2. I MPACT OF T EXTURAL P ROPERTIES OF M ESOPOROUS P ORPHYRINIC C ARBON
2.2. E XPERIMENTAL M ETHODS
2.2.1. Synthesis of Mesoporous Silica Templates
Mesoporous silica SBA-15 with hexagonal symmetry was synthesized as described in previous reports,61,63 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 aged reaction mixture was divided into three portions. Each portion of the mixture was then subjected to a hydrothermal treatment at a desired temperature (35, 100, or 150 °C) for 24 h. For samples treated at 100 or 150 °C, the reaction mixture was transferred to a Teflon®-lined autoclave. 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 3.0 g of HCl and 118.4 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. According to the hydrothermal treatment temperature, the resulting samples were denoted as SBA- 15-X (X = hydrothermal treatment temperature). Mesoporous silica KIT-6 with cubic Ia3d symmetry was synthesized following the previously reported method.62 17.5 g of Pluronic® P123, 33.8 g of HCl, and 625.0 g of DI water were added to a 1 L PP bottle, and the mixture was stirred at 35 °C until P123 was completely dissolved. To this solution, 22.4 g of butanol was added and stirred for 1 h. Next, 55.5 g of TEOS was added and the mixture was stirred at 35 °C for 24 h, and subsequently aged at 100 °C for 24 h under static conditions. Subsequent washing and calcination steps were carried out in the same manner as described above.
2.2.2. Synthesis of Fe-MPC Catalysts
One gram of a mesoporous silica template and a desired amount of FeIIITMPPCl were mixed in an agate mortar for 10 min. The amount of FeIIITMPPCl was adjusted depending on the total pore volume of a mesoporous silica template; 1.0 g of FeIIITMPPCl was used per 1.0 cm3 g−1 of pore volume of a silica template. 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% aqueous HF solution, and the slurry was stirred for 30 min, filtered, and washed with EtOH several times.13 The HF etching process was repeated in the same manner, and the resulting sample was dried at 60 °C to afford Fe-MPC samples.
2.2.3. Characterization Methods
SEM images were taken on a scanning electron microscope (S-4800, Hitachi) operating at 10 kV. TEM images were obtained using a transmission electron microscope (JEM-2100F, JEOL) with a
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probe-forming Cs corrector at an acceleration voltage of 200 kV. XRD patterns were measured with a high-power X-ray diffractometer (D/MAX2500V/PC, Rigaku) equipped with Cu Kα radiation, and operated at 40 kV and 200 mA. Small angle (2θ range of 0.6° to 5°) and high angle (15° to 65°) XRD patterns were obtained with scan rates of 0.5° min−1 and 2° min−1, respectively. 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 Brunauer–Emmett–Teller (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 423 K and 10−2 Pa for 12 h before measurements. The Fe content in Fe-MPC catalysts was analyzed using an ICP-OES analyzer (700-ES, Varian). Carbon, hydrogen, nitrogen, and oxygen contents in Fe-MPC catalysts were determined by an elemental analyzer (Flash 2000, Thermo Scientific), which is based on the dynamic flash combustion of the sample followed by reduction, trapping, complete separation by gas chromatography, and detection of the products by thermal conductivity detector. XPS measurements were performed on an ESCLAB 250XI (Thermo Scientific), equipped with a monochromatic Al Kα X-ray source (1486.6 eV). XPS 1s spectra of Fe-MPC catalysts were deconvoluted using XPS peak41 software. Deconvoluted chemical species of N 1s binding energy region were fitted to the spectra using the mixed Lorentzian-Gaussian-function (Lorentzian:
30% and Gaussian: 70%) after a Shirley-type background subtraction.
Fe K-edge XAS data were collected at the 8C beamline of the Pohang Accelerator Laboratory (PAL), Republic of Korea. The storage ring was operated at 3.0 GeV with the beam current of 300 mA. The beamline is equipped with a focusing Si (1 1 1) double crystal monochromator that was used to filter the incident photon, which was detuned by 20% to remove high-order harmonics and calibrated using a standard Fe foil. The catalyst powder was pelletized for the measurements. The background removal, normalization of the collected XAS data, and Fourier transform to the radial distribution functions were conducted using the Athena software.70
2.2.4. Electrochemical Characterizations
Electrochemical experiments were performed with a bipotentiostat (CHI760E, CH Instruments) and a rotator (AFMSRCE, Pine Research Instrumentation) at 25 °C using a three-electrode electrochemical cell. Ag/AgCl (RE-1B, ALS; saturated KCl filling solution) and Hg/HgO (CHI152, CH Instruments; 1 M KOH filling solution) reference electrodes were used for measurements in acidic and alkaline electrolytes, respectively, and a graphite rod was used as the counter electrode.
Potential was converted with respect to the RHE. To obtain the RHE conversion value, two electrode cell was built with Pt coil and a reference electrode soaked in a H2-saturated electrolyte. In this case, the Pt coil acts as the RHE. With continuous H2 bubbling, a stable open circuit voltage, which corresponds to the RHE conversion factor, could be acquired within 20 min. A catalyst ink coated disk
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in rotating ring-disk electrode (RRDE, AFE7R9GCPT, Pine Research Instrumentation) was used as the working electrode. Prior to every measurement, the RRDE was polished with a 1.0 µm alumina suspension and then 0.3 µm suspension to afford a mirror finish. A catalyst ink was prepared by mixing 30 mg of a catalyst powder with 100 µL of H2O, 1010 µL of EtOH, and 75 µL of Nafion (5 wt%). The mixture was ultrasonicated until catalyst was well dispersed. For 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 EtOH and 30 µL of Nafion (5 wt%). 6 µL of a 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 0.6 mg cm−2 (70 µg cm−2 for Pt/C). 0.1 M HClO4 and 0.1 M KOH electrolytes were prepared using 70% HClO4 (double distilled, Veritas, GFS chemicals) and 99.99% KOH pellet (Aldrich) with 18.2 MΩ∙cm Millipore water. Before the 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 (500 mV s−1 for Pt/C) in an N2-saturated electrolyte. Subsequently, cyclic voltammetry was performed in the potential rage of 0.05 to 1.2 V at a scan rate of 20 mV s−1 (50 mV s−1 for Pt/C).
ORR polarization curves were obtained by LSV from 1.2 to 0.2 V (from −0.01 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 rotation speed of 1600 rpm. To correct the non-Faradaic current (capacitive current) from the LSV curve, the same measurement was conducted in N2-saturated electrolyte. To investigate the kinetics, Tafel plot was generated according to Tafel equation
where E, b, ik, and C stand for the applied potential, the Tafel slope, the kinetic current, and constant, respectively. The kinetic current (ik) was extracted from the correction of diffusion-limited current.
where i and il signify the measured current and the diffusion-limited current (current at the plateau in the polarization curve), respectively. To measure peroxide production yield and electron transfer number (n), the applied potential of the Pt ring was held at 1.3 V (vs. RHE) during the potential sweep.
The peroxide yield and n were then calculated using the following relations
where N, id, and ir represent the collection efficiency (0.37, provided by manufacturer), the disk current, and the ring current, respectively. To measure solution resistance for iR-compensation, electrochemical impedance spectra were obtained at 0.68 V with AC potential amplitude of 10 mV
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from 100,000 to 1 Hz. In the carbon paper test, 40 µL of catalyst ink was deposited on the 1 × 1 cm2 carbon paper (EC-TP-060T, Toray), resulting in a catalyst loading of 1 mg cm−2. The measurement protocol with a carbon paper electrode was the same as that by the RRDE test. The measurements were independently repeated at least three times and the average data are presented.