1. G ENERAL I NTRODUCTION
1.5. R EFERENCES
above, the Lee, Choi, and Li groups reported that atomically dispersed Pt catalysts exhibit a selective 2e− pathway ORR with low activity.42,86,89 On the contrary, the Sun165 and Wang166 groups reported an efficient 4e− pathway ORR using atomically dispersed Pt catalysts with high ORR activity. These contradictory results continue to the present day. Han and co-workers demonstrated that atomically dispersed catalysts supported on TiC exhibited the 2e− pathway ORR,124 whereas the Ma group reported that single-atom Pt supported on MoC and MoN exhibited a 4e− pathway ORR.167
Regarding the 4e− pathway ORR exhibited by non-precious metal-based atomically dispersed M–N/C catalysts, geometrically isolated metal sites do not always guarantee the preservation of the oxygen bond during the reaction. From a thermodynamic perspective,94,168 ORR selectivity is determined by the oxygen binding energy of catalysts in the associative mechanism due to the scaling relationship among oxygen-related intermediates. As the oxygen binding energy of the catalyst reduces, the catalyst tends to generate H2O2. Conversely, H2O is preferentially produced when the oxygen binding energy of the catalyst becomes stronger. Thus, depending on the type of metal and ligand used, the oxygen binding energy of atomically dispersed catalysts can be modulated. Consequently, the ORR catalytic properties of atomically dispersed catalysts can be varied. However, this hypothesis is only based on thermodynamic considerations. Thus, relevant experimental studies are necessary to resolve the discrepancies among previous results and understand the distinct catalytic trends of atomically dispersed catalysts. This dissertation presents our efforts to reveal the origin of the unique catalytic behavior of atomically dispersed catalysts in the ORR.
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2
R OLES OF Fe–N
XAND Fe–Fe 3 C@C S PECIES IN Fe–
N/C E LECTROCATALYSTS FOR O XYGEN R EDUCTION
R EACTION
This chapter includes the published contents:
Kim, J. H.; Sa, Y. J.; Jeong, H. Y.; Joo, S. H. ACS Appl. Mater. Interfaces 2017, 9, 9567–9575. DOI:
10.1021/acsami.6b13417. Reproduced with permission. Copyright © 2017 American Chemical Society.
2.1.INTRODUCTION
Polymer electrolyte membrane fuel cells (PEMFCs) have been regarded as promising energy conversion devices, owing to their high efficiency and environmental benign property.1,2 In PEMFCs, the development of highly active and durable cathode catalysts for the oxygen reduction reaction (ORR) has been one of the most important challenges. Using plenty of Pt-based nanoparticle (NP) catalysts have been a preferred solution; however, the high cost and scarcity of Pt have hindered the widespread deployment of PEMFCs. Thus, several classes of non-precious-metal-based ORR catalysts, such as transition metal and nitrogen co-doped carbons,3-6 metal oxide/carbon composites,7-9 and heteroatom- doped carbons,10-12 have been extensively investigated as alternatives. In particular, Fe–N/C catalysts where single iron atoms are anchored on nitrogen-doped carbon surface (Fe–Nx sites) have been the most prominent type of non-precious-metal-based catalysts, because of their Pt-like ORR activity. To design high-performance Fe–N/C catalysts, it is essential to understand their active sites. While multiple active sites have been suggested for these catalysts, such as Fe–Nx sites,13,14 pyridinic or quaternary nitrogen,15,16 and defect sites of graphitic carbon,17 there has been a growing consensus that the Fe–Nx
sites are the major active centers.
Metallic iron and/or iron carbide encapsulated within graphitic carbon shell (Fe–Fe3C@C), recently, has also been newly proposed as a strong candidate of the active site in Fe–N/C catalysts.18-24 A broad spectrum of possible roles has been suggested for the Fe–Fe3C@C species. Some groups reported that high ORR activity could be achieved with catalysts containing only Fe–Fe3C@C sites.18-
21 In contrast, other groups pointed out that outstanding ORR activity of Fe–N/C catalysts originate from the synergistic effect of Fe–Fe3C@C sites in conjunction with the Fe–Nx sites.22-24 Another
viewpoint is that Fe–Fe3C@C sites are merely impurity phases. Hence, efforts were made to suppress their formation while maximizing the Fe–Nx sites.25-28 Such controversy on the role of Fe–Fe3C@C sites seems to stem from the difficulty in selectively synthesizing Fe–Nx and Fe–Fe3C@C sites in the preparation of Fe–N/C catalysts. Specifically, the synthetic procedure of Fe–N/C catalysts commonly involves the mixing of iron, nitrogen, and carbon precursors, followed by high-temperature pyrolysis.
18-28 In those synthetic conditions, both Fe–Nx and Fe–Fe3C@C sites can be easily generated.
Consequently, the catalytic roles of Fe–Nx and Fe–Fe3C@C species have been elusive.
Herein, we rationally designed model catalysts through an iron oxide conversion method. Starting with Fe3O4 NPs supported on carbon nanotubes (CNT), i.e., Fe3O4/CNT, three model catalysts were fabricated, which selectively contain atomically dispersed Fe–Nx, Fe–Fe3C@C, and N-doped carbon (C–Nx) sites. Physicochemical and electrochemical characterization of these catalysts demonstrated that the Fe–Nx sites efficiently catalyze the ORR via four-electron (4 e−) pathway, playing a major role for superb ORR activity; whereas the Fe–Fe3C@C sites mainly promote two-electron (2 e−) oxygen reduction and sequential 2-electron peroxide reduction (2 e− × 2 e− pathway), playing an auxiliary role for the ORR.
2.2.EXPERIMENTAL METHODS 2.2.1 Synthesis of Fe3O4/CNT
CNT (Carbon Nanomaterial Technology Company, MR 99) support was used after removing metal impurities by acid treatment. CNT (10 g) was dispersed in nitric acid (380 g, 63 wt%, Samchun) and deionized (DI) water (325 g, Millipore Milli-Q system, 18.2 MΩ cm). The solution was kept at 80 °C for 12 h. With vacuum filtration, the HNO3-treated CNTs were washed with sufficient amount of DI water, and dried in an oven at 60 °C. The second acid treatment-washing cycle was carried out similarly, except the used acid from HNO3 to HCl (390 g, 37 wt%, Samchun) in DI water (320 g).
Fe3O4 NPs were synthesized by using a metal-oleate complex.29 Hence, prior to the synthesizing Fe3O4 NPs, the iron-oleate complex was prepared by dissolving iron chloride (2.70 g, FeCl3·6H2O, Aldrich) and sodium oleate (9.13 g, 95%, TCI) 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, using a separatory funnel the upper organic solvent layer containing the iron-oleate complex was washed three times with copious amount of DI water. The remaining organic solvent was evaporated at 80 °C. For synthesizing Fe3O4 NPs, the prepared iron-oleate complex (3 g) and oleic acid (0.53 g, 90%, Aldrich) were dissolved in 1-octadecene (16.67 g, 90%, Aldrich) in a 100 mL three-neck flask. 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 was completed, the solution was cooled to room temperature (RT) and precipitated products were washed with acetone (99.9% Samchun) by
centrifugation. The precipitated Fe3O4 NPs were dispersed in chloroform (30 ml, 99.5%, Samchun). In a 250 mL erlenmeyer flask, CNT (1.13 g) was dispersed into chloroform (80 ml) and the solution was sonicated for 30 min. The as-prepared Fe3O4 NPs dispersed in chloroform (30 ml) were added to the CNT-dispersed chloroform solution and sonicated again for 1 h. The resulting Fe3O4/CNT was separated from solvent by centrifugation.
2.2.2 Synthesis of model catalysts
Using Fe3O4/CNT as a precursor, we prepared three model catalysts which selectively contain Fe–Fe3C@C, C–Nx, and Fe–Nx sites. Firstly, to fabricate catalyst containing Fe–Fe3C@C, C–Nx, and Fe–Nx sites altogether, urea-mediated conversion of Fe3O4/CNT was carried out.30 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. For a second model catalyst, we eliminated Fe–Nx sites in urea-mediated catalyst by acid leaching with the same procedure which was carried by CNT. Lastly, for the preparation of catalyst, which exclusively contains Fe–Fe3C@C sites, Fe3O4/CNT (0.24 g) was annealed in air at 300 °C for 1 h to eliminate capping agent enclosing the Fe3O4 NPs before the conversion reaction using CO. 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 Ar-balanced 30% CO (KOSEM) at a flow rate of 0.2 L min–1. After the reaction, cooling step was performed with N2 gas flow. The product was treated with acid following same protocol with CNT as described above.
2.2.3 Characterization Methods
X-ray diffraction (XRD) patterns were acquired by X-ray diffractometer (Rigaku D/Max 2500V/PC) equipped with a Cu Kα source operating at 40 kV and 200 mA. X-ray photoelectron spectroscopy (XPS) measurements were carried out with an ESCLAB 250Xi system (Thermo Scientific) equipped with a monochromatic Al Kα X-ray source (1486.6 eV). The obtained XPS spectra were analyzed using the XPSPeak41 software. Individual chemical components of the N 1s binding energy region were fitted to the spectra using the Gaussian (Gaussian 70, Lorentzian 30) function after Shirley- type background subtraction. Transmission electron microscope (TEM) images were obtained by a JEOL 2100 instrument under an accelerating voltage of 200 kV. Atomic resolution TEM images and electron energy loss spectroscopy (EELS) spectrum were attained by a FEI Titan3 G2 60-300 with an image-side spherical aberration (Cs) corrector at an accelerating voltage of 80 kV. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) analysis and elemental mapping by energy dispersive spectroscopy (EDS) were performed using a JEOL JEM 2100F instrument with a probe-forming Cs corrector under an accelerating voltage of 200 kV. Elemental analysis (EA) was performed by a Truspec Micro (Leco) instrument. All characterization experiments