORR. Currently, Pt and Pt-based catalysts are extensively used to speed up the ORR; however, platinum is a rare metal, and the catalysts are thus expensive, and therefore how to replace Pt catalysts with nonprecious metal catalysts is an issue. Recently, Kong et al. presented 3D ordered mesoporous Fe-porphyrin-like material obtained by nanocasting tetrapyridylporphyrin iron(III) chloride into SBA-15. After pyrolysis and HF etching, many Fe−Nx active sites were embedded in the mesoporous graphite backbones, and the electrical and mechanical contacts were improved. Therefore, the Fe−N−C catalysts showed exceptional ORR catalytic activities in both © 2017 American Chemical Society

alkaline solution and acid solution, which were in contrast to commercial Pt catalysts.1 Merzougui et al. synthesized Fe-based nonprecious metal catalysts by coating polyaniline on multiwalled carbon nanotubes, for which the catalysts showed excellent ORR activities in both acid and alkaline media. After 20 000 potential cycles, the half-wave potential only showed 40 mV decay in acid media.2 Chen et al. pyrolyzed FeSO4, poly(ethylenimine), and SiO2 template (Fe + N + S) to fabricate codoped N- and S-mesoporous carbon materials.3 The Fe + N + S catalysts showed half-wave potential and electron transfer number greater than 0.68 and 3.76, respectively, showing that the ORR performance was enhanced by the synergetic effect of the Fe+N+S combination. Zhang et al. used reduced graphene oxide to support nonprecious metal electrocatalysts (Fe−Nx/rGO) used in the ORR; tripyridyl triazine (TPTZ) and iron acetate were used as a ligand and metal precursor, respectively.4 After the pyrolysis, the catalyst structure contained framework defects, which were expected to influence both the ORR activity and stability. Because 2,6-bis(2pyridyl)-pyridine (TPY) has a structure similar to that of TPTZ, Wang et al. used TPY as a ligand to prepare Fe-TPY Received: July 27, 2016 Revised: February 14, 2017 Published: March 7, 2017 2897

DOI: 10.1021/acssuschemeng.6b01764 ACS Sustainable Chem. Eng. 2017, 5, 2897−2905

ACS Sustainable Chemistry & Engineering

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complexes for the ORR.5 The overall electron transfer number of the optimal active catalyst for the catalyzed ORR was 3.7, indicating that the atomic N contributing to the ORR ability of the Fe−N/C catalyst may originate from the pyridinic and graphitic nitrogen formed during pyrolysis. The single cell test performed that the maximum power density was 0.08 W cm−2 at 60 °C without back pressure. Furthermore, Peng et al. used poly-o-phenylenediamine (oPD), melamine (Mela), and iron as a hybrid precursor into pyrolysis.6 The catalyst had high surface area and a high amount of pyridinic and graphitic nitrogen and exhibited high ORR performance in both alkaline and acid solution. The maximum power density of Fe/oPD−Mela was 0.27 W cm−2. Since the investigation of cobalt phthalocyanine as the catalyst for the ORR by Jasinski in 1964,7 various studies have synthesized macrocyclic complexes by using transition metals.8−15 The inspiration for synthesizing such complexes was Heme in the blood. Chang et al. investigated using pyrolyzed vitamin B12 supported on carbon black material (py-B12/C) as a catalyst for the ORR in PEMFCs.16,17 From their analyzed results and calculations, they indicated that py-B12 with a low-symmetry Co-corrin structure had the preferable ORR path, which was not appropriate to a highsymmetry porphyrin structure. Wang et al. reported a carbonaceous oxygen reduction catalyst based on a simple and environmentally friendly strategy by using folic acid as a precursor, showing the enhanced activity and stability for the ORR in alkaline media because of its large surface area and good porosity.18 Additionally, erythropoiesis is the process through which red blood cells are produced, which requires nutrients such as iron, vitamin B12, vitamin B9, copper, erythropoietin, and testosterone. For the maturation of red blood cells, vitamin B12 and vitamin B9 are essential, and a lack of either of these nutrients causes maturation failure.19 The central metal ion in macrocyclic compounds appears crucial in the mechanism of ORR. Metal ion centers of Fe and Co have been discovered to show optimal catalytic features, which are due to the obvious redox features and have been suggested to serve as ORR active sites.20−24 In addition, the property of the metal−ligand interaction is crucial in the ORR activity of such complexes, too. Especially, the metal ion centers with high ionization potential have been regarded a critical factor affecting the ORR activity of the complexes.25−27 Studies on the ORR ability including activity and stability of nonprecious metal catalysts have been reviewed.22,24,28−31 Several factors, including the presence of transition metal nitrogen-containing complexes, the type of transition metal, the content of nitrogen, the surface properties of the carbon support, and the loading, affect the ORR characteristics. The present study proposes a simple approach for synthesizing the nonprecious metal catalyst for fuel cells; the approach involves mixing vitamin B9 (as a new nitrogen precursor), iron, and carbon black together. The pyrolyzed vitamin-B9-treated catalyst supported on carbon black material (py-Fe-FA/C) showed higher ORR catalytic activity through the direct fourelectron reduction pathway compared with other Fe-based nonprecious metal catalysts. The study first reports a pyrolyzed vitamin-B9-treated catalyst that can be used as the nonprecious metal catalyst for fuel cells. The facile, environmentally friendly, and renewable nonprecious metal catalyst had an efficient catalytic ability which made it a promising catalyst for a variety of energy technologies and applications.

Research Article

EXPERIMENTAL SECTION

Preparation of Pyrolyzed Fe-FA/C. The catalyst was prepared in accordance with the subsequent process: 0.25 g of vitamin B9 (99%, Aldrich) and 0.28 g of FeCl3·6H2O (99%, Acros) were dissolved in 10 mL of deionized water with 30 min of stirring at ambient temperature, and subsequently 0.05 g carbon black (Vulcan XC-72R) was introduced to the suspension for another 30 min stirring at ambient temperature. To eliminate the solvent, the mixture was heated to 80 °C. The obtained slurry was dried at ambient temperature in a vacuum oven for 12 h. For the pyrolysis, the slurry was filled into an aluminum oxide boat and introduced into a furnace in a quartz tube. The specific pyrolysis temperature (500, 700, and 900 °C) was raised at a rate of 20 °C min−1 in a nitrogen atmosphere, and the boat kept in the furnace for 3 h. Subsequently, the furnace cooled to ambient temperature naturally to get dried catalyst powder. When the sample was pyrolyzed at 700 °C, it was denoted as py-Fe-FA/C-700 in this study. Preparation of Pyrolyzed Iron(II) Phthalocyanine/C and Fe/ C. For the comparison, pyrolyzed iron(II) phthalocyanine/C (py-FePc/C) and Fe/C were also prepared in this study. Py-FePc/C was prepared as follows: 0.1 g of iron(II) phthalocyanine (FePc, 99%, Acros) and 0.4 g of carbon black (Vulcan XC-72R) were mixed and then dispersed in tetrahydrofuran (99.7%, Alfa Aesar) solvent. After 30 min of ultrasonication, it formed the homogeneous solution. This solution was heated to 80 °C to remove the solvent, and the obtained slurry was dried at ambient temperature in a vacuum oven for 12 h. Eventually, the py-FePc/C was pyrolyzed at 700 °C. Similarly, to prepare the Fe/C, 0.29 g of dried iron(III) chloride (FeCl3, 98%, Acros) and 0.4 g of carbon black (Vulcan XC-72R) were dispersed in deionized water, and the aforementioned py-FePc/C procedure was followed. Finally, the mixture was pyrolyzed in a 5% hydrogen atmosphere at 200 °C for 2 h, making sure that the catalyst was formed in the metallic state completely. Material Analysis. Raman spectroscopy (Jobin-Yvon LabRAM HR800 confocal micro-Raman spectroscopy, excitation source: 633 nm He−Ne laser) was used to study the structural change of pyrolyzed Fe-FA/C. X-ray photoelectron spectroscopy (XPS, VG ESCA Scientific Theta Probe, 1486.6 eV Al Kα source) was employed to investigate the elemental distribution and changes in the N 1s spectra of py-Fe-FA/C. The X-ray absorption near-edge structure (XANES) at the Fe K-edge was employed using the beamline 17C1 at the National Synchrotron Radiation Research Center (NSRRC), Hsinchu, Taiwan, which depends on a multipole wiggler source with 2.7 keV critical energy and a 1.5 GeV electron storage ring with a 120−200 mA beam current. All spectra were received in the transmission mode at room temperature, and the intensities of the incident and transmitted X-ray beams were estimated using gas-filled ionization chambers. An iron foil as the standard material was measured simultaneously in the third ionization chamber to enable calibrating the energy scale in each scan. For comparison, 5,10,15,20-tetrakiskis(4-methoxyphenyl)-21H,23H-porphine iron(III) chloride (FeTMPPCl) and pyrolyzed FeTMPPCl were prepared as the references. The py-FeTMPPCl was prepared as follows: FeTMPPCl (Sigma-Aldrich) was dispersed in tetrahydrofuran (99.7%, Alfa Aesar) solvent, which was then for 30 min ultrasonicated to form a homogeneous solution. This solution was heated to 80 °C to remove the solvent, and the obtained slurry was dried at room temperature in a vacuum oven for 12 h. At last, the heat treatment for py-FeTMPPCl was at 700 °C. In situ X-ray absorption spectroscopy (XAS) at the Fe K-edge were performed using the same beamline at NSRRC (17C1) and in a three-compartment cell by using a potentiostat/galvanostat instrument (Bio-Logic BiStat). The catalyst was painted by hand onto a 0.25 cm2 gold mesh working electrode by using Nafion solution. Pt foil and a saturated calomel electrode (0.242 V vs a normal hydrogen electrode, NHE) were used as the counter electrode and reference electrode, respectively. All potentials in this work are expressed relative to the reversible hydrogen electrode (RHE). The electrolyte was oxygen-saturated 0.1 M HClO4 solution. The IFEFFIT software was used to analyze the results. 2898


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Figure 1. (a) ORR curves for py-Fe-FA/C at different temperatures, (b) the n values, and (c) %H2O2 of the catalysts’ dependence on disk potentials. Rotating speed, 1600 rpm; scan rate, 10 mV s−1; ring potential, 1.2 V.

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Electrochemical Measurements. All electrochemical measurements were carried out in a three-compartment cell by using a potentiostat/galvanostat instrument (Bio-Logic BiStat). The working electrode was a rotating ring-disk electrode (RRDE, PINE AFE7R9GCPT) with a glassy carbon (GC) disk and platinum ring. The counter electrode was a Pt foil, and the reference electrode was saturated calomel electrode (0.242 V vs NHE). All potentials are versus RHE in this study. The electrolyte was oxygen-saturated 0.1 M HClO4 solution for all electrochemical measurments. The catalyst ink was prepared as follows: 160 mg of the catalyst was mixed with 20 mL of deionized water and subsequently 20 μL of the ink and 5 μL of 0.1 wt % Nafion solution were dropped onto the GC disk. Then, the catalyst ink was dried in air at ambient temperature. The linear sweep voltammetry was applied on the sample at the specific scan rate. To reduce the non-Faradaic current produced by the catalysts, the ORR curves for the GC disk were performed at a low scan rate of 10 mV s−1. To evaluate the yield of hydrogen peroxide in the ORR catalyzed by the catalyst on the GC disk, a potential of 1.2 V vs RHE was applied to the platinum ring to oxidize hydrogen peroxide to generate the oxidation current. Fuel Cell Test. The peformance of PEMFC was carried out by a 5 cm2 area of membrane electrode assembly (MEA), which was tested by the fuel cell test station (Asia Pacific Fuel Cell Technologies, Ltd.). The MEA was obtained by hot-pressing two electrodes on both sides of a Nafion 212 membrane (H+, DuPont) at 135 °C with a pressure of 130 kg cm−2 for 2 min. To prepare a specific cathode, py-Fe-FA/C was dispersed in 5 wt % Nafion solution, which was used as a catalyst ink of the cathode; the mass ratio of catalyst to dry Nafion was 1:2. The cathodic catalyst ink was painted by hand onto a carbon cloth with a microporous layer, attaining a py-Fe-FA/C loading of approximately 6.0 mg cm−2, and the cathodic electrode was then dried at ambient temperature for 6 h. The anode of the MEA was a commercial Pt/C electrode (E-TEK) with a 0.25 mg cm−2 metal loading. A polarization measurement was performed on the PEMFC at 80 °C by using hydrogen and oxygen flowing through the anode and cathode. Before entering the MEA, hydrogen and oxygen were passed through humidifiers at 80 °C. The back-pressure gages on the anode side and cathode side were fixed to 1 atm. The voltage and the current of the cell were recorded after reaching the steady values.

RESULTS AND DISCUSSION

The ORR pathway mainly involves the following reactions: Reaction 1: O2 + 4H+ + 4e− → 2H 2O...

E° = 1.229 V (1)

Reaction 2: O2 + 2H+ + 2e− → H 2O2 ...

E° = 0.695 V (2)

Reaction 3: H 2O2 + 2H+ + 2e− → 2H 2O...

E 0 = 1.763 V (3)

H2O2 can chemically decompose into O2 and H2O. Reaction 1 is a direct reduction pathway that includes a fourelectron transfer, and reaction 2 is an H2O2 pathway involving a two-electron transfer. Since the direct ORR pathway yields the higher thermodynamically reversible potential than an H2O2 pathway, reaction 1 is highly preferable to reaction 2 as the ORR in the PEMFC. At a specific pyrolysis temperature, the structures of transition metal complexes are partially or completely destroyed, resulting in new active sites with higher and more stable ORR activity than the untreated one. To understand the effect of the pyrolysis temperature, pyrolyzed Fe-FA supported on the carbon black material was prepared at 500 °C, 700 °C, and 900 °C to form the catalysts py-Fe-FA/C-500, py-Fe-FA/ C-700, and py-Fe-FA/C-900, respectively. Figure 1a depicts the ORR activity of the three compounds. The lower part of the figure plots the disk current (Id) as a function of the applied potential, and the upper part plots the ring current (Ir) against the applied potential. In particular, the py-Fe-FA/C-700 showed the highest absolute Id value and the lowest absolute Ir value. The overall electron transfer number (n) and yield 2899


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Figure 2. (a) ORR curves for py-Fe-FA/C, py-FePc/C, and Fe/C; (b) the n values; and (c) %H2O2 of the catalysts’ dependence on disk potentials. Rotating speed, 1600 rpm; scan rate, 10 mV s−1; ring potential, 1.2 V.

of hydrogen peroxide (%H2O2) in the catalyzed ORR were used: n=

Table 1. ORR Performances of Fe-based Nonprecious Catalysts Reported in the Papers

4Id Id +

%H 2O2 =

Ir N

(4) 2I r N

Id +

Ir N

× 100% (5)

where N, which denotes the RRDE collection efficiency, was determined to be 0.383. Figure 1b and c display the n values and %H2O2, respectively. Moreover, to understand the effect of nitrogen precursor/carbon ratio, the ORR activity of py-Fe-FA/ C-700 at different ratios of nitrogen precursor to carbon by the amount of mass and without carbon black support was determined as shown in Figure S1. The ORR activity of py-Fe-FA/C-700 is compared with other commonly iron-based nonprecious metal catalysts, and Figure 2a plots the ORR activity of the py-Fe-FA/C-700 (named py-Fe-FA/C here), py-FePc/C, and Fe/C. The typical ORR curve for acid solution reveals three dominant potential regionsthe kinetic range (>0.8 V), the mixed range (0.8−0.6 V), and the mass-transfer range (3.90 3.99 3.83 3.80−3.95 3.90 3.89 3.97 3.6−4.00 3.80 3.80 3.98

32 33 34 35 36 37 38 39 40 41 42 43 44 this work

Compared with other Fe-based catalysts, the catalytic quality of py-Fe-FA/C shows competitive results not only for onset potential but also electron transfer number. Figure 3 exhibits the Raman spectra of pristine folic acid and py-Fe-FA-700/C. At the pyrolyzed temperature of 700 °C, two strong peaks are found at 1330 and 1580 cm−1 from the py-Fe-FA/C, which are respectively related to the D and G peaks of carbon-like materials, indicating that the py-Fe-FA700/C forms a network structure of poly aromatic hydrocarbons. The presence of graphite-like carbon is likely to improve the corrosion resistance and electronic conductivity of the catalysts.22,45−48 Figure 4a and b show the XPS N 1s spectra of pristine Fe-FA/C and py-Fe-FA-700/C, respectively, and the corresponding fitting results are listed in Table 2. The XPS spectrum of pristine Fe-FA/C shows the peaks corresponding to cyanide, pyrrolic-type nitrogen, and quaternary-type nitrogen at 399.4, 2900


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Table 2. Fitting Results from XPS N 1s Spectra of Figure 4

N 1s (atomic %) Pristine Fe-FA/C Py-Fe-FA/C-700

quaternary- pyrrolic-type type nitrogen nitrogen cyanide (401.4 eV) (400.3 eV) (399.4 eV) 7.0% 23.3%

26.0% 30.2%

pyridinictype nitrogen (398.5 eV)

67.9% 46.5%

Fe-base catalysts.61 The method of the durability test was followed by previous studies.62,63 The 100-h durability PEMFC test by using the py-Fe-FA/C was performed at 0.4 V, flowing H2 and air fed into the anode and cathode, respectively, as shown in Figure 5b. The current density is decreased about approximately 40% in 100 h, although the reason is unclear. One of the possibilities is water flooding, which occurred at the cathode, resulting in the degradation of the current density. Figure 6a depicts the XANES spectra at the Fe K-edge of the pristine Fe-FA, py-Fe-FA-700, pristine iron(III) tetramethoxyphenylporphyrin chloride (FeTMPPCl), and py-FeTMPPCl700. The XANES spectra between 7100 and 7120 eV indicate a strong characteristic peak of the pristine FeTMPPCl and pristine Fe-FA at approximately 7113 eV. The pristine FeTMPPCl is a five-coordinated species, whereas the pristine Fe-FA is a sixcoordinated species because the FeCl3·6H2O in the pristine Fe-FA has an octahedral structure. By contrast, the peaks associated with the py-FeTMPPCl-700 and py-Fe-FA-700 shift to a higher energy and have a lower intensity, revealing that the characteristic peak of a species with a lower coordination number corresponds to a higher energy. This promotes the coordination of vacant sites with O2. Similar results have been reported in several studies of other M−N4 moieties in the ORR with the M−N4 moiety being destroyed partially or completely during pyrolysis.64,65 To acquire more structural information on the py-Fe-FA-700 compound, the Fe K-edge spectra are further resolved by using the extended X-ray absorption fine structure (EXAFS), and the accurate metal to ligand distances and coordination number of the central Fe in the py-Fe-FA-700 are obtained. Figure 6b shows the Fourier transformation of the k3-weighted K-edge EXAFS measurements for the pristine Fe-FA and py-Fe-FA-700 with clear differences. In the fitting results for the Fe interatomic lengths, the peak of the Fe−Cl bond of the pristine Fe-FA is at 2.06 Å, which is consistent with the XANES results.66 Moreover, two peaks are observed in the py-Fe-FA-700, one at 2.5 Å and the other, which is a considerably smaller peak, at approximately 1.95 Å. These peaks correspond to Fe−Fe and Fe−N bonds, respectively. Therefore, the XAS results (XANES and EXAFS) indicate that the octahedral structure of the FeCl3· 6H2O in the pristine Fe-FA changes after the pyrolysis, and that the iron atoms coordinate with the nitrogen atoms to form

Figure 3. Raman spectra of py-Fe-FA/C-700 and pristine folic acid.

400.3, and 401.4 eV, respectively. After the pyrolysis, it shows pyridinic-type nitrogen, pyrrolic-type nitrogen, and quaternarytype nitrogen at 398.5, 400.3, and 401.4 eV, respectively.49−51 Chung et al. suggested that quaternary-type nitrogen can lower the band gap energy of carbon and probably promotes catalytic activity.52 Liu et al. suggested that quaternary-type nitrogen is less susceptible to the protonation reaction under acid conditions; this low susceptibility explained the high stability after the pyrolysis.53 Wu et al. stated that different forms of nitrogen, especially pyridinic and quaternary, can serve as n-type carbon dopants in metal−nitrogen−carbon catalysts, assisting in the generation of disordered carbon nanostructures and donating electrons to the carbon, and that such CNx structures may play an important part in ORR active sites.54 Lately, studies on nitrogen-doped carbon catalysts have presented that the graphitic (quaternary) nitrogen structure shows ORR activity.55−60 In Fe-FA/C that has been pyrolyzed at a high temperature, the catalytic site shows considerably high activity and stability for the ORR. The N 1s region of the XPS spectra shows that the binding energies were altered by the pyrolysis process, indicating that the interaction among the iron, nitrogen, and carbon altered the profile of the adsorbed/desorbed energies of O2, thereby improving the activity of the ORR. In addition to understanding the nitrogen structure of Fe-FA/C before and after pyrolysis, the elemental distribution also analyzes from XPS results. Table S1 presents the elemental content for the py-Fe-FA/C-700 in accordance with the XPS fitting results. The Fe content of py-Fe-FA/C-700 is 0.82 atom %. Figure 5a plots the polarization curve of the PEMFCs with py-Fe-FA/C in the cathodes; the PEMFCs show the open circuit potential and maximum power density of 0.820 V and 330 mW cm−2, respectively, which is rather average value for

Figure 4. XPS showing N 1s spectra of (a) pristine Fe-FA/C and (b) py-Fe-FA-700/C. 2901


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Figure 5. (a) Polarization curves of the H2−O2 PEMFCs using py-Fe-FA/C as the cathodes. (b) The 100-h durability test of H2-Air PEMFC using py-Fe-FA/C. Operation temperature, 80 °C; back pressure, 1 atm; anode catalyst, 0.25 mg cm−2 of Pt/C; cathode catalyst, 6.0 mg cm−2 of py-Fe-FA/C; electrolyte, Nafion 212 (H+, DuPont).

Figure 6. (a) XANES spectra of pristine Fe-FA, py-Fe-FA-700, pristine FeTMPPCl, and py-FeTMPPC-700. (b) Fourier transforms of k3-weighted EXAFS data at the Fe K-edge for pristine Fe-FA and py-Fe-FA-700. Note: all the atomic distances given here are with phase correction.

bined density functional theory (DFT) calculations and EXAFS analysis and inferred that the weakening of the interplay between oxygen atoms and nonprecious metal catalysts on the basis of the ordered mesoporous porphyrinic carbons (M−OMPC; M = Fe, Co, or FeCo) compared with the interaction with Pt/C. They concluded that the high ORR ability of FeCo−OMPC was due to the relatively weak interplay between the catalyst and oxygen.66 To understand more structural information on the adsorbed oxygen molecule and metal and the adsorption mechanism during the ORR of the py-Fe-FA-700/C, the in situ Fourier transforms measurements of the Fe K-edge EXAFS during the ORR were obtained (Figure 7) by scanning from 1.2 to 0.2 V

the active sites. The EXAFS spectra including Fe foil, pristine FeTMPPCl, pristine Fe-FA, and py-Fe-FA-700 are shown in Figure S2 as a reference. Anderson et al. used quantum theory to predict the active site of pyrolyzed iron-N4 catalysts for the direct four-electron oxygen reduction. Initially, iron(II) is more favorable in the first step of the mechanism than iron(III). The bonding between H2O and iron(III) is stronger than that of iron(II), and it blocks the adsorption of oxygen, affecting the reversible potentials and activation energies of the ORR. Therefore, iron(II) is estimated to be the active site for the direct four electron transfer of the ORR.67 Schulenburg et al. proposed a model of the structure of the active site of Fe-based nonprecious catalysts to explain the catalytic mechanism.68 For geometrical reasons and due to the already occurring bonding between the nitrogen ligands and the graphene layers, it is not probable that more than four nitrogen atoms are coordinated to the iron(III) center. The missing ligands could have been adsorbed by oxygen or could have interacted with a neighboring graphene layer. According to time-of-flight secondary ion mass spectrometry results of a pyrolyzed FeTMPPCl catalyst, Lefèvre et al. have suggested that FeCxNy was responsible for the catalytic activity.69,70 Bron et al.71 and Bouwkamp-Wijnoltz et al.72 have proposed that the ORR activity was contributed by FeNx sites, and from EXAFS measurements, they found that each iron atom was coordinated with three or four nitrogen atoms. Ziegelbauer et al. confirmed the direct spectroscopic findings of the geometry of oxygen adsorbed on the pyropolymer active sites by using in situ EXAFS measurements. They observed that the specific geometrical adsorption of molecular oxygen relative to the plane of the M−Nx moieties extremely influenced the ORR pathway.73 Cheon et al. com-

Figure 7. Fourier transforms of k3-weighted EXAFS data at the Fe K-edge for py-Fe-FA/C at different potentials in 0.1 M HClO4 solution.

and back to 1.2 V, without bias. Two major characteristic peaks are observed, corresponding to Fe−N and Fe−Fe bonds, respectively. Furthermore, a peak with gradually enhanced 2902


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intensity is observed during the scan, corresponding to the Fe−O bond. The Fe−O bond is not observed at 1.2 V, but it appears at 1.0 V, and its intensity continuously is increased until the potential of 0.2 V is reached. After the scanning, the Fe−O bond remains, but with a lower intensity than that at 0.2 V. This observation indicates that oxygen is adsorbed with Fe and the ORR is initiated. During the reaction, a greater amount of oxygen is required in order to generate a higher reductive current and yield the desired electrochemical results. However, after the ORR is complete, the active sites are occupied continuously by some oxygen atoms. Chen et al. and Guo et al. have used DFT calculations to investigate the adsorption energy of O2, H2O, H2O2, and OH on different metallophthalocyanine (MPc; M = Fe, Co, Ni, and Mn) molecules.74,75 Fe-based catalysts were found to have higher adsorption energy compared with other catalysts, and this led to a rapid decay in current density in the durability test for the Fe-based catalysts. The presence of Fe oxide is the other possibility during the electrochemical measurement by the inclusion in carbon shells.76,77 Thus, the appearance of the Fe−O bond is affected by not only the adsorption between oxygen and Fe but also the retention of Fe oxide incorporated in carbon shells. Tylus et al. elucidated that the Fe−Nx−C catalysts contain some forms of metal nanoparticles and/or its oxides; the durability has been attributed to the presence of protective graphene-like layers surrounding the metal nanoparticles, which plays a possible direct role in enhancing ORR activity.76 These results elucidate the adsorption mechanism of oxygen molecules and the metal during the ORR.

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the National Synchrotron Radiation Research Center (Beamline 17C1), Hsinchu, Taiwan for X-ray adsorption spectrum (XAS) analysis facility.

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CONCLUSION The py-Fe-FA/C compound examined in this study exhibits excellent catalytic activity for the ORR and is a potential candidate cathode catalyst for PEMFCs. The py-Fe-FA/C exhibits a preference for the direct four-electron reduction pathway from O2 to H2O. The H2−O2 PEMFC with the py-Fe-FA/C in the cathode displays high performance and durability, which is attributable to the network structure of polyaromatic hydrocarbons, the quaternary type (graphitic) of nitrogen, and the coordination structure of the compound. The pyrolysis process alters the coordination structure of the Fe-FA, resulting in an increased ORR activity. Modification of the central metals and surrounding ligands should be studied through detailed in situ XAS measurements to obtain a comprehensive understanding of the ORR involving py-Fe-FA/C catalysts.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b01764. Electrochemical ORR results and the elemental content from XPS fitting results (PDF)

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REFERENCES

(1) Kong, A.; Dong, B.; Zhu, X.; Kong, Y.; Zhang, J.; Shan, Y. Ordered Mesoporous Fe-Porphyrin-Like Architectures as Excellent Cathode Materials for the Oxygen Reduction Reaction in Both Alkaline and Acidic Media. Chem. - Eur. J. 2013, 19 (48), 16170− 16175. (2) Merzougui, B.; Hachimi, A.; Akinpelu, A.; Bukola, S.; Shao, M. A Pt-free catalyst for oxygen reduction reaction based on Fe−N multiwalled carbon nanotube composites. Electrochim. Acta 2013, 107 (0), 126−132. (3) Shi, J.; Zhou, X.; Xu, P.; Qiao, J.; Chen, Z.; Liu, Y. Nitrogen and sulfur co-doped mesoporous carbon materials as highly efficient electrocatalysts for oxygen reduction reaction. Electrochim. Acta 2014, 145, 259−269. (4) Monteverde Videla, A. H. A.; Ban, S.; Specchia, S.; Zhang, L.; Zhang, J. Non-noble Fe−NX electrocatalysts supported on the reduced graphene oxide for oxygen reduction reaction. Carbon 2014, 76 (0), 386−400. (5) Wang, J.; Li, S.; Zhu, G.; Zhao, W.; Chen, R.; Pan, M. Novel nonnoble metal electrocatalysts synthesized by heat-treatment of iron terpyridine complexes for the oxygen reduction reaction. J. Power Sources 2013, 240 (0), 381−389. (6) Peng, H.; Hou, S.; Dang, D.; Zhang, B.; Liu, F.; Zheng, R.; Luo, F.; Song, H.; Huang, P.; Liao, S. Ultra-high-performance doped carbon catalyst derived from o-phenylenediamine and the probable roles of Fe and melamine. Appl. Catal., B 2014, 158−159 (0), 60−69. (7) Jasinski, R. A New Fuel Cell Cathode Catalyst. Nature 1964, 201, 1212−1213. (8) Savastenko, N. A.; Brüser, V.; Brüser, M.; Anklam, K.; Kutschera, S.; Steffen, H.; Schmuhl, A. Enhanced electrocatalytic activity of CoTMPP-based catalysts for PEMFCs by plasma treatment. J. Power Sources 2007, 165 (1), 24−33. (9) Bogdanoff, P.; H, I.; Hilgendorff, M.; Dorbandt, I.; Fiechter, S.; Tributsch, H. Probing Structural Effects of Pyrolysed CoTMPP-based Electrocatalysts for Oxygen Reduction via New Preparation Strategies. J. New Mater. Electrochem. Syst. 2004, 7 (2), 85−92. (10) Mocchi, C.; Trasatti, S. Composite electrocatalysts for molecular O2 reduction in electrochemical power sources. J. Mol. Catal. A: Chem. 2003, 204−205, 713−720. (11) Xie, X.-Y.; Ma, Z.-F.; Wu, X.; Ren, Q.-Z.; Yuan, X.; Jiang, Q.-Z.; Hu, L. Preparation and electrochemical characteristics of CoTMPPTiO2NT/BP composite electrocatalyst for oxygen reduction reaction. Electrochim. Acta 2007, 52 (5), 2091−2096. (12) Liu, H.; Song, C.; Tang, Y.; Zhang, J.; Zhang, J. High-surfacearea CoTMPP/C synthesized by ultrasonic spray pyrolysis for PEM fuel cell electrocatalysts. Electrochim. Acta 2007, 52 (13), 4532−4538. (13) Jingjie, W.; Haolin, T.; Mu, P.; Zhaohui, W.; Wentao, M. Novel methanol electro-oxidation catalyst assisting with functional phthalocyanine supports. Electrochim. Acta 2009, 54 (5), 1473−1477. (14) Lu, Y.; Reddy, R. G. The electrochemical behavior of cobalt phthalocyanine/platinum as methanol-resistant oxygen-reduction electrocatalysts for DMFC. Electrochim. Acta 2007, 52 (7), 2562− 2569. (15) Convert, P.; Coutanceau, C.; Crouïgneau, P.; Gloaguen, F.; Lamy, C. Electrodes modified by electrodeposition of CoTAA

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*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Chen-Hao Wang: 0000-0003-2350-3287 2903


Research Article

ACS Sustainable Chemistry & Engineering complexes as selective oxygen cathodes in a direct methanol fuel cell. J. Appl. Electrochem. 2001, 31 (9), 945−952. (16) Chang, S.-T.; Wang, C.-H.; Du, H.-Y.; Hsu, H.-C.; Kang, C.-M.; Chen, C.-C.; Wu, J. C. S.; Yen, S.-C.; Huang, W.-F.; Chen, L.-C.; Lin, M. C.; Chen, K.-H. Vitalizing fuel cells with vitamins: pyrolyzed vitamin B12 as a non-precious catalyst for enhanced oxygen reduction reaction of polymer electrolyte fuel cells. Energy Environ. Sci. 2012, 5 (1), 5305−5314. (17) Chang, S.-T.; Huang, H.-C.; Wang, H.-C.; Hsu, H.-C.; Lee, J.-F.; Wang, C.-H. Effects of structures of pyrolyzed corrin, corrole and porphyrin on oxygen reduction reaction. Int. J. Hydrogen Energy 2014, 39 (2), 934−941. (18) Wang, Y.; Jiang, X. Facile Preparation of Porous Carbon Nanosheets without Template and Their Excellent Electrocatalytic Property. ACS Appl. Mater. Interfaces 2013, 5 (22), 11597−11602. (19) Zittoun, J. Anemias due to disorder of folate, vitamin B12 and transcobalamin metabolism. La Revue du Praticien 1993, 43 (11), 1358−1363. (20) Vasudevan, P.; Santosh; Mann, N.; Tyagi, S. Transition metal complexes of porphyrins and phthalocyanines as electrocatalysts for dioxygen reduction. Transition Met. Chem. 1990, 15 (2), 81−90. (21) Zagal, J. H. Metallophthalocyanines as catalysts in electrochemical reactions. Coord. Chem. Rev. 1992, 119, 89−136. (22) Bezerra, C. W. B.; Zhang, L.; Lee, K.; Liu, H.; Marques, A. L. B.; Marques, E. P.; Wang, H.; Zhang, J. A review of Fe−N/C and Co−N/ C catalysts for the oxygen reduction reaction. Electrochim. Acta 2008, 53 (15), 4937−4951. (23) Serov, A.; Min, M.; Chai, G.; Han, S.; Seo, S.; Park, Y.; Kim, H.; Kwak, C. Electroreduction of oxygen over iron macrocyclic catalysts for DMFC applications. J. Appl. Electrochem. 2009, 39 (9), 1509−1516. (24) Jaouen, F.; Proietti, E.; Lefèvre, M.; Chenitz, R.; Dodelet, J. P.; Wu, G.; Chung, H. T.; Johnston, C. M.; Zelenay, P. Recent advances in non-precious metal catalysis for oxygen-reduction reaction in polymer electrolyte fuel cells. Energy Environ. Sci. 2011, 4 (1), 114− 130. (25) Shi, Z.; Zhang, J. Density Functional Theory Study of Transitional Metal Macrocyclic Complexes’ Dioxygen-Binding Abilities and Their Catalytic Activities toward Oxygen Reduction Reaction. J. Phys. Chem. C 2007, 111 (19), 7084−7090. (26) Beck, F. The redox mechanism of the chelate-catalysed oxygen cathode. J. Appl. Electrochem. 1977, 7 (3), 239−245. (27) Randin, J. P. Interpretation of the relative electrochemical activity of various metal phthalocyanines for the oxygen reduction reaction. Electrochim. Acta 1974, 19 (2), 83−85. (28) Chen, Z.; Higgins, D.; Yu, A.; Zhang, L.; Zhang, J. A review on non-precious metal electrocatalysts for PEM fuel cells. Energy Environ. Sci. 2011, 4 (9), 3167−3192. (29) Morozan, A.; Jousselme, B.; Palacin, S. Low-platinum and platinum-free catalysts for the oxygen reduction reaction at fuel cell cathodes. Energy Environ. Sci. 2011, 4 (4), 1238−1254. (30) Su, D. S.; Sun, G. Nonprecious-Metal Catalysts for Low-Cost Fuel Cells. Angew. Chem., Int. Ed. 2011, 50 (49), 11570−11572. (31) Gewirth, A. A.; Thorum, M. S. Electroreduction of Dioxygen for Fuel-Cell Applications: Materials and Challenges. Inorg. Chem. 2010, 49 (8), 3557−3566. (32) Hu, K.; Tao, L.; Liu, D.; Huo, J.; Wang, S. Sulfur-Doped Fe/N/ C Nanosheets as Highly Efficient Electrocatalysts for Oxygen Reduction Reaction. ACS Appl. Mater. Interfaces 2016, 8 (30), 19379−19385. (33) Gu, W.; Hu, L.; Li, J.; Wang, E. Iron and nitrogen co-doped hierarchical porous graphitic carbon for a high-efficiency oxygen reduction reaction in a wide range of pH. J. Mater. Chem. A 2016, 4 (37), 14364−14370. (34) Liu, T.; Zhao, P.; Hua, X.; Luo, W.; Chen, S.; Cheng, G. An FeN-C hybrid electrocatalyst derived from a bimetal-organic framework for efficient oxygen reduction. J. Mater. Chem. A 2016, 4 (29), 11357− 11364. (35) Wang, X.; Wang, B.; Zhong, J.; Zhao, F.; Han, N.; Huang, W.; Zeng, M.; Fan, J.; Li, Y. Iron polyphthalocyanine sheathed multiwalled

carbon nanotubes: A high-performance electrocatalyst for oxygen reduction reaction. Nano Res. 2016, 9 (5), 1497−1506. (36) Peera, S. G.; Arunchander, A.; Sahu, A. K. Cumulative effect of transition metals on nitrogen and fluorine co-doped graphite nanofibers: an efficient and highly durable non-precious metal catalyst for the oxygen reduction reaction. Nanoscale 2016, 8 (30), 14650− 14664. (37) Singh, K. P.; Bae, E. J.; Yu, J.-S. Fe−P: A New Class of Electroactive Catalyst for Oxygen Reduction Reaction. J. Am. Chem. Soc. 2015, 137 (9), 3165−3168. (38) Muthukrishnan, A.; Nabae, Y.; Hayakawa, T.; Okajima, T.; Ohsaka, T. Fe-containing polyimide-based high-performance ORR catalysts in acidic medium: a kinetic approach to study the durability of catalysts. Catal. Sci. Technol. 2015, 5 (1), 475−483. (39) Muthukrishnan, A.; Nabae, Y.; Chang, C. W.; Okajima, T.; Ohsaka, T. A high-performance Fe and nitrogen doped catalyst derived from diazoniapentaphene salt and phenolic resin mixture for oxygen reduction reaction. Catal. Sci. Technol. 2015, 5 (3), 1764− 1774. (40) Unni, S. M.; Illathvalappil, R.; Bhange, S. N.; Puthenpediakkal, H.; Kurungot, S. Carbon Nanohorn-Derived Graphene Nanotubes as a Platinum-Free Fuel Cell Cathode. ACS Appl. Mater. Interfaces 2015, 7 (43), 24256−24264. (41) Lin, L.; Zhu, Q.; Xu, A.-W. Noble-Metal-Free Fe−N/C Catalyst for Highly Efficient Oxygen Reduction Reaction under Both Alkaline and Acidic Conditions. J. Am. Chem. Soc. 2014, 136 (31), 11027− 11033. (42) Wang, G.; Jiang, K.; Xu, M.; Min, C.; Ma, B.; Yang, X. A high activity nitrogen-doped carbon catalyst for oxygen reduction reaction derived from polyaniline-iron coordination polymer. J. Power Sources 2014, 266, 222−225. (43) Zhu, Y.; Zhang, B.; Liu, X.; Wang, D.-W.; Su, D. S. Unravelling the Structure of Electrocatalytically Active Fe−N Complexes in Carbon for the Oxygen Reduction Reaction. Angew. Chem., Int. Ed. 2014, 53 (40), 10673−10677. (44) Bukola, S.; Merzougui, B.; Akinpelu, A.; Laoui, T.; Hedhili, M. N.; Swain, G. M.; Shao, M. Fe-N-C Electrocatalysts for Oxygen Reduction Reaction Synthesized by Using Aniline Salt and Fe3+/ H2O2 Catalytic System. Electrochim. Acta 2014, 146, 809−818. (45) Wang, C.-H.; Chang, S.-T.; Hsu, H.-C.; Du, H.-Y.; Wu, J. C.-S.; Chen, L.-C.; Chen, K.-H. Oxygen reducing activity of methanoltolerant catalysts by high-temperature pyrolysis. Diamond Relat. Mater. 2011, 20 (3), 322−329. (46) Lalande, G.; Côté, R.; Tamizhmani, G.; Guay, D.; Dodelet, J. P.; Dignard-Bailey, L.; Weng, L. T.; Bertrand, P. Physical, chemical and electrochemical characterization of heat-treated tetracarboxylic cobalt phthalocyanine adsorbed on carbon black as electrocatalyst for oxygen reduction in polymer electrolyte fuel cells. Electrochim. Acta 1995, 40 (16), 2635−2646. (47) Yang, W.; Zhang, Y.; Liu, C.; Jia, J. Dual-doped carbon composite for efficient oxygen reduction via electrospinning and incipient impregnation. J. Power Sources 2015, 274 (0), 595−603. (48) Zhao, J.; Li, H.; Liu, Z.; Hu, W.; Zhao, C.; Shi, D. An advanced electrocatalyst with exceptional eletrocatalytic activity via ultrafine Ptbased trimetallic nanoparticles on pristine graphene. Carbon 2015, 87 (0), 116−127. (49) Jiménez Mateos, J. M.; Fierro, J. L. G. X-ray Photoelectron Spectroscopic Study of Petroleum Fuel Cokes. Surf. Interface Anal. 1996, 24 (4), 223−236. (50) Wang, C.-H.; Huang, H.-C.; Chang, S.-T.; Lin, Y.-C.; Huang, M.-F. Pyrolysis of melamine-treated vitamin B12 as a non-precious metal catalyst for oxygen reduction reaction. RSC Adv. 2014, 4 (9), 4207−4211. (51) Yang, D.-S.; Song, M. Y.; Singh, K. P.; Yu, J.-S. The role of iron in the preparation and oxygen reduction reaction activity of nitrogendoped carbon. Chem. Commun. 2015, 51 (12), 2450−2453. (52) Chung, H. T.; Johnston, C. M.; Artyushkova, K.; Ferrandon, M.; Myers, D. J.; Zelenay, P. Cyanamide-derived non-precious metal 2904


Research Article

ACS Sustainable Chemistry & Engineering catalyst for oxygen reduction. Electrochem. Commun. 2010, 12 (12), 1792−1795. (53) Liu, G.; Li, X.; Ganesan, P.; Popov, B. N. Studies of oxygen reduction reaction active sites and stability of nitrogen-modified carbon composite catalysts for PEM fuel cells. Electrochim. Acta 2010, 55 (8), 2853−2858. (54) Wu, G.; Zelenay, P. Nanostructured Nonprecious Metal Catalysts for Oxygen Reduction Reaction. Acc. Chem. Res. 2013, 46 (8), 1878−1889. (55) Lee, K.; Zhang, L.; Lui, H.; Hui, R.; Shi, Z.; Zhang, J. Oxygen reduction reaction (ORR) catalyzed by carbon-supported cobalt polypyrrole (Co-PPy/C) electrocatalysts. Electrochim. Acta 2009, 54 (20), 4704−4711. (56) Ozaki, J.-i.; Kimura, N.; Anahara, T.; Oya, A. Preparation and oxygen reduction activity of BN-doped carbons. Carbon 2007, 45 (9), 1847−1853. (57) Ikeda, T.; Boero, M.; Huang, S.-F.; Terakura, K.; Oshima, M.; Ozaki, J.-i. Carbon Alloy Catalysts: Active Sites for Oxygen Reduction Reaction. J. Phys. Chem. C 2008, 112 (38), 14706−14709. (58) Niwa, H.; Horiba, K.; Harada, Y.; Oshima, M.; Ikeda, T.; Terakura, K.; Ozaki, J.-i.; Miyata, S. X-ray absorption analysis of nitrogen contribution to oxygen reduction reaction in carbon alloy cathode catalysts for polymer electrolyte fuel cells. J. Power Sources 2009, 187 (1), 93−97. (59) Sidik, R. A.; Anderson, A. B.; Subramanian, N. P.; Kumaraguru, S. P.; Popov, B. N. O2 Reduction on Graphite and Nitrogen-Doped Graphite: Experiment and Theory. J. Phys. Chem. B 2006, 110 (4), 1787−1793. (60) Lai, L.; Potts, J. R.; Zhan, D.; Wang, L.; Poh, C. K.; Tang, C.; Gong, H.; Shen, Z.; Lin, J.; Ruoff, R. S. Exploration of the active center structure of nitrogen-doped graphene-based catalysts for oxygen reduction reaction. Energy Environ. Sci. 2012, 5 (7), 7936−7942. (61) Shao, M.; Chang, Q.; Dodelet, J.-P.; Chenitz, R. Recent Advances in Electrocatalysts for Oxygen Reduction Reaction. Chem. Rev. 2016, 116 (6), 3594−3657. (62) Lefèvre, M.; Proietti, E.; Jaouen, F.; Dodelet, J. P. Iron-Based Catalysts with Improved Oxygen Reduction Activity in Polymer Electrolyte Fuel Cells. Science 2009, 324 (5923), 71−74. (63) Proietti, E.; Jaouen, F.; Lefévre, M.; Larouche, N.; Tian, J.; Herranz, J.; Dodelet, J.-P. Iron-based cathode catalyst with enhanced power density in polymer electrolyte membrane fuel cells. Nat. Commun. 2011, 2, 416. (64) Manzoli, M.; Boccuzzi, F. Characterisation of Co-based electrocatalytic materials for O2 reduction in fuel cells. J. Power Sources 2005, 145 (2), 161−168. (65) Pylypenko, S.; Mukherjee, S.; Olson, T. S.; Atanassov, P. Nonplatinum oxygen reduction electrocatalysts based on pyrolyzed transition metal macrocycles. Electrochim. Acta 2008, 53 (27), 7875−7883. (66) Cheon, J. Y.; Kim, T.; Choi, Y.; Jeong, H. Y.; Kim, M. G.; Sa, Y. J.; Kim, J.; Lee, Z.; Yang, T.-H.; Kwon, K.; Terasaki, O.; Park, G.-G.; Adzic, R. R.; Joo, S. H. Ordered mesoporous porphyrinic carbons with very high electrocatalytic activity for the oxygen reduction reaction. Sci. Rep. 2013, 3, DOI: 10.1038/srep02715. (67) Anderson, A. B.; Sidik, R. A. Oxygen Electroreduction on FeII and FeIII Coordinated to N4 Chelates. Reversible Potentials for the Intermediate Steps from Quantum Theory. J. Phys. Chem. B 2004, 108 (16), 5031−5035. (68) Schulenburg, H.; Stankov, S.; Schunemann, V.; Radnik, J.; Dorbandt, I.; Fiechter, S.; Bogdanoff, P.; Tributsch, H. Catalysts for the Oxygen Reduction from Heat-Treated Iron(III) Tetramethoxyphenylporphyrin Chloride: Structure and Stability of Active Sites. J. Phys. Chem. B 2003, 107 (34), 9034−9041. (69) Lefèvre, M.; Dodelet, J. P.; Bertrand, P. Molecular Oxygen Reduction in PEM Fuel Cells: Evidence for the Simultaneous Presence of Two Active Sites in Fe-Based Catalysts. J. Phys. Chem. B 2002, 106 (34), 8705−8713. (70) Lefèvre, M.; Dodelet, J. P.; Bertrand, P. O2 Reduction in PEM Fuel Cells: Activity and Active Site Structural Information for Catalysts

Obtained by the Pyrolysis at High Temperature of Fe Precursors. J. Phys. Chem. B 2000, 104 (47), 11238−11247. (71) Bron, M.; Radnik, J.; Fieber-Erdmann, M.; Bogdanoff, P.; Fiechter, S. EXAFS, XPS and electrochemical studies on oxygen reduction catalysts obtained by heat treatment of iron phenanthroline complexes supported on high surface area carbon black. J. Electroanal. Chem. 2002, 535 (1−2), 113−119. (72) Bouwkamp-Wijnoltz, A. L.; Visscher, W.; van Veen, J. A. R.; Boellaard, E.; van der Kraan, A. M.; Tang, S. C. On Active-Site Heterogeneity in Pyrolyzed Carbon-Supported Iron Porphyrin Catalysts for the Electrochemical Reduction of Oxygen: An In Situ Mössbauer Study. J. Phys. Chem. B 2002, 106 (50), 12993−13001. (73) Ziegelbauer, J. M.; Olson, T. S.; Pylypenko, S.; Alamgir, F.; Jaye, C.; Atanassov, P.; Mukerjee, S. Direct Spectroscopic Observation of the Structural Origin of Peroxide Generation from Co-Based Pyrolyzed Porphyrins for ORR Applications. J. Phys. Chem. C 2008, 112 (24), 8839−8849. (74) Chen, R.; Li, H.; Chu, D.; Wang, G. Unraveling Oxygen Reduction Reaction Mechanisms on Carbon-Supported Fe-Phthalocyanine and Co-Phthalocyanine Catalysts in Alkaline Solutions. J. Phys. Chem. C 2009, 113 (48), 20689−20697. (75) Guo, J.; He, H.; Chu, D.; Chen, R. OH−-Binding Effects on Metallophthalocyanine Catalysts for O2 Reduction Reaction in Anion Exchange Membrane Fuel Cells. Electrocatalysis 2012, 3 (3−4), 252− 264. (76) Tylus, U.; Jia, Q.; Strickland, K.; Ramaswamy, N.; Serov, A.; Atanassov, P.; Mukerjee, S. Elucidating Oxygen Reduction Active Sites in Pyrolyzed Metal−Nitrogen Coordinated Non-Precious-Metal Electrocatalyst Systems. J. Phys. Chem. C 2014, 118 (17), 8999−9008. (77) Jaouen, F.; Marcotte, S.; Dodelet, J.-P.; Lindbergh, G. Oxygen Reduction Catalysts for Polymer Electrolyte Fuel Cells from the Pyrolysis of Iron Acetate Adsorbed on Various Carbon Supports. J. Phys. Chem. B 2003, 107 (6), 1376−1386.

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