Chapter 3: Sphere-to multipod transmorphic change of nanoconfined platinum
3.2. Experimental section
3.2.1. Graphene oxide (GO)
GO was prepared from purified natural graphite (SP-1, Bay Carbon) using the m odified Hummers method. Graphite powder (2.0 g) was added to concentrated H2SO4 (4 6 ml), and KMnO4 (6.0 g) was added gradually with stirring and cooling, while the te mperature of the mixture was maintained below 20 °C. The mixture was then stirred at 35 °C for 2 h, and distilled water (92 ml) was added. After 15 min, the reaction was t erminated by the addition of a large amount of distilled water (280 ml) and a 30% H2
O2 solution (5.0 ml), after which the color of the mixture changed from black to bright yellow. The mixture was centrifuged at 4000 rpm and washed with a 1:10 HCl solution (500 ml) in order to remove metal ions. The graphite oxide product was suspended in d istilled water producing a viscous, brown dispersion, which was subjected to dialysis to completely remove metal ions and acids. To obtain the GO dispersion, the graphite oxid e was exfoliated by ultrasonication (Sonic Dismembrator Model 500, Fisher Scientific) at 100 W for 15 min and then centrifugation at 4000 rpm for 10 min.
3.2.2. Pt-loaded reduced GO (Pt/FL-rGO)
The colloidal mixture of GO and H2PtCl6∙6H2O were prepared and their weight r atio was 1:0.5 with the concentration of GO in colloidal mixture fixed at 0.5 mg/ml. T hen, the mixture was poured into a glass Petri dish with a diameter of 9.5 cm and a d epth of 1.6 cm, and incubated at 80 °C for 24 h. The GO/H2PtCl6∙6H2O film was caref ully peeled off from the glass dish. Then, FL-RGO/Pt was fabricated by followed therm al annealing at 900 °C (ramp: 5 °C/min) in a vacuum for 3 hr.
3.2.3. Crumpled rGO (CR-rGO)
GO dispersion was atomized through a 7.0 μm nozzle of a spray dryer (Buchi B-90) to micrometer-size droplets by 30 mbar. The droplets were dried by hot air at 120 °C. The dry powders were settled by a cyclone separator to remove tiny particles. The powders were thermally annealed at 900 °C in a ramp of 5 °C min-1 in vacuum for 3 h.
3.2.4. Pt-loaded holey CR-rGO (Pt/hCR-rGO)
The colloidal mixture of GO and H2PtCl6∙6H2O were used as starting materials for porous and crumpled rGO-supported Pt catalyst, and their weight ratio was 1 : 0.5 with the concentration of GO in colloidal mixture fixed at 0.5 mg ml-1. Then, crumpled rGO/Pt was fabricated by the spray dryer (Buchi B-90 spray dryer) followed by thermal annealing at 900 °C (ramp: 5 °C /min)
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in a vacuum for 3 hr. In this process, micrometer-sized droplets of dispersion were produced by the atomizer (nozzle in 7.0 μm) of the spray dryer. The droplets were carried away and dried by inflow warm air (120 °C) to produce powder particles. The dry powders were settled by a cyclone separator and the air was discharged from the separator along with small particles. The atomizer pressure was about 30 mbar and the hot air temperature was 120 °C. In all experiments, 30-40 % of the spray rate was used for spraying the dispersions.
The porous crumpled rGO/Pt samples were prepared in a single-step air oxidation process by directly heating the starting crumpled rGO/Pt sample in static air at an elevated temperature in an open-ended tube furnace. The samples were prepared at 350 °C and held for 3 h, and then, thermally reduced at 900 °C (ramp: 5 °C/min) in a vacuum for 3 h to remove oxidized carbon in rGO.
3.2.5. Electrochemical measurements
Cyclic voltammetry (CV) as well as linear sweep voltammetry (LSV) for ORR measurements were conducted by a potentiostat (BioLogic VMP 3). Three-electrode syste ms were configured with graphite rod as a counter electrode and a commercial Hg/HgO for alkaline media or a commercial Ag/AgCl (3 M KCl) for acid media as a reference electrode. Aqueous solution of 0.1 M KOH or HClO4 was used as electrolyte. The rotat ing ring disk electrodes (RRDEs) were polished consecutively with two different alumina powders (0.3 and 0.05 μm) and cleaned by 20 min sonication in deionized water for r emoving alumina powders from electrodes. 20 mg of catalyst powders was dispersed in a mixture of 900 μl of ethanol and 100 μl of 5 wt. % nafion suspension in alcohol (Si gma- Aldrich) by sonication for 30 min. 6 μl droplet of the catalyst ink placed on disk of polished RRDEs was dried in an oven at 80 °C for 5 min to evaporate solvent. Elec trolytes were purged with nitrogen for measuring the background currents while they we re saturated by oxygen for measuring ORR currents. All potentials were reported versus reversible hydrogen electrode (RHE) (VRHE = V versus RHE).
3.2.6. Physicochemical characterization
TEM characterization. BFTEM images were acquired by a JEOL JEM 2100F at an accelerating voltage of 200 kV. Atomic-resolution TEM images were performed by u sing a FEI Titan3 G2 60-300 equipped with a double-side spherical aberration corrector at an accelerating voltage of 80 kV. Change in binding energy of Pt after durability test was measured by X-ray photoelectron spectroscopy (XPS; Thermo Scientific K-Alpha usi ng a monochromic Al Kα X-ray source). X-ray absorption fine spectra (XAFS) were inv
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estigated by the Beamline 6D of the Pohang Accelerator Laboratory (PAL) at 3 GeV be am energy and 300 mA currents. Diffraction patterns of the samples were collected on a Bruker D8 advance diffractometer using Cu-Kα radiation (40 kV, 40 mA). Samples w ere scanned over a 2θ range 10-80◦ in a step scan for 0.05◦/step. Thermogravimetric ana lysis (TGA) of the samples was conducted on a Thermal Analysis TA Q500 instrument.
The experiment was performed at a temperature range of 25 – 800 ℃ with a heating ra te of 10 ℃ min-1 in constant airflow.
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