2. I MPACT OF T EXTURAL P ROPERTIES OF M ESOPOROUS P ORPHYRINIC C ARBON
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4
S TRUCTURAL E VOLUTION OF A TOMICALLY
D ISPERSED F E S PECIES IN F E –N/C C ATALYSTS
P ROBED BY X- RAY A BSORPTION AND 57 F E
M ӦSSBAUER S PECTROSCOPIES
This chapter includes the published contents:
Woo, J.; Choi, H.; Sa, Y. J.; Kim, H. Y.; Lim, T.; Jang, J.-H.; Yoo, S. J.; Kim, J. Y.; Joo, S. H. J.
Phys. Chem. C 2021, Accepted for Publication. Reproduced with permission. Copyright © 2021 American Chemical Society.
4.1.INTRODUCTION
In current low-temperature polymer-electrolyte fuel cell (PEFC) technologies, the development of high-performance cathode catalysts that can overcome the intrinsically slow kinetics of the oxygen reduction reaction (ORR) is one of the most critical issues. Notwithstanding their long history of development, the mainstay of cathode catalysts still relies on Pt-based catalysts. However, they suffer from high cost, scarcity, and uneven global distribution.1–6 This has triggered a recent drive toward the pursuit of non-precious metal-based catalysts (NPMCs).7–43 Among the diverse classes of NPMCs, metal and nitrogen-codoped carbon (M–N/C) catalysts are considered the most promising alternatives to Pt-based catalysts due to their excellent ORR activity, even rivaling those of commercial Pt/C catalysts.12–35 The high activity of M–N/C catalysts originates from the presence of catalytically active and atomically dispersed metal–nitrogen-coordinated (M–Nx) sites.14,15,36–43
Typical preparation methods for M–N/C catalysts involve high-temperature (600–1100 °C) pyrolysis to endow electrical conductivity and structural integrity,44,45 during which a significant portion of metallic species is aggregated into less-active metal particles.46 Consequently, a number of synthetic strategies have been developed to prepare M–N/C catalysts comprising a high density of active M–Nx sites even after high-temperature pyrolysis.29,40,47–52 Notable methods rely on repetitive heating-and-acid treatments or heat treatments under NH3 or diluted H2 gas environments to remove inactive metal or metal carbide particles, making the resulting catalysts enriched with M–Nx sites.47–49 The controlled thermal conversion of metal–organic frameworks (MOFs) has also been demonstrated to generate M–N/C catalysts with a high density of active sites.22,50–52 Another prominent method entails the addition or coating of inorganic materials, such as silica and NaCl.29,34,52–56 The inorganic
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materials are generally added before high-temperature pyrolysis and are removed by leaching afterward. Based on this principle, we recently developed a “silica-protective-layer-assisted” strategy that could preferentially generate catalytically active Fe–Nx sites, while suppressing Fe clusters.29 This strategy is generally effective for Fe–N/C catalysts synthesized under a wide range of synthesis conditions, employing different precursors, carbon supports, Fe contents, and pyrolysis temperatures.34 Regarding the role of inorganic materials, it has been suggested that they may suppress the formation of metallic particles and prevent the rapid decomposition of precursors.
However, the detailed role of inorganic materials in generating M–Nx sites remains elusive.
This study was undertaken to reveal the role of the inorganic silica layer in the formation of atomically dispersed Fe-based species during the silica coating-mediated synthesis of Fe–N/C catalysts by X-ray absorption spectroscopy (XAS) and 57Fe Mӧssbauer spectroscopy. Among a variety of spectroscopic and microscopic methods, XAS and Mӧssbauer spectroscopy have been demonstrated to provide the most detailed structural information on the active site structures of Fe–
N/C catalysts. From XAS and Mӧssbauer spectroscopy, qualitative and even quantitative information regarding Fe oxidation states, spin states, and coordination environments of various Fe–Nx sites could be gleaned.57–59 In this study, we synthesized a carbon nanotube coated with a thin layer of porphyrinic carbon (CNT/PC) using an isotope 57Fe-enriched porphyrin precursor. The preparatory steps of the silica coating-mediated synthesis of 57Fe-enriched Fe–N/C catalysts were monitored by X- ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), and Mӧssbauer spectroscopy. From the XAS data of the intermediate sample after silica coating, it was found that new atoms, other than the four surrounding N atoms, were axially bonded to Fe. In addition, this interaction was confirmed as the Fe–Si doublet using Mӧssbauer spectroscopy. Upon comparing the Fe species of the CNT/PC and that synthesized without silica coating (CNT/PC_w/o SiO2), it was observed that most of the Fe in the CNT/PC were in the form of atomically dispersed Fe sites (91%), whereas the CNT/PC_w/o SiO2 had significant Fe-based particles (55%). We believe that the silica protective layer forms a Fe–Si interaction with the center Fe atom and inhibits the formation of agglomerated crystalline Fe particles during high-temperature pyrolysis. The ORR kinetic current density at 0.9 V of CNT/PC was twice as high as that of CNT/PC_w/o SiO2 in the 0.1 M KOH electrolyte. Therefore, the silica coating strategy is advantageous for enhancing the ORR activity by enabling the development of a catalyst with a higher density of Fe–Nx active sites.
85 4.2.EXPERIMENTAL METHODS
4.2.1. Synthesis of 57CNT/PC Catalysts
The synthesis of CNT/PC, employing 57FeTMPPCl as the Fe and N precursors, was carried out following the silica-coating-mediated synthesis process with some modifications.29 0.75 g of
57FeTMPPCl was ground in a mortar for 3 min, and the resulting fine powder was mixed with 1.5 g of acid-treated CNT for 20 min. The 57FeTMPPCl–CNT mixture was heated to 400 °C at a ramping rate of 2 °C min−1 and maintained at 400 °C for 1 h under N2 gas (99.999%) at a flow rate of 1 L min−1. The resulting composite was mixed with 7.5 mL of TEOS in a mortar, followed by mixing with 7.5 mL of formic acid. The paste-like mixture was kept at room temperature (RT) for 12 h in a fume hood and was then heated to 800 °C at a ramping rate of 2 °C min−1 and maintained at that temperature for 3 h. To etch the silica, the pyrolyzed composite was mixed with 1:1 (v/v) = ethanol:10% aqueous HF solution. The slurry was stirred for 30 min, filtered, and washed with ethanol several times. The HF etching process was repeated in the same manner, and the product was oven-dried at 60 °C. The CNT/PC_w/o SiO2 catalyst was also prepared for comparison, and it was produced in the same way as the CNT/PC, but without the silica coating step.
4.2.2. Characterization Methods
X-ray diffraction (XRD) patterns were obtained with a high-power X-ray diffractometer (D/MAX2500V/PC, Rigaku) equipped with Cu Kα radiation operated at 40 kV and 200 mA. The XRD patterns were measured for the 2θ range from 15° to 65° at a scan rate of 2° min−1. X-ray photoelectron spectroscopy (XPS) measurements were performed on a K-Alpha spectrometer (Thermo Fisher Scientific) equipped with a monochromatic Al Kα X-ray source (1486.6 eV). The N 1s XPS peak deconvolution was carried out using the XPSPeak41 software. A Gaussian-Lorentzian (70:30) mixed function was used to fit the curve after a Shirley-type background removal. The Fe in the samples was analyzed using an inductively coupled plasma optical emission spectrometry (ICP- OES) analyzer (700-ES, Varian), whereas the carbon, hydrogen, nitrogen, and oxygen contents of the samples were determined with an elemental analyzer (Flash 2000, Thermo Fisher Scientific).
4.2.3. XAS Experiments
Fe K-edge XAS profiles were collected at RT in the 10C beamline of the Pohang Accelerator Laboratory (PAL). The storage ring was operated at 3.0 GeV with a beam current of 300 mA in the decay mode. The beamline is equipped with a focusing Si (1 1 1) double-crystal monochromator that is used to filter the incident photons. The X-ray was detuned by 30% to remove high-order harmonics and was calibrated using a standard Fe foil. The X-ray intensities were monitored using standard N2- filled ion chambers and an Ar-filled detector. The catalyst powder was pelletized in a sample holder (1 cm in width) to an adequate thickness to obtain a significant transmission signal. Background removal
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and normalization of the collected XAS data and Fourier-transform of the radial distribution functions were conducted using the Athena software64 with Rbkg of 1.2 and a Hanning-type window.
4.2.4. 57Fe Mössbauer Spectroscopy
The Mӧssbauer spectra were recorded in the transmission mode with a 57Co source in a rhodium matrix, and the sample temperature was maintained at 295 K. The electromechanical type Mӧssbauer spectrometer had a fixed absorber and was operated source on the constant-acceleration mode. Isomer shifts were calibrated relative to the centroid of the spectrum of a metallic α-Fe foil at RT. Mӧssbauer spectra were fitted by the least squares method, and provided the hyperfine field (Hhf), isomer shift (δiso), electric quadrupole splitting (ΔEQ), and relative area of Fe ions.
4.2.5. Electrochemical Characterizations
Electrochemical experiments were performed with a bipotentiostat (CHI760E, CH Instruments) and rotator (AFMSRCE, Pine Research Instrumentation) at RT using a three-electrode electrochemical cell. A Hg/HgO (CHI152, CH Instruments; 1 M KOH filling solution) electrode and a graphite rod were used as the reference and counter electrodes, respectively. The Hg/HgO reference electrode was calibrated with respect to the RHE before use. The RHE calibration was performed in a H2-saturated 0.1 M KOH solution, with a platinum coil as the working electrode and Hg/HgO as the reference electrode. With continuous H2 bubbling, a stable open circuit potential was obtained within 20 min, which corresponded to the RHE conversion value. A rotating ring-disk electrode (RRDE, AFE7R9GCPT, Pine Research Instrumentation) coated with a catalyst ink was used as the working electrode. Prior to every measurement, the RRDE was polished with a 1.0-µm-thick alumina suspension followed by a 0.3-µm-thick suspension to yield a mirror finish. The catalyst ink was prepared by mixing 15 mg of the catalyst powder with 50 µL of H2O, 505 µL of ethanol, and 37.6 µL of Nafion (5 wt%). For a commercial Pt/C catalyst (20 wt% Pt, HiSPEC-3000, Johnson-Matthey), a catalyst ink was prepared by mixing 3.5 mg of Pt/C with 100 μL of H2O, 1070 μL of ethanol, and 30 μL of Nafion. The mixture was ultrasonicated until the catalyst was well-dispersed. Additionally, 8 µL (6 µL for Pt/C) of the catalyst ink was pipetted with a micro-pipette onto the glassy carbon disk (0.247 cm2) of the RRDE, resulting in a catalyst loading of 800 µg cm−2 (70 µg cm−2 for Pt/C). A 0.1 M KOH electrolyte was prepared by dissolving high-purity KOH in 18.2 MΩ·cm of Millipore water. Before performing linear sweep voltammetry (LSV), the catalyst was cleaned by cycling the potential between 0.05 and 1.2 V (vs. RHE) for 20 cycles at a scan rate of 100 mV s−1 (50 cycles at a scan rate 500 mV s−1 for Pt/C) in a N2-saturated electrolyte. Subsequently, cyclic voltammetry (CV) was performed in the potential range from 0.05 to 1.2 V for three cycles at a scan rate of 20 mV s−1 (50 mV s−1 for Pt/C). To measure the solution resistance for iR-compensation, electrochemical impedance spectra were obtained at 0.68 V with an AC potential amplitude of 10 mV from 100 kHz to 1 Hz with
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a rotation speed of 1600 rpm. The LSV polarization curves for the ORR were obtained by sweeping the potential from 1.2 to 0.2 V (from −0.01 V to 1.1 V for Pt/C) at a scan rate of 5 mV s−1 (20 mV s−1 for Pt/C) in an O2-saturated electrolyte at a rotating speed of 1600 rpm. To correct the non-Faradaic current (capacitive current) from the LSV curve, the same measurement was conducted in a N2- saturated electrolyte. During the potential sweep the applied potential of the Pt ring was held at 1.3 V (vs RHE) to measure the 4-electron selectivity. The electron transfer number (n) was calculated from the following equation
where N, id, and ir represent the collection efficiency (0.37, provided by the manufacturer), the disk current, and the ring current, respectively. The measurements were independently repeated at least three times, and the average data were obtained.
4.2.6. AEMFC Performances
AEMFC tests were conducted based on the membrane electrode assemblies (MEAs). An MEA was fabricated using the catalyst-coated membrane (CCM) method by spraying a catalyst slurry onto FAA-3-50 membrane (Fumatech GmbH). CNT/PC and Pt/C (46 wt% Pt; Tanaka Co.) catalysts were used for the cathode and anode catalysts, respectively. For the catalyst slurry of CNT/PC, 9 mg of CNT/PC were mixed with 38.89 mg of FAA-3 ionomer (10 wt % FAA-3 in N-methyl-2-pyrrolidone, Fumatech GmbH), 0.3 mL of H2O, and 1.5 mL of IPA. For the Pt/C catalyst, a catalyst slurry was prepared by mixing 5.15 mg of Pt/C with 22.07 mg of FAA-3 ionomer and 1 mL of IPA. The ionomer content in each slurry was 30 wt%. The catalyst slurry of CNT/PC was deposited onto the membrane with a loading of 2.0 mg cm−2 on the cathode. The anode was prepared using Pt/C catalyst with a loading of 0.5 mgPt cm−2. For the benchmark, we also prepared Pt/C cathode-based AEMFC. The Pt loading on both electrodes for the Pt/C cathode-based AEMFC layers was 0.5 mgPt cm–2. The as- prepared CCM was soaked for 12 h in 1 M NaOH to improve the ionic conductivity of the membrane through replacing anions with OH– ions. Then, the treated CCM was rinsed with DI water until the complete elimination of Na+ ions. For the single-cell tests, the CCM was placed between two gas diffusion layers (GDL, SGL 39BC) made of carbon paper. The microporous layer of the GDLs was placed facing the catalyst layers on the CCM. With a gasket on both sides of the CCM–GDL complex, the whole MEA was inserted into a single cell module consisting of two graphite plates, each with a serpentine gas flow channel of 1 cm2 geometric area. The single-cell performance was evaluated using a single-cell test station (CNL Energy Co., Korea) with polarization tests through the current sweep method. The polarization curves were obtained at 60 °C, supplying fully humidified H2 (0.2 mL min–
1) and O2 (0.4 mL min–1) gases.
88 4.3.RESULTS AND DISCUSSION
4.3.1. Synthesis of 57CNT/PC Catalysts
Figure 4.1 illustrates the overall synthesis procedure for CNT/PC and CNT/PC_w/o SiO2
catalysts. To synthesize the CNT/PC catalyst, 57FeTMPPCl and CNT were physically mixed and heated at 400 °C to generate a composite of CNTs coated with thermally decomposed 57FeTMPPCl. A silica layer was then formed on the surface of the composite by condensation polymerization of TEOS in the presence of a formic acid catalyst, and the resulting material was pyrolyzed at 800 °C. Finally, the silica layer was etched to obtain the CNT/PC. The intermediate samples obtained after low- temperature annealing at 400 °C, silica coating, and high-temperature pyrolysis at 800 °C were designated as CNT/PC(LT), CNT/PC(SiO2), and CNT/PC(HT), respectively. The CNT/PC_w/o SiO2
catalyst was synthesized using the same procedure as that for the CNT/PC, except for the absence of the silica coating step. The intermediate sample after pyrolysis at 800 °C was denoted as CNT/PC_w/o SiO2(HT). More detailed synthesis methods were described in the Materials and Methods section.
Figure 4.1. Synthetic scheme for the preparation of CNT/PC catalysts.
4.3.2. XAS Analysis
First, we observed the evolution of the coordination environment of Fe centers with XAS.
Figure 4.2a shows the XANES spectra of the CNT/PC and its intermediate samples, as well as references (FePc, FeTMPPCl, and Fe foil). In the pre-edge region (7110–7120 eV), the reference FePc evidently showed peaks at 7114 eV (peak A) and 7118 eV (peak B), whereas FeTMPPCl exhibited only peak A. This difference between the two references originates from the coordination structure. In FePc, a central Fe atom is coordinated with four N atoms in a square planar structure, whereas FeTMPPCl has an additional Cl atom that is axially coordinated to the central Fe atom to form a square pyramidal structure. Hence, the presence or absence of peak B can serve as a fingerprint to distinguish the coordination structure of the central Fe: four-coordination square planar structure or