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5
L AYER N UMBER -C ONTROLLED F E –N/C C ATALYSTS
R EVEAL AN U NEXPECTED T REND IN O XYGEN
R EDUCTION R EACTION A CTIVITY
5.1.INTRODUCTION
In light of ever-increasing climate change, the development of hydrogen-based energy cycle that can replace the current hydrocarbon energy in a carbon-neutral manner is imperative. Overall performance of hydrogen energy cycle depends critically on the efficiency of energy conversion electrocatalysts that catalyze interconversions between H2 and O2 and H2O.1–4 For fuel cells that convert chemical energy stored in the fuel (e.g. H2) to electrical energy, the development of highly active electrocatalysts that can overcome intrinsically sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode has been an issue of foremost importance. Despite continued, decades- long efforts, platinum (Pt) or Pt-based alloy materials are still mainstays as cathodic catalysts.5–10 However, they are expensive, scarce, and unevenly distributed, which have hindered their widespread deployment.1–4,11,12 In this context, multifaceted efforts have been made to develop inexpensive non- precious metal catalysts (NPMCs) for the ORR.
Among the classes of NPMCs, Fe–N/C catalysts comprising an atomically dispersed Fe–Nx
species as an active site are the most promising alternatives to commercial Pt catalysts due to their high ORR activity.13–21 Various synthetic strategies have been developed to synthesize catalysts containing a high density of Fe–Nx active sites. Representative synthetic strategies include (i) the exploitation of metal–organic frameworks (MOFs) as hosts or precursors,15,17,18 (ii) the use of sacrificial templates,16 and (iii) the preferential generation of active sites.19–21 However, it is difficult to construct a rationally controlled catalyst system because most of the synthetic methods are based on wetness impregnation, physical mixing, or ball milling of randomly mixed metal/nitrogen/carbon precursors. In addition, during the high-temperature heat treatment required for structural integrity of the active sites, unintended aggregation of metals, collapse of a carbon support, and evaporation of nitrogen species take place simultaneously.22–24 We consider the formation of coordinated structure between metal, nitrogen, and carbon precursors can help develop rationally designed Fe–N/C catalysts.
We noted that Fe2+ ions and phenanthroline (Phen) easily form a Fe(Phen)3 complex in the presence of a solvent. However, previous studies have revealed that inactive Fe clusters are readily formed when
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precursors were mixed under wet conditions during the synthesis of Fe–N/C catalysts.25
To address this problem, we polymerized Fe2+ and phenanthroline under dry conditions using silanol groups on the pore surfaces of a mesoporous silica. This allowed for the gradual growth of Fe–
N-containing carbon layer from the surface of silica pore walls. As the amount of precursor was increased, the framework structure of catalysts changed from a tube-type with 3–4 carbon layers to a rod-type with 20 carbon layers. The tube-type catalyst exhibited a larger specific surface area and CO adsorption capacity than the rod-type catalyst. Surprisingly, the rod-type catalyst showed much greater ORR activity than the tube-type catalyst with 31 times higher turnover frequency (TOF). It was found that the enhancement of ORR activity in the rod-type catalyst with a larger layer number is related to the change in oxygen binding mode. In the rod-type catalyst, O2 is expected to be adsorbed and dissociated between two metal centers, whereas, in the tube-type catalyst, O2 is expected to be adsorbed on on-top site with a single metal center. The optimized rod-type Fe–N/C catalysts demonstrated excellent ORR activity with half-wave potentials of 0.92 V and 0.80 V in alkaline and acidic electrolytes, respectively. The ORR activity in terms of half-wave potential is comparable to that of the best Fe−N/C catalysts to date.
5.2.EXPERIMENTAL METHODS 5.2.1. Synthesis of Fe-Phen_X Catalysts
The synthesis of Fe-Phen_X (X = weight ratio of Fe, N precursors to silica template) was carried out through dry mixing of FeAc, Phen, and mesoporous silica SBA-15. Mesoporous silica SBA-15 with hexagonal symmetry was synthesized as described in previous reports,37,38 with some modifications. 18.5 g of Pluronic P123, 110.0 g of HCl, and 580.0 g of deionized (DI) water were added to a 1 L polypropylene (PP) bottle, and the mixture was stirred at 35 °C until P123 was completely dissolved. 39.0 g of TEOS was added to P123 solution, and the mixture was stirred for 5 min and aged at 35 °C without stirring for 24 h. The mixture was subjected to a hydrothermal treatment at 150 °C for 24 h. After the hydrothermal treatments, the resulting white-colored precipitates were filtered, washed twice with DI water, and dried in an oven at 60 °C. To remove P123 structure-directing agent, the dried powder was added to a solution containing 10.0 g of HCl and 395.0 g of EtOH. The mixture was stirred for 30 min, filtered, washed with EtOH, and dried in an oven at 60 °C. Finally, the dried sample was calcined in air at 550 °C for 5 h, resulting in a mesoporous SBA-15 silica template. One gram of a mesoporous silica template and desired amounts of FeAc and Phen (FeAc to 1,10-phenanthroline molecular ratio was 1:3.) were mixed in an agate mortar for 10 min. For example, 1.0 g of precursors are used for the synthesis of Fe-Phen 1.0, and the precursors consists of 0.243 g of FeAc and 0.757 g of Phen. The mixture was heated from RT to 800 °C for 6 h, and maintained at that temperature for 3 h under N2 gas flow (1.0 L min−1). To remove the silica template, the resulting carbon-silica composite was mixed with 1:1 (v/v)=EtOH:10%
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aqueous HF solution, and the slurry was stirred for 30 min, filtered, and washed with EtOH several times. The HF etching process was repeated in the same manner, and the resulting sample was dried at 60 °C to afford Fe-Phen_X samples.
The Fe-Phen_C catalyst is synthesized through mixing of mesophase pitch, Fe–Nx@SBA-15 composite, and EtOH. The Fe–Nx@SBA-15 composite is obtained after 800 °C pyrolysis of the Fe- Phen_1.0 precursor mixture. 0.125 g of aromatic mesophase pitch (Mitsubishi Gas Chemical Company), 0.5 g of Fe–Nx@SBA-15 composite, and 4 mL of EtOH were added in an agate mortar and mixed using a pestle for 10 min. The black slurry was dried at 60 °C for 8 h. The dried powder was heated to 300 °C at a ramping rate of 1.4 °C min−1 under 1 L min−1 N2 flow and maintained at that temperature for 4 h. The temperature was subsequently elevated to 900 °C at a heating rate of 2.5 °C min−1 and maintained for 2 h. The resulting Carbon@Fe–Nx@SBA-15 composite was mixed with 1:1 (v/v)=EtOH:10% aqueous HF solution, and the slurry was stirred for 30 min, filtered, and washed with EtOH several times. The HF etching process was repeated in the same manner, and the resulting sample was dried at 60 °C to afford Fe-Phen_C sample. The Fe-Phen_C_Fe catalyst (layer-by-layer catalyst) is synthesized through dry mixing of FeAc, Phen, and Carbon@Fe–Nx@SBA-15 composite.
0.061 g of FeAc, 0.189 g of Phen, and 0.367 g of Carbon@Fe–Nx@SBA-15 composite were mixed in an agate mortar for 10 min. The subsequent steps after precursor mixing were the same as those used for the Fe-Phen_X catalysts described above.
5.2.2. Characterization Methods
Fourier-transform infrared spectroscopy (FT-IR) spectra were measured using a Varian model 670 equipped with a liquid nitrogen cooled germanium detector in the range 4,000–400 cm−1. HRTEM and HAADF-STEM images were acquired using a JEOL JEM-2100F and a FEI Titan3 G2 60-300 TEM equipped with a double-sided spherical aberration (Cs) corrector operating at an accelerating voltage of 200 kV. SEM images were taken on a scanning electron microscope (S4800, Hitachi) operating at 10 kV. The textural properties of the samples were analyzed using a nitrogen physisorption analyzer (BELSORP-Max, BEL JAPAN, Inc.) operated at 196 °C. Specific surface areas were calculated using the BET equation in the relative pressure range of 0.05–0.2, while pore size distributions of the samples were obtained using the BJH method based on adsorption branches of their isotherm. All samples were pre-evacuated at 150 °C and 10−2 Pa for 12 h before measurements.
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 4° 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
111
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).
5.2.3. 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 and normalization of the collected XAS data and Fourier-transform of the radial distribution functions were conducted using the Athena software39 with Rbkg of 1.2 and a Hanning-type window.
5.2.4. CO cryo Adsorption
Temperature-programmed desorption (TPD) profiles were collected using a BELCAT II instrument (MicrotracBEL) equipped with a thermal conductivity detector. Prior to each CO TPD experiment, 0.03 g of sample was heated up to 600 °C with a ramp of 10 °C min−1 followed by a hold 15 min under a He flow to remove pre-adsorbed O2 or H2O molecules.29 The second step is CO pulse chemisorption carried out at −80 °C using a liquid nitrogen. Ten consecutive 0.5 mL CO pulses at 600 s intervals were passed over the samples, and CO uptake per mole was monitored and quantified using a thermal conductivity detector (TCD). The last step is five consecutive CO pulse chemisorption after He purging for 20 min to quantify the amount of physisorbed CO.
5.2.5. O2 Temperature Programmed Desorption
Prior to each O2 TPD experiment, 0.05 g of sample was degassed at 350 °C for 1 h (ramping rate: 10 °C min−1) under a He flow to remove pre-adsorbed O2 or H2O molecules.31 After degassing, the sample was cooled to −50 °C and O2 adsorption was carried out for 1 h under 10% O2/He gas conditions, followed by He purging for 2 h at −50 °C to remove weakly adsorbed O2. Then, the TPD experiment was performed by heating the sample to 500 °C (ramping rate: 10 °C min−1) under a He flow.
5.2.6. Electrochemical Characterizations
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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 the Pt/C catalyst (20 wt % Pt, HiSPEC-3000, Johnson-Matthey), a catalyst ink was prepared by mixing 3.5 mg of Pt/C catalyst 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 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
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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.
The hydrogen peroxide (H2O2) reduction measurements were carried out in an H2O2-containing 0.1 M KOH and 0.1 M HClO4. In an N2-saturated electrolyte (200 mL), two cycles of CV without H2O2 were conducted in the voltage range from 0.2 to 1.2 V at a scan rate 50 mV s−1. The H2O2
reduction currents were measured by CV after adding successively 20.2, 81, 101.2, 809.8, and 1012.2 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.33
5.3.RESULTS AND DISCUSSION 5.3.1. Synthesis of Fe-Phen_X Catalysts
As shown in the Figure 5.1, Fe-Phen_X (X = weight ratio of Fe, N precursors to silica template) is synthesized through dry mixing of Fe acetate (FeAc), Phen, and mesoporous silica. The precursor mixture was pyrolyzed at 800 °C for 3 h under N2 gas (99.999%) at a flow rate of 1 L min−1. Finally, the silica template was etched to obtain the Fe-Phen_X. The Fe(Phen)3 complex is formed spontaneously when Fe2+ and Phen are dissolved together in a solvent. However, inactive Fe clusters are easily formed during the pyrolysis of Fe–N/C catalysts when using precursor mixture prepared with wetness conditions.25 Thus, we synthesized Fe(Phen)3 under dry conditions using silica template containing a large amount of hydrophilic silanol groups.
Figure 5.1. Schematic illustration for the preparation of Fe-Phen_X catalysts.
114 5.3.2. Fe(Phen)3 Complex Formation in Precursor Mixture
Through FT-IR, it was confirmed that Fe2+ and Phen formed the Fe(Phen)3 complex with silica even in dry conditions. We compared the FT-IR peaks of the mixtures of Fe-Phen_X with the those of comparative groups (Figure 5.2). As the comparative groups, pristine Phen, mixture of FeAc and Phen in dry condition, and Fe(Phen)3 complex synthesized in the ethanol were prepared. The peak of 738 cm−1 observed in the pristine Phen means C–H out-of-plane deformation (ring torsion). It was reported that the peak position shifts to a smaller wavenumber when nitrogen in Phen coordinates with metal species.26 Compared to Phen, no peak shift was observed in the mixture of FeAc and Phen, which means that Fe2+ and Phen do not bind in the absence of silica or solvent. In contrast, Fe(Phen)3
complex synthesized in the ethanol showed a peak at a wavenumber of 725 cm−1, indicating that bonds were formed between Fe2+ and Phen in the presence of ethanol. The FT-IR peaks of Fe-Phen_X mixtures, in which FeAc and Phen were dry-mixed with silica, were observed in the region between those of Fe(Phen)3 complex and Phen, meaning the formation of Fe(Phen)3. The FT-IR peaks of the Fe-Phen_X mixtures became similar to that of Phen with the amount of FeAc and Phen precursors increased. This can be attributed to an increase in the amount of Phen that does not form a Fe(Phen)3
complex as relatively large amount of the precursors were added.
Figure 5.2. The region (675–775 cm−1) of FT-IR spectra for the precursor mixtures of the Fe-Phen_X samples.