C Electrocatalysts

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Roles of Fe–N and Fe–FeC@C Species in Fe–N/ C Electrocatalysts for Oxygen Reduction Reaction Jae Hyung Kim, Young Jin Sa, Hu Young Jeong, and Sang Hoon Joo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13417 • Publication Date (Web): 28 Feb 2017 Downloaded from http://pubs.acs.org on March 1, 2017

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ACS Applied Materials & Interfaces

Roles of Fe–Nx and Fe–Fe3C@C Species in Fe–N/C Electrocatalysts for Oxygen Reduction Reaction Jae Hyung Kim,† Young Jin Sa, ‡ Hu Young Jeong,§ and Sang Hoon Joo*,†,‡ †

School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan 44919, Republic of Korea.

‡

Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan 44919, Republic of Korea. §

UNIST Central Research Facilities (UCRF), Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan 44919, Republic of Korea.

*E-mail: [email protected]

1 ACS Paragon Plus Environment


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ABSTRACT Iron and nitrogen co-doped carbons (Fe–N/C) have emerged as promising nonprecious metal catalysts for replacing Pt-based catalysts for oxygen reduction reaction (ORR). While Fe–Nx sites have been widely considered as active species for Fe–N/C catalysts, very recently, iron and/or iron carbide encased with carbon shells (Fe–Fe3C@C) has been suggested as a new active site for ORR. However, most of synthetic routes to Fe–N/C catalysts involve high-temperature pyrolysis, which unavoidably yield both Fe–Nx and Fe–Fe3C@C species, hampering the identification of exclusive role of each species. Herein, in order to establish the respective roles of Fe–Nx and Fe–Fe3C@C sites we rationally designed model catalysts via the phase conversion reactions of Fe3O4 nanoparticles supported on carbon nanotubes. The resulting catalysts selectively contained Fe–Nx, Fe–Fe3C@C, and N-doped carbon (C–Nx) sites. It was revealed that Fe–Nx sites dominantly catalyze ORR via 4-electron (4 e−) pathway, exerting a major role for high ORR activity, whereas Fe–Fe3C@C sites mainly promote 2 e− reduction of oxygen followed by 2 e− peroxide reduction, playing an auxiliary role.

KEYWORDS electrocatalysis, Fe–N/C catalyst, active site, model system, oxygen reduction reaction


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INTRODUCTION Polymer electrolyte fuel cells (PEFCs) have emerged as next-generation energy conversion devices, because of their high efficiency and environmental benignity.1,2 In PEFCs, the development of high-performance cathode catalysts for the oxygen reduction reaction (ORR) has been one of the most important challenges. Pt-based nanoparticles have been a favored solution; however, the high cost and scarcity of Pt have hampered the widespread deployment of PEFCs. To surmount this obstacle, several classes of non-precious metal ORR catalysts, such as transition metal and nitrogen co-doped carbons,3-10 metal oxide/carbon composites,11-14 and heteroatom-doped carbons,15-20 have been extensively investigated as alternatives. In particular, catalysts composed of iron, nitrogen, and carbon (Fe–N/C) have been the most prominent type of non-precious metal catalysts, owing to their outstanding ORR activity. To design highperformance Fe–N/C catalysts, it is of prime importance to understand their active sites. While multiple active sites have been suggested for these catalysts, including Fe–Nx sites,21-27 pyridinic or quaternary nitrogen,28-29 and defect sites of graphitic carbon,30 there has been a growing consensus that the Fe–Nx sites are the dominant active centers. Recently, iron and/or iron carbide encased within carbon shells (Fe–Fe3C@C) have also been suggested as an active site in Fe–N/C catalysts.31-43 However, a broad spectrum of possible roles has been proposed for the Fe–Fe3C@C species. Some groups reported that high ORR activity could be achieved with catalysts containing only Fe–Fe3C@C sites.35,37,41,43 Others suggested that the Fe–Fe3C@C sites play a synergistic role in conjunction with the Fe–Nx sites.34,39,42 Another viewpoint is that Fe–Fe3C@C sites are merely an impurity phase, and hence efforts were made to suppress their formation while maximizing the Fe–Nx sites by post forming gas treatment or silica-layer overcoating.44-47 Such a discrepancy on the role of Fe–Fe3C@C sites



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seems to stem from the difficulty in preparing Fe–N/C catalysts containing only these sites. Specifically, the synthesis of catalysts containing Fe–Fe3C@C sites commonly involves the mixing of metal, nitrogen, and carbon precursors, followed by high-temperature pyrolysis.33-47 This procedure is very similar to that for high-performing Fe–N/C catalysts with Fe–Nx sites. Hence, it is likely that these prepared Fe–N/C catalysts contain Fe–Nx as well as Fe–Fe3C@C sites. This situation may render the identification of catalytic roles of Fe–Nx and Fe–Fe3C@C species elusive. Herein, we rationally designed model catalysts via an iron oxide conversion method. Starting with Fe3O4 nanoparticles (NPs) supported on carbon nanotubes (CNTs), i.e., Fe3O4/CNT, three different types of catalysts were prepared, which selectively contain Fe–Nx, Fe–Fe3C@C, and N-doped carbon (C–Nx) sites. Electrochemical and physicochemical characterization of these catalysts demonstrated that the Fe–Nx sites catalyze the ORR via 4electron (4 e−) pathway, playing a key role for high ORR performance; whereas the Fe–Fe3C@C sites mainly promote 2-electron oxygen reduction and sequential peroxide reduction (2 e− × 2 e− pathway), playing an auxiliary role in the ORR.


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RESULTS AND DISCUSSION

Figure 1. (a) Schematic illustration of the iron oxide conversion process and the generated active sites in each catalyst. TEM images of (b) Fe–Fe3C@C/CNT_CO_A, (c) Fe–Fe3C@C/CNT_Urea, and (d) Fe–Fe3C@C/CNT_Urea_A. HAADF-STEM images of (e) Fe–Fe3C@C/CNT_CO_A, (f) Fe–Fe3C@C/CNT_Urea, and (g) Fe–Fe3C@C/CNT_Urea_A. (h) Atomic resolution TEM image of Fe–Fe3C@C/CNT_Urea. (i) EELS spectrum obtained from the black box region in panel (h).



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In most currently reported synthetic approaches to high-performing Fe–N/C catalysts, during high-temperature pyrolysis the iron precursor is uncontrollably converted to multiple iron-related species: iron oxide, iron nitride, metallic Fe, Fe–Nx, Fe–Fe3C@C, etc.33,35,39,41 In order to circumvent this problem, in this work, we exploited the phase conversion reaction of Fe3O4 NPs to selectively generate different catalytic sites (Figure 1a).48,49 Utilizing two different conversion agents, namely the nitrogen-free CO gas and nitrogen-containing urea, Fe3O4 NPs were converted into different target active species. When CO gas was used, most of Fe3O4 NPs were converted into Fe–Fe3C@C species.50 On the other hand, the use of urea resulted in the partial dissolution of Fe3O4 NPs by in-situ-generated cyanic acid and ammonia gas,51 yielding Fe–Nx as well as Fe–Fe3C@C sites. Along with the two sites, the decomposed ammonia also reacts with the carbon surface to produce C–Nx sites. A third model catalyst was prepared by treating the urea-mediated catalyst in hot acid, dramatically reducing the amount of Fe–Nx sites, from the second catalyst. Fe3O4 NPs of diameter ~12 nm were prepared following a previously reported method (Figure S1a,c),52 and deposited on CNTs to yield a Fe3O4/CNT “precursor” (Figure S1b,d). The conversion reaction using CO gas at 900 °C produced the product labeled Fe–Fe3C@C/CNT_CO. X-ray diffraction (XRD) pattern of Fe–Fe3C@C/CNT_CO (Figure 2) revealed that Fe3O4 phase was mostly converted into Fe–Fe3C phase, but a trace amount of Fe3O4 NPs still remained. To selectively remove the residual Fe3O4 NPs, Fe–Fe3C@C/CNT_CO was subject to sequential acid leaching with 6 M HCl and 6 M HNO3 at 80 °C to form Fe–Fe3C@C/CNT_CO_A. Since the Fe– Fe3C particles were protected by the graphitic carbon layers from acid, the acid treatment could only remove the Fe3O4 NPs.37 The content of Fe analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES) revealed that, although some Fe species were lost during the


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transformations due to the incomplete conversion of Fe3O4 nanoparticles, considerable amounts of Fe remained in Fe–Fe3C@C/CNT_CO_A sample (Table S1). The resulting XRD pattern (Figure 2) confirmed the etching of Fe3O4 phase as well as the preservation of Fe–Fe3C phase. TEM images of Fe–Fe3C@C/CNT_CO_A (Figures 1b and S2a) also show that Fe–Fe3C@C sites were supported on CNTs. The X-ray photoelectron spectroscopy (XPS) spectrum for Fe– Fe3C@C/CNT_CO_A (Figure 3) showed no peaks for nitrogen-containing species. In addition, the high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) observation of Fe–Fe3C@C/CNT_CO_A in the region free of Fe–Fe3C@C particles (Figures 1e and S3a) revealed no other Fe-containing species. These observations confirmed that the catalyst contained only Fe–Fe3C@C sites.

Figure 2. XRD patterns of Fe3O4/CNT, Fe–Fe3C@C/CNT_CO, Fe–Fe3C@C/CNT_CO_A, Fe– Fe3C@C/CNT_Urea, and Fe–Fe3C@C/CNT_Urea_A. 7 ACS Paragon Plus Environment


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Figure 3. XPS N 1s spectra of Fe phthalocyanine, Fe–Fe3C@C/CNT_CO_A, Fe– Fe3C@C/CNT_Urea, and Fe–Fe3C@C/CNT_Urea_A.

In the urea-mediated conversion reaction,53 Fe3O4/CNT was mixed with urea and agar, and subjected to pyrolysis at 900 °C under N2 flow, to yield Fe–Fe3C@C/CNT_Urea catalyst. Its XRD pattern (Figure 2) indicated that the Fe3O4 phase was fully converted into mixed Fe and Fe3C phases. TEM images of Fe–Fe3C@C/CNT_Urea (Figures 1c and S2b) also revealed that Fe–Fe3C@C sites were generated and dispersed on the CNT support. In contrast to the conversion with CO, Fe–Nx and C–Nx sites could be additionally generated in the urea-mediated conversion as discussed above. Elemental analysis results confirmed the generation of nitrogen species in Fe–Fe3C@C/CNT_Urea (Table S2). The detailed analysis of nitrogen species by XPS 8 ACS Paragon Plus Environment

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(Figure 3) indicated three types of nitrogen species with deconvoluted peaks at 398.1 eV, 398.9 eV, and 400.8 eV, which can be ascribed to pyridinic Nβ (bond with outer carbon of Fe phthalocyanine), pyrrolic Nα (Fe–N bond), 54 and graphitic nitrogen moieties (bond with three six-membered carbon ring),55,56 respectively. The XPS analysis clearly indicates the formation of Fe–Nx species and N-doped carbon species. We note that the nitrogen can also exist in the carbon shells encapsulating Fe–Fe3C particles. The HAADF-STEM image of Fe– Fe3C@C/CNT_Urea (Figures 1f and S3b) shows very small dots of just few Angstrom (shown within dotted red circles). The ultrasmall dots appear to be atomically dispersed Fe-based species. Further characterization by electron energy loss spectroscopy (EELS) spectrum (Figure 1i) taken at dotted square section of atomic-resolution TEM image (Figure 1h) shows the presence of Fe and N species, suggesting the existence of Fe–Nx species. The HAADF-STEM image also shows relatively large Fe nanoparticles of 1-3 nm without encapsulation by carbon layers. In order to identify the catalytic role of Fe–Nx sites, these sites were selectively etched from Fe–Fe3C@C/CNT_Urea by hot acid washing. This treatment leaves the C–Nx and Fe– Fe3C@C sites intact,37 whereas most of Fe–Nx species, if not all, can be etched.22,57 The TEM (Figures 1d and S2c) and HAADF-STEM (Figures 1g and S3c) images of Fe– Fe3C@C/CNT_Urea_A confirmed that the white dots observed in Fe–Fe3C@C/CNT_Urea were almost all removed after acid washing, while the Fe–Fe3C@C species remained. The absence of Fe–Nx sites in Fe–Fe3C@C/CNT_Urea_A could be further corroborated by the N 1s XPS spectrum (Figure 3). The peak area of pyridinic Nβ was nearly preserved and that for graphitic nitrogen species was slightly increased, whereas the area of pyrrolic Nα peak for Fe–Nx sites was diminished significantly after the hot acid treatment. Therefore, the Fe–Nx sites in Fe– Fe3C@C/CNT_Urea were mostly removed by the acid leaching, while the Fe–Fe3C@C and C–



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Nx sites were preserved. In addition, after the acid leaching, the content of oxygen was increased as revealed by elemental analysis results (Table S2). O 1s XPS analysis (Figure S4) also showed that the peak areas for C–O and C=O bonding were increased in Fe–Fe3C@C/CNT_Urea_A, compared to Fe–Fe3C@C/CNT_Urea, implying that additional oxygen species were introduced on carbon surfaces after the acid treatment. The catalytic roles of different active species were examined with rotating ring disk electrode (RRDE) experiments using the three model catalysts in 0.1 M KOH (Figures 4, S5, and S6). The ORR polarization curves (Figure 4a) revealed the superior ORR activity of Fe– Fe3C@C/CNT_Urea catalyst over the other two. Its half-wave potential (E1/2 = 0.87 V) was higher than those of Fe–Fe3C@C/CNT_Urea_A (0.81 V) and Fe–Fe3C@C/CNT_CO_A (0.66 V), and was compared favorably to commercial Pt/C (0.90 V). This excellent ORR activity of Fe3C@C/CNT_Urea catalyst was further corroborated by very low HO2− yield (less than 3%) in all potential range (Figure 4b and S5a) and near 4-electron selectivity (Figure S6).


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Current Density (mA cm-2)

(a)

0

Fe-Fe3C@C/CNT_Urea Fe-Fe3C@C/CNT_Urea_A

-1

Fe-Fe3C@C/CNT_CO_A Pt/C

-2 -3 -4 -5 -6

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Potential (V vs RHE)

(b) 100 90 80

HO2 Yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fe-Fe3C@C/CNT_Urea Fe-Fe3C@C/CNT_Urea_A Fe-Fe3C@C/CNT_CO_A

70 60 50 40 30 20 10 0 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Potential (V vs RHE)

Figure 4. (a) ORR polarization curves of Pt/C and Fe–Fe3C@C/CNT catalysts. (b) HO2‒ yield of Fe–Fe3C@C/CNT catalysts. The role of Fe–Nx sites could be accessed by comparing the catalytic activity before and after their removal (Fe3C@C/CNT_Urea and Fe3C@C/CNT_Urea_A, respectively). The E1/2 of Fe–Fe3C@C/CNT_Urea_A is negatively shifted by 60 mV from that of Fe–Fe3C@C/CNT_Urea. The decreased activity of Fe–Fe3C@C/CNT_Urea_A catalyst is accompanied by an increased HO2− yield (up to 20%). Kinetic data of the catalysts were deduced from Tafel analysis (Figure S7). Fe–Fe3C@C/CNT_Urea exhibited lower Tafel slope (58 mV/dec) than that of Fe– Fe3C@C/CNT_Urea_A (64 mV/dec). The Tafel slope data suggest that Fe–Nx sites play a 11 ACS Paragon Plus Environment


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significant role for fast reaction kinetics for the ORR. Mass-transfer-corrected kinetic current densities of the catalysts showed a similar trend with Tafel slopes. We normalized kinetic currents by two different areal factors: geometric area of the electrode and electrochemical active surface area (ECSA) which is estimated from electrochemical double-layer capacitance (Figure S8). At 0.8 V, Fe–Fe3C@C/CNT_Urea showed much higher kinetic current densities (29.0 mA cmgeo–2 and 83.9 µA cmECSA–2) than those of Fe–Fe3C@C/CNT_Urea_A (5.88 mA cmgeo–2 and 13.5 µA cmECSA–2), confirming fast reaction kinetics of Fe–Nx site for the ORR (Tables S3 and S4). We also compared charge transfer resistance between Fe–Fe3C@C/CNT_Urea and Fe– Fe3C@C/CNT_Urea_A through electrochemical impedance spectroscopy (EIS) analysis (Figure S9). The impedance spectra were recorded from 10 kHz to 0.05 Hz at 0.8 V (vs RHE), at which oxygen reduction reaction (ORR) takes place. While the two catalysts showed virtually the same solution resistance values, the Fe–Fe3C@C/CNT_Urea_A catalyst clearly exhibited larger charge transfer resistance than Fe–Fe3C@C/CNT_Urea. The decrease of Fe–Nx sites results in the increase of charge transfer resistance, confirming that the ORR proceeds predominantly at the Fe–Nx sites. Given that the main difference between the two catalysts is the number of Fe–Nx sites, this result clearly indicates that the main role of Fe–Nx sites in ORR is catalyzing direct, 4electron reduction of oxygen. We also note that pyridinic and graphitic nitrogen or Fe–Fe3C@C sites may play an insignificant role in the ORR. Further investigation of the functions of Fe–Nx sites was performed by poisoning experiments using potassium cyanide (KCN) as a probe molecule (Figure S10). As reported earlier,5,36,42 CN− ions completely block Fe–Nx sites, resulting in a huge decline of ORR activity. When 10 mM KCN was added in the electrolyte, the half-wave potential of resulting Fe– Fe3C@C/CNT_Urea_KCN showed a dramatic negative shift of 100 mV, compared to that of Fe–


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Fe3C@C/CNT_Urea. Hence, the CN− poisoning experiments further corroborates that the Fe–Nx sites trigger the 4 e− pathway oxygen reduction. We note that the CN− poisoning induced a more severe loss of catalytic activity than the acid treatment. Previously, Mukerjee et al. revealed that there are two types of Fe–Nx sites: one type (Type I) is unrecoverable after acid treatment, while the other type (Type II) can be recovered by heating above 300 °C or base washing.22,57 Similarly, in our system, Type I Fe–Nx sites were eliminated from acid leaching, while Type II Fe–Nx sites were preserved, resulting in existence of residue Fe–Nx sites. We also performed the CN− poisoning

experiment

with

Fe–Fe3C@C/CNT_Urea_A,

and

the

resulting

Fe–

Fe3C@C/CNT_Urea_A_KCN exhibited a substantial decrease in ORR activity, similar to the case

of

Fe–Fe3C@C/CNT_Urea_KCN.

Interestingly,

the

catalytic

activity

of

Fe–

Fe3C@C/CNT_Urea_A_KCN was worse than that of Fe–Fe3C@C/CNT_Urea_KCN. If the acid treatment deteriorates just Fe–Nx sites but does not affect other catalytic sites, the catalytic activity of Fe–Fe3C@C/CNT_Urea_A_KCN should be very similar to that of Fe– Fe3C@C/CNT_Urea_KCN. We suppose that the acid treatment could modify C–Nx sites toward unfavorable state for the ORR. One possibility to account for the modification is alternation of electron donating power (basicity) in C–Nx sites during acid treatment. The electron donating power from catalyst to the adsorbed oxygen has been suggested as one of activity descriptor for the ORR.58 The acid treatment could also change the basicity of carbon surface by introducing acidic oxygen functional groups (i.e. carboxyl groups, lactones, phenol and lactol groups), exerting an adverse impact on C–Nx sites for the ORR.59 Compared

to

Fe–Fe3C@C/CNT_Urea

and

Fe–Fe3C@C/CNT_Urea_A,

Fe–

Fe3C@C/CNT_CO_A showed substantially lower ORR activity, manifested by its lower E1/2, higher HO2− yield, and lower electron transfer number (Figures 4a,b and S5). The lower activity



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of Fe–Fe3C@C/CNT_CO_A could be further substantiated by its higher Tafel slope (67 mV/dec), lower kinetic current densities at 0.8 V (0.11 mA cmgeo–2 and 0.54 µA cmECSA–2), and higher charge transfer resistance, compared to those of Fe–Fe3C@C/CNT_Urea and Fe– Fe3C@C/CNT_Urea_A (Figure S7, Tables S3 and S4, Figure S9), representing slow reaction kinetics of Fe–Fe3C@C sites. We note that the amount of Fe in Fe–Fe3C@C/CNT_CO_A (6.3 wt%) sample was higher than that in Fe–Fe3C@C/CNT_Urea_A (4.1 wt%), as revealed in ICPOES analysis results (Table S1). Given most of the remaining Fe-based species in the both samples after the acid treatment would be Fe or Fe3C nanoparticles encapsulated within carbon layer, this indicates that Fe–Fe3C@C sites in Fe–Fe3C@C/CNT_CO_A outnumbered that in Fe– Fe3C@C/CNT_Urea_A. However, ORR catalytic activity of Fe–Fe3C@C/CNT_CO_A is far lower than that of Fe–Fe3C@C/CNT_Urea_A, suggesting that higher ORR activity of Fe– Fe3C@C/CNT_Urea_A could originate from other sites such as C–Nx sites or Fe–Nx residues rather than Fe–Fe3@C sites. This implies that Fe–Fe3C@C sites exhibits slower kinetics for catalyzing ORR than Fe–Nx sites or C–Nx sites in low overpotential region.


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Figure 5. ORR polarization curves of (a) Fe–Fe3C@C/CNT_CO_A and CNT, and (c) Fe– Fe3C@C/CNT_Urea_KCN (with 10 mM KCN) and N-CNT. HO2‒ reduction current density of (b) CNTs and Fe–Fe3C@C/CNT_CO_A, and (d) Fe–Fe3C@C/CNT_Urea_KCN and N-CNT, with various H2O2 concentrations at 0.3 V vs reversible hydrogen electrode (RHE) in N2saturated 0.1 M KOH electrolyte.

To further clarify the role of the Fe–Fe3C@C sites, the ORR activity of acid-treated bare CNTs was measured and compared with Fe–Fe3C@C/CNT_CO_A. Fe–Fe3C@C/CNT_CO_A exhibited relatively higher E1/2 (Figure 5a), lower HO2− yield, and higher electron transfer number (Figure S11) than the bare CNTs. This result may suggest that the inner metallic Fe– Fe3C particles are responsible for promoting facile 2-electron pathway ORR and sequential peroxide reduction (2 e− × 2 e− oxygen reduction). Indeed, in peroxide reduction experiments in 15 ACS Paragon Plus Environment


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N2-saturated electrolyte, Fe–Fe3C@C/CNT_CO_A exhibited higher reduction current than bare CNTs at all tested H2O2 concentrations, indicating that peroxide reduction is also more facilitated on Fe–Fe3C@C sites than on the hollow carbon layers of the CNTs (Figures 5b and S12). Previously, Bao and co-workers proposed that electron transfer from the inner Fe particles to surface carbon could activate the carbon surfaces by decreasing the local work function, thereby enhancing the catalytic activity for ORR.35 In our model catalysts, such a phenomenon was experimentally confirmed, and the unique core-shell structure promotes facile 2 e− × 2 e− ORR. The above-mentioned possible role of Fe–Fe3C particles during the ORR was further confirmed in the N-doped catalysts, by comparing the catalytic activities of Fe– Fe3C@C/CNT_Urea_KCN and nitrogen-doped CNTs (N-CNT). As mentioned above, Fe– Fe3C@C/CNT_Urea_KCN is completely free of Fe–Nx sites and comprises Fe–Fe3C@N-C and C–Nx sites, while N-CNT contains only the C–Nx species. Consistent with the nitrogen-free system, Fe–Fe3C@C/CNT_Urea_KCN showed better ORR activity and higher electron transfer number than N-CNT (Figures 5c and S13). In peroxide reduction experiments (Figures 5d and S14), Fe–Fe3C@C/CNT_Urea_KCN also produced relatively higher reduction currents. Therefore, it was concluded that regardless of the presence of nitrogen on the outer carbon layers, the inner Fe–Fe3C in Fe–Fe3C@C sites activates the carbon layers to catalyze facile 2 e− × 2 e− oxygen reduction, performing an auxiliary role for the ORR. We note that similar results on the role of Fe-based species were drawn from infrared reflection-absorption spectroscopy based study.60


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Scheme 1. Illustration of the role of Fe–Nx and Fe–Fe3C@C sites for ORR.

CONCLUSIONS In conclusion, model Fe–N/C catalysts were prepared and studied to understand the catalytic role of Fe–Nx and Fe–Fe3C@C sites. The synthesis exploited the iron oxide conversion method, which selectively yielded Fe–Nx, Fe–Fe3C@C, and C–Nx sites in the resulting catalysts. As illustrated in scheme 1, the physicochemical and electrochemical characterization results established that the Fe–Nx sites play the major role in catalyzing the ORR with the 4 e− pathway, whereas the role of Fe–Fe3C@C sites is mainly auxiliary by promoting the 2 e− × 2 e− oxygen reduction pathway. We believe that these results about the roles of Fe–Nx and Fe–Fe3C@C species can provide a guideline for designing high-performing Fe–N/C catalysts.

MATERIALS AND METHODS



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Acid Treatment of CNTs. Two acid treatment-washing cycles were used to remove metal impurities from the CNTs (Carbon Nanomaterial Technology Company, MR 99). CNTs (10 g) were dispersed in HNO3 (380 g, 63 wt%, Samchun) and deionized (DI) water (325 g, Millipore Milli-Q system, 18.2 MΩ cm), and the solution was kept at 80 °C for 12 h. After filtration, the nitric acid-treated CNTs were washed with copious amount of DI water, and the filter cake was dried in an oven at 60 °C. The second acid treatment-washing cycle was carried out similarly, except the acid was HCl (390 g, 37 wt%, Samchun) in DI water (320 g). The obtained CNTs were used for the subsequent experiments. Preparation of Fe3O4 NPs. Fe3O4 NPs were synthesized by using a metal-oleate complex.47 In the typical preparation of the iron-oleate complex, iron chloride (2.70 g, FeCl3·6H2O, Aldrich) and sodium oleate (9.13 g, 95%, TCI) were dissolved in a solvent mixture composed of ethanol (20 ml, 95%, Samchun), DI water (15 ml), and n-hexane (35 ml, 95%, J.T.Baker). The resulting solution was kept at 70 °C for 4 h. After the reaction was completed, the upper organic layer containing the iron-oleate complex was washed three times with sufficient amount of DI water in a separatory funnel. Then, n-hexane was evaporated off at 80 °C. In a 100 mL three-neck flask, the prepared iron-oleate complex (3.0 g) and oleic acid (0.53 g, 90%, Aldrich) were dissolved in 1-octadecene (16.67 g, 90%, Aldrich). The solution was heated to 320 °C with a ramping rate of 3.3 °C min–1, and kept at 320 °C for 30 min. After the reaction, the solution was cooled to room temperature and precipitated with acetone (99.9% Samchun) by centrifugation. The precipitated Fe3O4 NPs were dispersed in chloroform (30 ml, 99.5%, Samchun).


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Preparation of Fe3O4/CNT. In a 250 mL Erlenmeyer flask, CNTs (1.13 g) was added into chloroform (80 ml) and sonicated for 30 min. After the sonication, the as-prepared Fe3O4 NPs dispersed in chloroform (30 ml) were added to the solution and the sonication was continued for 1 h. The impregnated Fe3O4/CNT was separated from solvent by centrifugation. Preparation of Fe–Fe3C@C/CNT_CO. Prior to the conversion reaction using CO, Fe3O4/CNT (0.24 g) was annealed in air at 300 °C for 1 h to eliminate capping agent enclosing the Fe3O4 NPs,. The sample was then heated to 900 °C with a 9.7 °C min–1 ramping rate, and kept at 900 °C for 2 h under CO gas (30%/Ar balance, KOSEM) at a flow rate of 0.2 L min–1. After the reaction, the product was cooled with N2 gas flow. Preparation of Fe–Fe3C@C/CNT_Urea.

For the urea-mediated conversion,48

Fe3O4/CNT (0.24 g), urea (2.57 g, 99%, JUNSEI), and agar (51.4 mg, Aldrich) were mixed in a mortar. The mixture was then heated to 900 °C with a ramping rate of 9.7 °C min–1, and kept at 900 °C for 30 min under N2 gas (99.99%, KOSEM) at a flow rate of 1 L min–1. Preparation of Fe–Fe3C@C/CNT_CO_A and Fe–Fe3C@C/CNT_Urea_A. Fe– Fe3C@C/CNT_CO and Fe–Fe3C@C/CNT_Urea samples were treated with acid following the same protocol as for the bare CNTs described above. The acid-treated samples were denoted as Fe3C@C/CNT_CO_A and Fe–Fe3C@C/CNT_Urea_A, respectively. Preparation of N-CNT. CNTs (70 mg), urea (0.75 g), and agar (15 mg) were mixed in a mortar until homogeneous. The mixed powder was heated to 700 °C with a ramping rate of 9.7 °C min–1 and kept at 700 °C for 1 h under N2 flow. Physical Characterization. All characterization experiments were carried out at the Ulsan National Institute of Science and Technology Central Research Facilities (UCRF) Center. XRD patterns were obtained using an X-ray diffractometer (Rigaku D/Max 2500V/PC) equipped



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with a Cu Kα source operating at 40 kV and 200 mA. XPS measurements were performed on an ESCLAB 250Xi system (Thermo Scientific) equipped with a monochromatic Al Kα X-ray source (1486.6 eV). Individual chemical components of the N 1s binding energy region were fitted to the spectra using the mixed (Gaussian 70, Lorentzian 30) function after Shirley-type background subtraction. TEM images were obtained by a JEOL 2100 instrument. Atomic resolution TEM images and EELS spectrum were obtained by a FEI Titan3 G2 60-300 with an image-side spherical aberration (Cs) corrector at an accelerating voltage of 80 kV. HAADFSTEM analysis was performed using a JEOL JEM 2100F instrument with a probe-forming Cs corrector at 200 kV. Elemental analysis was performed by a Truspec Micro (Leco) instrument. The Fe content of each sample was analyzed using an ICP-OES analyzer (Varian). Electrochemical Characterization. Half-cell tests using RRDE (ALS) were conducted using an IviumStat electrochemical analyzer at room temperature (25 °C) with a three-electrode electrochemical cell. A graphite rod and an Hg/HgO electrode (CHI152, CH Instruments; 1 M KOH filling solution) were used as the counter and reference electrodes, respectively. Prior to each measurement, the RRDE was polished first with a 1.0 µm alumina suspension and then with a 0.3 µm suspension to afford a mirror finish. In a typical preparation of the catalyst ink, the catalyst (5 mg), DI water (50 µL), 5 wt% Nafion® (20 µL, D521, DuPont), and ethanol (530 µL, 99.9%, Samchun) were mixed and ultrasonicated for 20 min. For the benchmark Pt/C catalyst (20 wt% Pt, HiSPEC-3000, Johnson-Matthey), a catalyst ink was prepared with the catalyst (3.5 mg), DI water (0.1 mL), ethanol (1.07 mL), and Nafion® (0.03 mL). The catalyst ink (3 µL for Pt/C, 9 µL for all other catalysts) was dropped with a micro-pipette onto the glassy carbon disk (area: 0.1257 cm2) of the RRDE, and dried at 70 °C for 1 min. The resulting catalyst loading was 600 µg cm−2 for the Fe–N/C catalysts and 14 µgPt cm−2 for Pt/C. All electrochemical


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measurements were carried out in 0.1 M KOH (99.99%, Aldrich) solution. In the poisoning experiments, 10 mM KCN (97%, Aldrich) was added to the 0.1 M KOH aqueous solution. Before the electrochemical measurements, the catalyst was cleaned by cycling the potential between 0.05 and 1.2 V (vs. RHE) for 50 cycles at a scan rate of 100 mV s−1 (500 mV s−1 for Pt/C) in an N2-saturated electrolyte. Subsequently, cyclic voltammetry (CV) was performed in the potential range of 0.05 to 1.2 V at a scan rate of 20 mV s−1 (50 mV s−1 for Pt/C). Linear sweep voltammetry (LSV) polarization curves for the oxygen reduction reaction (ORR) were obtained by sweeping the potential from 1.2 V to 0.2 V (from −0.01 V to 1.1 V for Pt/C), in an O2-saturated electrolyte with O2 purging at the electrode rotating speed of 1600 rpm. In order to correct the non-Faradaic current (capacitive current) in the LSV curve, the same measurement was also conducted in N2-saturated electrolyte. To measure the solution resistance for iRcompensation and charge transfer resistance, electrochemical impedance spectra were obtained at 0.8 V with AC potential amplitude of 10 mV from 10 kHz to 0.05 Hz. ORR measurements were independently repeated at least three times and the average data were presented. The electrochemical double-layer capacitance for estimating ECSA was measured by non-Faradaic capacitive current with CV scans. The range of CV scan was ± 0.05 V centered at the open circuit potential on N2 condition to minimize Faradaic current, which indicate that all current from CV can be assumed double-layer charging. The scan rate was 10, 20, 30, 40, and 50 mV/s and third cycle of CV scans were collected at each scan speed. The double-layer charging current, ic satisfies below equation. =



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where v and CDL indicate scan rate and electrochemical double-layer capacitance, respectively. Thus, a slope in a plot of ic as a function of v is equal to CDL. ECSA of catalysts can be calculated by dividing the CDL with specific capacitance (Cs) of the sample as below equation. ECSA =

Cs value have been measured for a variety of metal electrodes in acidic and alkaline solutions and typical values reported are 0.022−0.130 mF cm–2 in NaOH and KOH solutions.61 In this work 0.040 mF cm–2 was used. The hydrogen peroxide (H2O2) reduction measurements were carried out in an H2O2containing 0.1 M KOH. In an N2-saturated 0.1 M KOH solution (80 mL), two cycles of CV without H2O2 were conducted in the voltage range from 0.2 V to 1.2 V at a scan rate 50 mV s–1. The H2O2 reduction currents were measured by CV after adding successively 8, 32, 40, 320, and 400 µL (1, 5, 10, 50, and 100 mM, respectively) of 30% H2O2 (Fluka) solution. The capacitive current was eliminated in the same manner as with ORR. To explore the reaction selectivity of the catalysts, The HO2– yield and electron transfer number (n) during the ORR were calculated from the following equations using RRDE measurements,

= 200% ×

+

,

=

4

1 + ×

where iD and iR are the disk and ring currents, respectively, and N is the ring collection efficiency. However, in the presence of KCN, the Pt ring on RRDE is poisoned so that the ring current cannot be measured. Thus, the electron transfer number was instead calculated using the


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Koutecky-Levich plots with different rotating speeds. The Koutecky-Levich equation relates the inverse current density with the inverse square root of the rotating speed as follows. 1 1 1 1 1 = + = + , ! "#$/

&ℎ()( " =

/1

0.62,-./ 0./ 2$/3

where i is the experimentally determined current, ik is the kinetic current, id is the diffusionlimited current, F is the Faraday constant (96485 C mol–1), A is the geometric area of the electrode (0.1257 cm2), CO2 is the O2 concentration in the electrolyte (1.26 ×10–3 mol L–1), DO2 is the diffusion coefficient of O2 in the KOH solution (1.93 × 10–5 cm2 s–1), and 2 is the viscosity of the electrolyte (1.01 × 10–2 cm2 s–1).

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Tables for elemental analysis and ICP-OES results and Figures for additional TEM and HAADF-STEM images, O 1s XPS spectra, and additional electrocatalysis data.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.



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ACKNOWLEDGEMENT This work was supported by the National Research Foundation (NRF) of Korea (2015M1A2A2056560) and the KEIT funded by the MOTIE (10050509).


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TOC Graphic


C Electrocatalysts

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