Enhanced Activity for Oxygen Reduction Reaction on. and Sandwich-Segregation Structures

Transcription

1 Supplementary Materials for Enhanced Activity for Oxygen Reduction Reaction on Pt 3 Co Nanoparticles: Direct Evidence of Percolated and Sandwich-Segregation Structures Shuo Chen, Paulo J. Ferreira, Wenchao Sheng, Naoaki Yabuuchi, Lawrence F. Allard, and Yang Shao-Horn*, Electrochemical Energy Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, Materials Science and Engineering Program, The University of Texas at Austin, Austin, Texas 78712, and Oak Ridge National Laboratory, Oak Ridge, Tennessee shaohorn@mit.edu S1

2 Sample Description Number-averaged d n *(TEM) (nm) Volume-surfacearea averaged d v/a ** (TEM) (nm) Electrochemic al surface area (m 2 /g Pt ) Specific Activity i s (macm -2 Pt ) Electrochemical surface area Mass Activity i m (Amg -1 Pt ) Pt-4nm 3.6 (200 particles) Acid-treated Pt 3 Co Annealed Pt 3 Co 4.1 (199 particles) 4.9 (214 particles) 4.8 (200 particles) 5.4 (199 particles) 9.2 (214 particles) *d n = n i=1 n d i **d v / a = n d i 3 i=1 n 2 d i i=1 Table S1. List of average particle diameters and ORR activities of the samples studied in this work. ORR activity was evaluated in O 2 -saturated 0.1 M HClO 4 at room temperature at a sweeping rate of 20 mv/s. The specific and mass activities were compared at 0.9 V vs. RHE at 1600 rpm, which were calculated based on the electrochemical surface area obtained from cyclic voltammograms and Pt loading, respectively. S2

3 (a) dn=4.1 nm 5 nm (b) dn=4.9 nm 5 nm (c) dn=3.6 nm 5 nm Figures S1a-c Histograms and representative images of acid-treated Pt3Co (a), annealed Pt3Co (b) and heat-treated Pt nanoparticles supported on carbon (c), having Pt loading of 46 wt% (Tanaka Kikinzoku Kogyo Co. Ltd.). The number-averaged diameters dn are included from measurements of nearly 200 particles. The annealed sample was obtained by heat-treating acid-treated Pt3Co at 1000 K for 3 hours at a low pressure of Torr and then being cooled in the furnace to 373 K with an average heating rate of 30 K/min and an average cooling rate of 5 K/min. S3

4 Figures S2. Polarization curves for Pt-4nm, acid-treated Pt 3 Co and annealed Pt 3 Co samples, collected in O 2 -saturated 0.1 M HClO 4 electrolyte, on a rotating disk electrode in the positive-going scan at 1600 rpm and a scanning rate of 10 mv/s. The loading of these samples on glassy carbon electrodes were 9 μg Pt /cm 2 GCE, 11 μg Pt /cm2 GCE and 12 μg Pt /cm2 GCE for Pt-4nm, acid-treated, and annealed Pt 3 Co, respectively. 200 μc/cm 2 Pt and 180 μc/cm2 Pt were used to determine the effective surface areas of acidtreated Pt 3 Co and annealed Pt 3 Co catalysts and specific oxygen reduction activity 1 and 210 μc/cm 2 Pt were used for Pt-4nm. S4

5 (a) 1 st layer 14 th layer (b) Position=0 2 2 nm nm (c) (d) (e) (f) 111 FCC 002 FCC 111 FCC [110] Figures S3a-d. Cs-corrected HAADF image (a) of an acid-treated Pt 3 Co nanoparticles and the corresponding fast Fourier transform result (f). (b) A 3D model of the particle, whose shape is assumed as a truncated octahedron. (c) Normalized intensity variation of columns of atoms in the HAADF image (solid curve) along the dash line in (a) and normalized thickness variations of the truncated octahedron in (b) (dash curve). The image intensity, which was measured with Digital Micrograph, and thickness variation of the truncated octahedron, which was measured with AutoCAD are plotted along the white dotted line in (a) and (b), respectively. (d) Normalized integrated intensity from (c) and normalized thickness of the truncated octahedral in (b). (e) The normalized atomic number averaged over each column of atoms along the beam direction, which is defined as the square root of the ratio of normalized integrated intensity and normalized thickness in (d). The 1 st and 14 th colunns in (e) are plotted with gray background as relatively larger uncertainty in the normalized atomic number near the edges. S5

6 (a) (b) 5 nm 5 nm Figures S4a-b. Cs-corrected HAADF images of acid-treated Pt 3 Co nanoparticles. The contrast of individual particles of 10 nm indicates the existence of a percolated structure as a result of chemical composition variations associated Co dissolution during acid treatments. S6

7 (a) (b) Pt/Co ratios nm Figures 5a-b. (a) HAADF STEM image of an acid-treated Pt 3 Co nanoparticle. The image was taken with VG 603 STEM. Energy Dispersive spectroscopy spot capture was carried out in four different positions of the particle with a 2 nm diameter electron beam for 30 s, as shown by the white dots and number 1-4. Insert of (a): Co atomic percentage at each position. (b): the spectra of positions 1 to 4 (from bottom to top). Co K α and Pt L α, which were used for quantification, are labeled. The peaks at approximate 8 kev correspond to K α of Cu, which came from the TEM copper grid. S7

8 (a) (100) (b) (100) (331) (221) 2 nm (331) (113) (100) (110) 2 nm (100) (113) Figures S6a-b. Typical HRTEM images of Pt-4nm (a) and acid-treated Pt 3 Co (b) nanoparticles. Surface atoms are defined by white dots and the surface planes are marked. Comparable fractions of low and high index planes projected in the [110] direction were found on HRTEM data analyzed over 20 nanoparticles from each of the two samples. S8

9 (a) (b) 002 FCC 001 FCC 311 FCC Position=0 [130] 1 st layer (c) (d) (e) (f) Pt-rich (001) Co-rich Figures S7a-f. (a) Cs-corrected HAADF images of annealed Pt 3 Co nanoparticles. Insert of (a): fast Fourier transform result of the particle outlined at bottom. (b) A 3D model of the particle, whose shape is assumed as a truncated octahedron. (c) Normalized intensity variation in the HAADF image (solid curve) and normalized thickness variation of the truncated octahedron in (b) (dash curve). The image intensity, which was measured with Digital Micrograph, and thickness variation of the truncated octahedron, which was measured with AutoCAD are plotted along the white dotted line in (a) and (b), respectively. (d) Normalized integrated intensity from (c) and normalized thickness of the truncated octahedral in (b). (e) The normalized atomic number of each atomic layer, which is defined as the square root of the ratio of normalized integrated intensity and normalized thickness in (d). (f) Cross section view of a proposed atomic model with Pt enrichment in the outermost layer, Co enrichment in the second layer and ordering of Pt-rich and Co-rich layers throughout the particle. S9

10 Reference (1) Stamenkovic, V. R.; Mun, B. S.; Mayrhofer, K. J. J.; Ross, P. N.; Markovic, N. M. J. Am. Chem. Soc. 2006, 128, S10