Truncated Octahedral Pt 3 Ni ORR Electrocatalysts

Transcription

1 Truncated Octahedral Pt Ni ORR Electrocatalysts Jianbo Wu, Junliang Zhang, Zhenmeng Peng, Shengchun Yang, Frederick T. Wagner and Hong Yang*, Department of Chemical Engineering, University of Rochester, Gavett Hall 06, Rochester, New York, 1467 Electochemical Energy Research Lab, General Motors Research and Development, Honeoye Falls, NY 1447 * CORRESPONDING AUTHOR hongyang@che.rochester.edu Telephone: (585) ; Fax: (585) S1

2 Experimental Details Synthesis of Pt Ni Nanoparticles. A mixture of borane-tert-butylamine complex (TBAB, Aldrich, 97%, 1.14 mmol), adamantanecarboxylic acid or adamantaneacetic acid (ACA or AAA, Aldrich, 99%, 1. mmol), hexadecanediol (Aldrich, 96%, 6. mmol), one of the following long alkane chain amines-hexadecylamine (HDA, TCI, 90% 8.8 mmol), dodecylamine (DDA, Aldrich, 98%, 8.8 mmol), or octadecylamine (ODA, Aldrich, 97%, 8.8% mmol)-and diphenyl ether (DPE, Aldrich, 90%, ml) was added into a 5-mL three-neck round-bottle flask under argon protection. The reaction mixture was maintained at 190 C using an oil bath. Platinum acetylacetonate (Pt(acac), Strem, 98%, 0.17 mmol) and nickel acetylacetonate (Ni(acac), Aldrich, 95%, mmol) were dissolved in -ml DPE at 60 C followed by rapid injection into flask. The reaction was maintained at 190 C for 1 h. After the reaction, 00 μl of the product was mixed with 800 μl of chloroform in a plastic vial (1 ml), followed by the addition of 1 ml of ethanol. The precipitate was separated from the mixture by centrifugation at 5000 rpm for 5 min. The supernatant was decanted and the black product was dispersed in 1 ml of chloroform. This process was repeated three times. Preparation of Carbon-Supported Catalysts. Carbon black (Vulcan XC-7) was used as support for making platinum nickel catalysts (Pt Ni/C). In a standard preparation, carbon black particles were dispersed in hexane and sonicated for 1 h. A designed amount of platinum nickel nanoparticles were added to this dispersion at the nanoparticle-to-carbon-black mass ratio of 0:80. This mixture was further sonicated for 0 min and stirred overnight. The resultant solids were precipitated out by centrifugation and dried under an argon stream. The solid product was then re-dispersed in n-butylamine at a concentration of 0.5 mg-catalyst/ml. The mixture was kept under stirring for days and then collected using a centrifuge at a rate of 5000 rpm for 5 min. The precipitate was re-dispersed in 10 ml methanol by sonicating for 15 min and then separated by centrifugation. This procedure was repeated three times. The final samples were dispersed in ethanol for further characterization. Characterization. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) images were taken on a FEI TECNAI F-0 field emission microscope at an accelerating voltage of 00 kv. Scanning transmission electron microscopy (STEM) and elemental maps were carried out using the high-angle annular dark field (HAADF) mode on the same microscope. The optimal resolution of this microscopy is 1 Å under TEM mode and 1.4 Å under STEM mode. Energy dispersive X-ray (EDX) analysis of particles was also carried out on a field emission scanning electron microscope (FE-SEM, Zeiss-Leo DSM98) equipped with an EDAX detector. Powder X-ray diffraction (PXRD) patterns were recorded using a Philips MPD diffractometer with a Cu Kα X-ray source (λ= Å). Electrochemical Measurement. The alloy mass of each Pt Ni/C catalyst was determined by thermogravimetric analysis (TGA) using an SDT-Q600 TGA/DSC system S

3 from TA Instruments, Inc at a ramp rate of 10 C/min to 600 C in air followed by annealing at 600 C for 0 min under a forming gas of 5 % hydrogen in argon at a flow rate of 50 ml/min. A three-electrode cell was used to do the electrochemical measurements. The working electrode was a glassy-carbon Rotating Disk Electrode (RDE) (diameter: 5mm and area: cm ) from Pine Instruments. The procedure for the RDE measurements followed those established in literature. 1- A platinum foil with the size of 1cm 1cm was used as the counter electrode and an Ag/AgCl (-M Cl - ) was used as the reference electrode. To effectively prevent chloride ions from cross-contaminating the electrolyte in the working cell, the reference electrode was placed in a separate compartment via a stopcock that was wetted with the electrolyte. All potentials in this paper are referenced to the Reversible Hydrogen Electrode (RHE), which was calibrated with H oxidation/evolution on a Pt polycrystalline RDE electrode. All the liquid junction potentials were cancelled out in adjusting for the RHE scale. The electrolyte used for all the measurements was 0.1-M HClO 4, diluted from 70% double-distilled perchloric acid (GFS Chemicals, USA) with Millipore ultra pure water. To prepare the working electrode, 10 mg of the Pt Ni/C catalyst was dispersed in 0 ml of a mixed solvent and sonicated for 5 mins. The solvent contained a mixture of DI water, isopropanol, 5% Nafion in the volume ratio of 4:1: μl of the suspension was added onto the RDE by a pipette and dried in flowing argon. The Pt loading on the RDE was calculated as 9. μg Pt /cm. The electrochemical active surface area (ECSA) measurements were determined by integrating the hydrogen adsorption charge on the cyclic voltammetry (CV) at room temperature in nitrogen saturated 0.1-M HClO 4 solution. The potential scan rate was 0 mv/s for the CV measurement. Oxygen reduction reaction (ORR) measurements were conducted in a 0.1-M HClO 4 solution which was purged with oxygen for 0 min prior to, and during, the measurement. The scan rate for ORR measurement was 10 mv/s in the positive direction. The ORR polarization curves were collected at 1600 rpm. Due to the small currents measured at 0.9 V on RDE, the effect of ir compensation is expected to be negligible, thus the data presented in this paper were used without ir-drop correction. For comparison, Pt/C (TKK, 50wt%Pt on Vulcan carbon) was used as the baseline catalyst, and the same procedure as described above was used to conduct the electrochemical measurement, except that the Pt loading was controlled at 11 μg Pt /cm. Calculation for (111) Surface Ratio. A simple cube has only {100} facets and no {111} facets, i.e. the fraction of (111) surface over the entire surface area, S cube (111)%, is equal to 0. A truncated octahedron can be generated by cutting off six vertices from the octahedron, generating {100} facets (Scheme S1). Thus we can calculate the (111) surface area by subtracting the corresponding (111) surfaces of six square pyramids from the original octahedron, S octahedron (111), which can be derived from the following equation: 1 0 S octahedron ( 111) = 8 ( a) sin 60 = 4 a where a is the distance from the corner to the body center of the (untruncated) octahedron. Assuming the height of each square pyramid is b, then the sum of {111} surface area of six square pyramids being removed should be: Ssquare pyramid 1 (111) = 6 4 ( b) sin 60 0 S = 1 b, b a

4 and the total area of the (100) surfaces of a truncated octahedron being created should be: {100} 6 ( b) = 1b, b S t, o = a Thus, the total (111) surface area of a truncated octahedron can be calculated by the following equations: S t, o 111) = Soctahedron(111) Ssquare pyramid (111) = 4 a 1 ( b Thus, the ratio of (111) surface over the entire surface area of a truncated octahedron can be calculated by the following formula: S S t, o(111) = (111) + S (100) 4 4 a 1 b a 1 b + 1b b 1 a = b 1 ( ) a t, o( 111)% = St, o t, o The overall fraction of (111) surface areas over the entire surface areas of a catalyst that is composed of given percentages of truncated octahedral (α t,o ) and cubic (α cube ) shapes, is the weighted average given by the following equation: S catalyst( 111)% St, o(111)% t, o = α + S (111)% α cube The average value of ( b a) is 0.11 for those truncated octahedral Pt Ni nanocrystals in all three samples based on the TEM measurement of about 00 nanocrystals, while the population of truncated octahedra, α t,o, is 70%, 90% and 100%, respectively. The population of cubes, α cube, in the samples was based on the TEM images and obtained by counting about 00 nanocrystals. The rest of nanocrystals were truncated octahedra and the population of truncated octahedra, α t,o, could be derived using the following formula: α t, o = 1 α cube Thus, the fraction of (111) surfaces is for the catalyst with 70% truncated octahedral shape, for that with 90%, and for the one without cubes. Reference: (1) Schmidt, T. J.; Gasteiger, H. A.; Stab, G. D.; Urban, P. M.; Kolb, D. M.; Behm, R. J. J. Electrochem. Soc. 1998, 145, () Paulus, U. A.; Schmidt, T. J.; Gasteiger, H. A.; Behm, R. J. J. Electroanal. Chem. 001, 495, () Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Appl. Catal. B-Environ. 005, 56, 9-5. cube, b a S4

5 Scheme S1. Illustration of generation of truncated octahedron. S5

6 Table S1. ECSA, Mass- and Area- Specific ORR Activities of Pt Ni and Pt Catalysts* sample name Pt loading [μg Pt /cm disk] ECSA [m /g Pt ] mass activity [A/mg Pt ] specific activity [ma/cm Pt] 100% t,o-pt Ni 90% t,o-pt Ni 70% t,o-pt Ni Pt/C (TKK) *: The activity was measured at 0.9 V (vs. RHE). S6

7 Figure S1. TEM image of Pt Ni nanoparticles produced with octadecylamine. Figure S. HR-TEM image of a Pt Ni alloy nanocubes. S7

8 Figure S. TEM image of Pt Ni nanoparticles made using TBAB as the reducing agent. Figure S4. a) STEM image and its corresponding b) Pt (M line) and c) Ni (K line) elemental maps, and d) EDX spectrum of t,o-pt Ni nanoparticles. S8

9 Figure S5. EDX spectra of (a) 70% and (b) 90% truncated octahedral nanoparticles showing the metal composition of Pt Ni. Figure S6. TEM images of carbon-supported Pt Ni particles with various shapes: (a) 70%, (b) 90%, (c) 100% truncated octahedral Pt Ni nanoparticles, respectively. Figure S7. TEM image of the carbon-supported Pt catalyst (TKK, 50% loading). S9