Nitrogen-Doped Highly Crystalline Carbon

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Bimodal porous iron-nitrogen doped highly crystalline carbon nanostructure as a cathode catalyst for oxygen reduction reaction in an acid medium Seul Lee, Da-Hee Kwak, Sang-Beom Han, Young-Woo Lee, Jin-Yeon Lee, In-Ae Choi, Hyun-Suk Park, Jin-Young Park, and Kyung-Won Park ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02721 • Publication Date (Web): 27 Jun 2016 Downloaded from http://pubs.acs.org on June 30, 2016

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Bimodal Porous Iron-Nitrogen Doped Highly Crystalline Carbon Nanostructure as a Cathode Catalyst for Oxygen Reduction Reaction in an Acid Medium Seul Lee,1 Da-Hee Kwak,1 Sang-Beom Han,1 Young-Woo Lee,1,2 Jin-Yeon Lee,1 In-Ae Choi,1 Hyun-Suk Park,1 Jin-Young Park,1 and Kyung-Won Park1,* 1

Department of Chemical Engineering, Soongsil University, Seoul 156743, Republic of Korea

2

Department of Engineering Science, University of Oxford, Oxford OX1 3PJ, United Kingdom

KEYWORDS Doped carbon, Porous structure, Bimodal porous, Oxygen reduction reaction, Acid medium

ABSTRACT: Doped carbon nanomaterials as nonprecious metal catalysts for oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells have received intensive attraction. The improvement of ORR performance for the doped porous carbon nanostructures with high specific surface areas is mainly attributed to multi-doped electrochemical active sites provided by the metallic (Fe, Co) and non-metallic species (N, B, and S). Here, we prepared porous iron/nitrogen doped carbon nanostructured materials via a simple synthesis process using silicate

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beads (500 and 50 nm diameter) as templates in the presence of 5,10,15,20-tetrakis(4methoxyphenyl)-21H,23H

porphyrin

(TMPP)

or

5,10,15,20-tetrakis(4-methoxyphenyl)-

21H,23H-porphyrin iron(III) chloride (FeTMPP). The resulting samples exhibited a bimodal porous structure, homogeneous heteroatomic doping, and a fairly large specific surface area. In particular, the sample prepared using both 500 and 50 nm silicate beads with FeTMPP (FeTMPP-C-500/50) exhibited much improved ORR performance in an acid solution. The enhanced ORR properties of FeTMPP-C-500/50 could result from the fairly large specific surface area, mixed macro-/meso-porous structure, high crystallinity, and co-doping of metal and nitrogen.

Introduction Proton electrolyte membrane fuel cells (PEMFCs) have received attractions as environmentfriendly electrochemical energy sources due to relatively high power density, excellent energy conversion efficiency, extremely low emission of pollutants. In PEMFCs, since the ORR at the cathode is much sluggish compared to the anodic oxidation reaction, the cathode needs a large quantity of Pt as a representative metal catalyst in order to expedite the ORR [1-5]. However, the high loading amount of Pt cathode catalyst can hinder the commercialization of PEMFCs. Thus, the main issue in the cathode catalyst has been overcome by systematically designing Pt-based alloy or non-precious metal (NPM) cathode materials in PEMFCs [6-11]. Recently, various types of NPM or metal-free catalysts for ORR have been suggested such as transition metal complex (i.e., carbides, nitrides, and sulfides) [12-15], nitrogen-doped carbon nanostructures [16-19], carbon nitride [20], polymer-based nanomaterials [21], and metal-NX-macrocycles [22-24]. To further design NPM catalysts with excellent ORR performance, many intensive researches have been carried out to fabricate nitrogen, boron, and/or sulfur doped carbon nanostructures. In


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particular, the doped carbon nanostructure catalysts with particular electronic, physical, and chemical properties showed much improved ORR activity [19,25-32]. Although the recent suggestions of elaborate chemical raw materials and selection of the synthetic procedure might appear to be breakthroughs, comparable properties of the NPM catalysts to the commercial cathode catalyst need to be achieved for commercialization of the PEMFCs [19]. The performance of the NPM catalysts are strongly affected by the distribution active sites, the nature of the active species, pore structure, and the electrochemical active surface area [19,33,34]. For practical applications of the catalysts, research on the fabrication and design of the membraneelectrode-assembly (MEA) can be essential [35]. In the PEMFC, the catalyst layer fabricated by NPM catalysts can be thicker than the normal case in the MEA. The cell performance of the microporous carbon-based nanostructure catalyst decreased at high current range. On the other hand, the meso- and macro-pores of the nanostructure catalyst could provide easier access of reactants and products to active sites [34-36]. Thus, the porous NPM catalysts for the high performance fuel cell applications should be developed by considering the mass transport phenomena in the MEA. Herein, we synthesized the macro/mesoporous carbon nanomaterials doped by iron-nitrogen as a cathode catalyst for ORR via a facile synthesis process using silicate beads as templates in the presence of TMPP as nitrogen and carbon sources or FeTMPP as iron, nitrogen, and carbon sources. To obtain the bimodal porous doped carbon nanostructures, we used the following farbication process: the first step involves using the laminated molding of SiO2 beads as a wellstacked template (50 and 500 nm) in a mixed solution with TMPP or FeTMPP and dimethylformamide (DMF) as a solvent; in the second step, the solvent is evaporated at 80 oC; the third step involves heat treatment of the mixed TMPP or FeTMPP coated on the SiO2 beads


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at 900 oC for 3 h under an N2 atmosphere; and finally, the SiO2 template is removed through a hydrofluoric acid washing process.

Experimental Section Synthesis of the porous doped carbon nanostructures. To synthesize the porous doped carbon nanostructures, as shown in Figure 1, the 0.2 g 5,10,15,20-tetrakis(4-methoxyphenyl)21H,23H-porphyrin (TMPP, Sigma-Aldrich) or 0.2 g 5,10,15,20-tetrakis(4-methoxyphenyl)21H,23H-porphyrin iron(III) chloride (Fe-TMPPCl, Sigma-Aldrich) were dissolved in the presence of 0.2 g silica beads (500 nm, Alfa Aesar) and/or 0.23g silica beads solution (50 nm, with 30% in ethylene glycol (Alfa Aesar)) as a template and 20 mL N,N-Dimethylformamide (DMF solution, Sigma-Aldrich) in a 100 mL vial. 0.2 g TMPP was mixed with 0.2 g silica beads (500 nm, Alfa Aesar) and/or 0.23 g silica beads solution (50 nm, with 30% in Ethylene glycol (Alfa Aesar)) template in 20 mL DMF solution. 0.2 g Fe-TMPPCl was dissolved in the presence of 0.2 g silica beads (500 nm, Alfa Aesar) and/or 0.23 g silica beads solution (50 nm, with 30% in Ethylene glycol (Alfa Aesar)) template in 20 mL DMF solution. After completely stirring, the solutions were placed into glass Petri dishes to achieve a well-stacked structure. The as-prepared solutions were evaporated in an oven at 80 oC. The evaporated powders were heated under N2 flowing at 900 oC for 3 h. The products were stirred in 10 vol.% hydrofluoric acid solution for over 1 h and then washed with de-ionized water and ethanol several times. The resulting samples were obtained by driying in a 50 oC oven for 24 h. Structural analysis. The structure of the as-prepared samples was observed using field emission-transmission electron microscopy (FE-TEM, a Philips Tecnai F20 system) and energy dispersive X-ray spectroscopy (EDX) [6]. Field emission-scanning electron microscopy (FE-


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SEM) was performed using a Carl Zeiss SIGMA system with a working voltage of 5 kV. Crystal structure of the carbon nanomaterials was carried out using the XRD method (Bruker, D2 Phase System) with a Cu Kα (λ = 0.15406 nm) and a Ni filter. Raman spectra were obtained on a high resolution micro Raman spectrometer (Horiba Jobin Yvon). X-ray photoelectron spectroscopy (XPS) analysis was performed using the Al Kα X-ray source (1486.8 eV), with the working pressure of < 7.8×10-9 Torr and beam power of 200 W (Thermo Scientific, K-Alpha) [37]. The surface area and porosity of the samples were obtained using nitrogen sorption measurement (Micromeritics ASAP 2020 adsorption analyzer) [6]. Thermogravimetric analysis (TGA) was carried out using a thermal analyzer (SDTA851, Mettler Toledo) between 30 and 850 oC at a heating rate of 5 oC min-1 and an air flow of 60 cm3 min-1 [38]. Electrochemical analysis. The electrochemical characteristics and performance of the catalysts were evaluated using a potentiostat (PGSTAT302N, AUTOLAB) in a three-electrode cell with the Pt wire as a counter electrode and Ag/AgCl (in saturated KCl) as a reference electrode. The catalyst ink was prepared by mixing catalyst powder in de-ionized water, isopropanol, and 5wt% Nafion solution (Aldich) [39]. The ink was dropped on a rotating disk electrode (RDE) with an area of 0.070685 cm2 and dried in a 50 oC oven for 10 min. After complete drying, the loading amount of the catalyst was 400 µg cm-2. The commercial Pt/C catalyst (20 wt.% Pt on carbon black, Premetek Co.) with an average particle size of 3.5-4 nm and an electrochemically active surface area of 105.25 m2 g-1 was used as a cathode. The Pt loading amount was 40 µg cm-2. Cyclic voltammograms (CVs) and linear sweep voltammograms (LSVs) of the catalysts were obtained in a 0.5 M H2SO4 solution in order to evaluate the ORR performance. The ORR electron-transfer number (n) of the electrocatalysts was measured using a rotating ring disk electrode (RRDE, CH Instrument, CHI 700C). The ORR stability test was conducted by


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sweeping between 0.6 and 1.0 V (vs. Ag/AgCl) for 2000 cycles with a scan rate of 50 mV s-1 in an O2-saturated 0.5 M H2SO4 [6]. The ORR activity of the electrocatalysts for methanoltolerance was evaluated in O2-saturated 0.5 M H2SO4 containing 0.5 M CH3OH. The LSVs of the samples were obtained in O2-saturated 0.5 M H2SO4 in the absence and presence of 10 mM KCN in order to characterize active sites of metal species in the electrocatalysts in the ORR. All electrode potentials were converted into a reversible hydrogen electrode (RHE).

Results and Discussion Figures 2(a)-2(e) show FE-SEM images of the samples synthesized via a template method using silicate beads (500 and/or 50 nm in diameter) in the presence of TMPP or FeTMPP. The samples synthesized using pure 500 nm silicate beads (denoted as TMPP-C-500) and a mixture of 500 and 50 nm (denoted as TMPP-C-500/50) silicate beads as a template in the presence of TMPP exhibited hollowed spherical shapes, implying the formation of homogeneous porous carbon by TMPP sources on the SiO2 templates with the heating process at 900 oC in an N2 atmosphere (Figures 2(a) and 2(b)). Furthermore, the samples synthesized using only 50 nm silicate beads (denoted as FeTMPP-C-50), only 500 nm silicate beads (denoted as FeTMPP-C-500) and a mixture of 500 and 50 nm silicate beads (denoted as FeTMPP-C-500/50) in the presence of FeTMPP showed hollow spherical shapes, demonstrating the formation of homogeneous porous carbon by FeTMPP sources on the SiO2 templates (Figures 2(c) and 2(e)). In particular, TMPPC-500/50 and FeTMPP-C-500/50 contained both macropores and mesopores formed with 500 and 50 nm beads, respectively, whereas TMPP-C-500 and FeTMPP-C-500 contained pure macropores prepared using the 500 nm beads. Figures 2(f)-2(j) show FE-TEM images of the samples prepared using a template method. All samples exhibited a porous spherical shape with


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a wall of ~12 nm thickness, demonstrating the formation of the carbon nanostructures on SiO2 beads. Furthermore, as shown in the high-resolution (HR) TEM images, the d-spacings of FeTMPP-C-50, FeTMPP-C-500, and FeTMPP-C-500/50 are 0.340, 0.340, and 0.341 nm, respectively. However, the HR-TEM images of TMPP-C-500 and TMPP-C-500/50 were not clear enough to measure the d-spacing due to the low graphitic degree of the samples. It is significant that FeTMPP-C-50, FeTMPP-C-500, and FeTMPP-C-500/50, formed in the presence of FeTMPP, exhibited the d-spacings close to the (002) of graphitic carbon, implying a high crystalline structure of the carbon nanomaterials, compared to TMPP-C-500 and TMPP-C500/50, that were formed in the presence of TMPP. The elemental distributions of the asprepared catalysts were confirmed through STEM mapping images obtained by high-angle annular dark-field scanning TEM-EDX spectroscopy (Figure S1). From the STEM images, it was observed that the carbon, nitrogen, and iron as ORR active sites were homogeneously distributed in the carbon nanostructures [40,41]. Figure 3 and Figure S2 show nitrogen adsorption-desorption isotherms and pore size distributions of the as-prepared samples. In particular, TMPP-C-500/50 and FeTMPP-C-500/50 exhibited a type IV isotherm, which is characteristic of mesoporosity (Figures 3(b), 3(e)) and a pore size distribution containing ~40 nm formed with the 50 nm SiO2 beads (the insets of Figures S2(b), S2(e)). As a result, the mesoporous structure having ~40 nm pore size for TMPP-C500/50 and FeTMPP-C-500/50, respectively, is attributed to the formation of porous carbon caused by the mixture of the SiO2 templates during the pyrolysis of TMPP and Fe-TMPP under a nitrogen atmosphere. The BET surface areas of TMPP-C-500, TMPP-C-500/50, FeTMPP-C-50, FeTMPP-C-500, and FeTMPP-C-500/50 were 374.7, 571.0, 328.9, 228.1, and 468.3 m2 g-1, respectively. In particular, the samples synthesized using a mixture of 500 and 50 nm silicate


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beads showed higher BET surface areas than those of the samples synthesized using pure 500 nm silicate beads. The samples having high-surface-areas can provide sites that are more active, thus enhancing the ORR activity in the kinetic controlled region. Furthermore, the mass transport region during the ORR activity is mainly related to the pore structure and distribution. The macro- and meso-porosity in the carbon nanostructures might lead to the efficient mass transport [42-44]. Consequently, the porous high-surface-area carbon nanostructures could be responsible for an enhanced ORR performance facilitating an enhanced accessibility of oxygen and transporting of by-products. XRD patterns of TMPP-C-500, TMPP-C-500/50, FeTMPP-C-50, FeTMPP-C-500, and FeTMPP-C-500/50 are shown in Figure 4(a). The (002) and (101) peaks of the samples represent a formation of graphitic carbon nanostructures, which is close to the d-spacing of graphite. In particular, FeTMPP-C-50, FeTMPP-C-500 and FeTMPP-C-500/50 exhibited a carbon structure with an improved crystallinity due to the carbonization process in the presence of iron as a catalyst in FeTMPP, compared with TMPP-C-500 and TMPP-C-500/50. As shown in the Raman spectra of the as-prepared samples (Figure 4(b)), the two characteristic peaks consist of a G-band, at ~1591 cm−1, which corresponds to the graphitic carbon structure and D-band at ~1351 cm−1, which is related to the structural disorder on the graphitic structure as a result of the heteroatomic doping [6,45]. The intensity ratio (ID/IG) values of TMPP-C-500, TMPP-C-500/50, FeTMPP-C50, FeTMPP-C-500, and FeTMPP-C-500/50 are 1.04, 1.05, 1.17, 1.25, and 0.78, respectively. The high ID/IG might result from the atomic disorder produced by the heating process with TMPP or FeTMPP as dopants and carbon sources [6]. However, it is significant that FeTMPP-500/50 exhibits an excellent crystallinity with low ID/IG and 2D band, indicating a high crystalline carbon structure [46]. Furthermore, in particular, the doped carbon nanostructures exhibited


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much higher conductivity (17.8-71.3 S cm-1) than those of carbon black (Vulcan XC-72, 3.9 S cm-1). The improved electronic conductivity of the as-prepared samples is attributed to the wellformed graphitic carbon crystalline arrangement despite the relatively low heating temperature of 900 oC, leading to high catalytic properties in ORR [47]. XPS analysis was performed to examine the chemical elements and states of the as-prepared catalysts. The measured spectra display the presence of C, O, and N as shown in Figure S3(a). Using the XPS quantitative analysis, the contents of nitrogen species contained in TMPP-C-500, TMPP-C-500/50, FeTMPP-C-50, FeTMPP-C-500, and FeTMPP-C-500/50 were determined to be 3.84, 4.19, 2.52, 1.55, and 2.23 at.%, respectively. Despite the acid treatment during the synthesis of the doped porous carbons, the iron amounts in FeTMPP-C-50, FeTMPP-C-500 and FeTMPP-C-500/50 were 0.47, 0.12 and 0.34 at.% of Fe, respectively (Figures S3(b)-S3(d)), which can provide the metal-Nx as an ORR electrocatalytic active site. Furthermore, the N1s spectra of the catalysts were obtained as shown in Figure 5. The N1s spectra contained characteristic peaks at 398.0-399.5 (pyridinic N), 400.1-400.9 (pyrrolic N), 401-402 (graphitic N), and 402-410 (oxidized N) eV [1,48,49]. TMPP-C-500 and TMPP-C-500/50 had a higher graphitic N portion than the pyridinic and pyrrolic N states, whereas the relative ratios of pyridinic and pyrrolic N in FeTMPP-C-50, FeTMPP-C-500, and FeTMPP-C-500/50 increased, compared to TMPP-C-500 and TMPP-C-500/50, due to the metal doping forming Fe-N4 or FeN3 macrocycles among the nitrogen species [50,51]. The Fe-Nx macrocycles in the FeTMPP-C50, FeTMPP-C-500, and FeTMPP-C-500/50 might withdraw an electron, thus resulting in an enhancement of positively charged carbon atoms [52,53]. The charge polarization and bimodal pore structure can adsorb and reduce the O2 species on the surfaces in the doped carbon samples accompanied by a complete transferred electron reaction [54,55]. Accordingly, the relatively


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increased portion of pyrrolic and pyridinic N states with the existence of iron in FeTMPP-C-50, FeTMPP-C-500 and FeTMPP-C-500/50 might be responsible for an improved electrocatalytic activity towards ORR. As shown in the TGA curve of the FeTMPP-C-500/50 nanostructures (Figure S4), the normalized weight reduction of FeTMPP-C-500/50 is ~93 wt%, which results from an oxidation of the doped sample, and the content of Fe is measured to be ~4.9 wt% [47]. CVs of the doped carbon samples were measured in Ar- and O2-saturated 0.5 M H2SO4. The oxygen reduction peak potentials of TMPP-C-500, TMPP-C-500/50, FeTMPP-C-50, FeTMPPC-500, and FeTMPP-C-500/50 in O2-saturated 0.5 M H2SO4 were 0.593, 0.570, 0.725, 0.768, and 0.729 V, respectively (Figure S5). The samples prepared using a pyrolysis process with FeTMPP as both doping and carbon sources exhibited higher peak potentials than the samples prepared using TMPP. Thus, it is evident that pyrrolic and pyridinic N states conjugated with iron can result in an improved catalytic activity of FeTMPP-C-50, FeTMPP-C-500 and FeTMPP-C-500/50 towards ORR compared with TMPP-C-500 and TMPP-C-500/50 as Fe-free catalysts. All doped porous carbon nanostructure catalysts showed a typical ORR characteristic curve containing activation, ohmic, and diffusion polarization regions in applied potential ranges. Especially, among these doped carbon nanostructure catalysts, FeTMPP-C-500/50 exhibited an excellent ORR performance, i.e. improved half-wave potential (0.83 V) and relatively high current density (3.68 mA cm-2 at 0.8 V) even compared to commercial Pt/C (0.84 V and 4.46 mA cm-2), as shown in Figures 6(a) and 6(b). As shown in Figure 6(c), the electron-transfer number (n) of the catalysts for the ORR is determined using the following equation:

= 4 ×

+


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where ID, IR, and N are the current on the carbon electrode, the current from Pt ring-disk, and the collecting coefficient number (-0.4245), respectively [56,57]. As shown in Figure 6(c), the n value for FeTMPP-C-500/50 was determined to be approximately 4.0 in the overall potential range, demonstrating a complete ORR process in an acid medium. The enhanced ORR activity of FeTMPP-C-500/50 is believed to be dominantly attributed to sites that are more active, i.e. a high population of N states conjugated with iron, provided by a high surface area and both macro- and meso-porous structures. However, the order of the samples in the ORR activity was as follows: FeTMPP-C-500/50 > FeTMPP-C-500 > FeTMPP-C-50 (Figure 6(a)). The poor ORR activity of FeTMPP-C-50 could be attributed to the non-uniform stacking and arrangement, leading to reduced electrolyte accessibility and decreased electron transfer. In particular, the enhanced ORR performance of the FeTMPP-C-500/50 can result from the mixed pore structure, excellent crystallinity, high specific surface areas, and heteroatomic doping [23,26,51,58,59]. The FeTMPP-C-500/50 having enhanced electrochemical reactions exhibits the following characteristics: (1) Homogeneous heteroatomsic arrangement in the doped sample. (2) The improved electronic conductivity with the high crystalline structure. (3) The relatively high portion of particular N sates with an existence of iron. However, for evaluation of the ORR properties of the nanostructures as a cathode catalyst in the single cell, an optimization of the nanostructures for the MEA fabrication should be carried out as future work. In this study, the catalytic role of iron of the Fe-Nx macrocycles in the bimodal porous doped samples remains an essential issue. Typically, it has been reported that CN− ions strongly adsorb with iron and poison the sites in the chemical compounds [6,60-62]. The LSVs of FeTMPP-C50, FeTMPP-C-500 and FeTMPP-C-500/50 were compared in O2-saturated 0.5 M H2SO4 in the absence and presence of 10 mM KCN. FeTMPP-C-50, FeTMPP-C-500 and FeTMPP-C-500/50


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exhibited a remarkable difference of 25-79 mV between half-wave potentials before and after the poisoning experiment and a decreased diffusion-limiting current, compared to TMPP-C-500 and TMPP-C-500/50 (Figure 7) [63]. This demonstrates that the iron in the porous doped nanomaterials can be an essentially electrocatalytic active site for oxygen reduction process. As a result, the superior electrocatalytic performance of FeTMPP-C-500/50 as an ORR electrocatalyst in an acid medium might result from the synergistic effect between the high content of the iron element and particular nitrogen species, and the bimodal porous carbon nanostructure. For an evaluation of the ORR stability of the catalysts, a stability test was carried out by linearly sweeping in the range of 0.6 and 1.0 V for 2000 cycles in O2-saturated 0.5 M H2SO4. The LSV curves of the catalysts were obtained before and after the ORR stability test as shown in Figure S6. The doped carbon nanostructure catalysts showed half-wave potential differences of 10~47 mV between before and after the stability test, representing an improved stability of the bimodal porous doped carbon nanomaterials towards the oxygen reduction reaction. In particular, after the stability test, FeTMPP-500/50 maintained a comparable activity to commercial Pt/C due to a high crystalline bimodal porous carbon nanostructure (Figure S7). Typically, it is well known that a methanol-crossover in direct methanol fuel cells (DMFCs) can deteriorate the performance of DMFCs owing to both the reduced fuel efficiency in the anode and the mixed potential in the cathode [64]. Accordingly, in order to maintain an ORR activity in DMFCs, methanol-tolerant nanostructure cathode materials such as doped carbon nanostructure catalysts are required. The as-prepared carbon nanostructures exhibited constant performance with methanol-contained O2-saturated 0.5 M H2SO4. On the other hand, the commercial Pt/C catalyst exhibited a seriously decreased performance with methanol oxidation


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current. This implies that the porous doped cathode catalyts are tolerant with methanol during the ORR, thus avoiding the mixed potential in the DMFCs (Figure S8).

Conclusions In summary, we prepared macro- and meso-porous iron-nitrogen doped high crystalline carbon nanomaterials as cathode catalysts using a facile synthesis process with silicate bead templates in the presence of TMPP or FeTMPP as doping and carbon sources. The samples showed fairly large surface area, well-formed pore nanostructure, high crystallinity, and uniform heteroatomic distribution in the matrix. Especially, the bimodal porous structure of FeTMPP-C500/50 with the co-existence effect of Fe and N and excellent crystalline carbon structure can be responsible for a superior ORR performance. Thus, FeTMPP-C-500/50 can be expected to be utilized as a cathode catalyst in PEMFCs.


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FIGURES

Figure 1. Schematic illustration of synthetic procedure of doped carbon nanostructures prepared using the template method.


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Figure 2. FE-SEM, FE-TEM, and HR-TEM images of (a), (f), (k) TMPP-C-500, (b), (g), (l) TMPP-C-500/50, (c), (h), (m) FeTMPP-C-50, (d), (i), (n) FeTMPP-C-500, and (e), (j), (o) FeTMPP-C-500/50.


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Figure 3. Nitrogen gas adsorption-desorption isotherms of (a) TMPP-C-500, (b) TMPP-C500/50, (c) FeTMPP-C-50, (d) FeTMPP-C-500, and (e) FeTMPP-C-500/50.


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Figure 4. (a) XRD patterns and (b) Raman spectra of TMPP-C-500, TMPP-C-500/50, FeTMPPC-50, FeTMPP-C-500, and FeTMPP-C-500/50.


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Figure 5. XPS N1s spectra of (a) TMPP-C-500, (b) TMPP-C-500/50, (c) FeTMPP-C-50, (d) FeTMPP-C-500, and (e) FeTMPP-C-500/50.


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Figure 6. (a) ORR polarization curves of the samples in O2-saturated 0.5 M H2SO4 with a scan rate of 5 mV s-1 and 1600 rpm. (b) Comparison of ORR activity for the samples. (c) Electrontransfer number (n) vs. potential of the samples.


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Figure 7. LSVs of (a) TMPP-C-500, (b) TMPP-C-500/50, (c) FeTMPP-C-50, (d) FeTMPP-C500, and (e) FeTMPP-C-500/50 with a scan rate of 5 mV s-1 and 1600 rpm in O2-saturated 0.5 M H2SO4 in the absence and presence of 10 mM KCN.


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AUTHOR INFORMATION Supporting Information. Additional STEM images, N2 isotherms, XPS spectra and electrochemical data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the International Collaborative Energy Technology R&D Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea. (No. 20138520030800).


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Nitrogen-Doped Highly Crystalline Carbon

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