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
were carried out at the Ulsan National Institute of Science and Technology Central Research Facilities (UCRF) Center.
2.2.4 Electrochemical Characterization
In a three-electrode electrochemical cell, electrochemical characterization was performed using rotating ring disk electrode (RRDE, ALS) with IviumStat electrochemical analyzer at RT. 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 with a 1.0 µm and 0.3 µm alumina suspension, sequentially, to afford a mirror finish. The catalysts were loaded on the electrode by drop-casting of catalyst ink. In a 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 by the ultrasonication 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 catalyst loading was 600 µg cm−2 for the Fe-N/C catalysts and 14 µgPt
cm−2 for Pt/C. The ORR performance measurements were performed in 0.1 M KOH (99.99%, Aldrich) solution, except poisoning experiments. The poisoning experiments were carried out in the 0.1 M KOH aqueous solution with 10 mM KCN (97%, Aldrich). Before the ORR performance tests, the catalysts were cleaned by cycling the potential between 0.05 and 1.2 V (vs. reversibly hydrogen electrode, 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 for 3 cycles at a scan rate of 20 mV s−1 (50 mV s−1 for Pt/C) in an N2-saturated electrolyte. Linear sweep voltammetry (LSV) polarization curves for the 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 at the electrode rotating speed of 1600 rpm. With applying 1.3 V of potential on the ring electrode during LSV, ring currents were also attained to identify ORR selectivity. In order to rule out capacitive currents from the LSV curve, LSV was additionally conducted in N2-saturated electrolyte, and the obtained current subtracted from the current acquired under O2-saturated condition. To measure the solution resistance for iR-compensation, electrochemical impedance spectra were obtained at 0.68 V with AC potential amplitude of 10 mV from 10000 Hz to 1 Hz at the rotating speed of 1600 rpm. The hydrogen peroxide (H2O2) reduction reaction performance tests were carried out in the 0.1 M KOH electrolyte with adding 30% H2O2 (Fluka) solution.
In an N2-saturated 0.1 M KOH solution (80 mL), two cycles of CV were conducted without H2O2 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 solution. The capacitive current was eliminated in the same manner as with ORR. All the
electrochemical measurements were repeated three times and the average data were presented.
2.2.5 Analysis of ORR selectivity
To evaluate the ORR selectivity of the catalysts, the HO2− yield during the ORR were obtained by the following equations using RRDE measurements.
𝐻𝑂2− = 200% × 𝐼𝑅 𝑁 (𝐼𝑅
𝑁) + 𝐼𝐷
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 was poisoned. Thus, the electron transfer number was instead calculated using the Koutecky-Levich plots by performing LSV 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 𝑖 = 1
𝑖𝑘+ 1 𝑖𝑑 = 1
𝑖𝑘+ 1
𝐵𝜔1/2, 𝑤ℎ𝑒𝑟𝑒 𝐵 =0.62𝑛𝐹𝐴𝐶𝑂2𝐷𝑂
2
2/3
𝜂1/6
where I is the experimentally measured current, ik is the kinetic current, id is the diffusion-limited 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 𝜂 is the viscosity of the electrolyte (1.01 × 10–2 cm2 s–
1).
2.3.RESULTS AND DISCUSSION