Supplementary Figure S1: Particle size distributions of the Pt ML /Pd 9 Au 1 /C

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1 a 2 15 before cycle test mean particle size: 3.8 ± 1.2 nm b 2 15 after.6v - 1.V 1k cycle test mean particle size: 4.1 ± 1.5 nm Number 1 total number: 558 Number 1 total number: Particle size (nm) Particle size (nm) c Number after.6v - 1.4V 2k cycle test mean particle size: 3.6 ± 1.2 nm total number: Particle size (nm) Supplementary Figure S1: Particle size distributions of the Pt ML /Pd 9 Au 1 /C electrocatalyst. (a) Before the stability test. (b) After 1, potential cycle test between.6 V and 1. V. (c) After 2, potential cycle test between.6 V and 1.4 V. S1

2 Intensity (a.u.) Au(1 1 1) Pt(1 1 1) Pd(1 1 1) Au(2 ) Pt(2 ) Pd(2 ) Angle (2θ) Supplementary Figure S2: Synchrotron X-ray diffraction (XRD) pattern of Pt ML /Pd 9 Au 1 /C nanoparticles with λ = Å. The reflection peaks of (111) and (2) planes for Au, Pt, and Pd are designated, respectively. The measurement was done at BL8B2 beamline, Spring-8 (Hyogo, Japan). S2

3 a FT magnitude (Å -3 ) Au L3 Pd 9 Au 1 fit r (Å) b FT magnitude (Å -3 ) Pd K Pd 9 Au 1 fit r (Å) Supplementary Figure S3: Fourier-transformed extended X-ray absorption fine structure (EXAFS) data (red) with first-shell fits (dotted blue). (a) Au L3 and (b) Pd K edges from Pt ML /Pd 9 Au 1 /C electrocatalysts in 1 M HClO 4 at a potential of.41 V. S3

4 3 2 Current (ma) before cycle test after 1, cycle test Potential (V RHE) Supplementary Figure S4: Voltammetry curves of the Pt ML /Pd 9 Au 1 /C electrocatalyst. The curves were measured before and after 1, cycles between.6 V and 1. V in the membrane electrode assembly (MEA) cell at a sweep rate of 5 mv s -1 at room temperature. S4

5 a b Supplementary Figure S5: Two Pd 9 Au 1 core models. Schematic of subcore models (a) Model I viewed in two different directions and (b) Model II. The quadrilaterals in purple are the (1) facets used for modeling the diffusion of Au or Pd from the Pd 9 Au 1 subcore to a vertex defective site on the Pt shell (Au-Pd-Pd-Pd and Pd-Pd-Pd-Pd, respectively) as compiled in Supplementary Table S3. S5

6 a b Supplementary Figure S6: Imperfection of the Pt monolayer. Schematic of a sphere-like nanoparticle without (a) and with (b) defects ( D ). Here the defect is represented by a Pt vacancy. S6

7 Electrochemical surface area (m 2 g Pt -1 ) Pt ML /Pd 9 Au 1 /C Pt/C -42% -81% Number of potential cycles Supplementary Figure S7: Electrochemical surface area (ECSA) of the Pt ML /Pd 9 Au 1 /C electrocatalyst as a function of number of potential cycle during fuel cell testing. The limits of the potential cycle were.6 and 1.4 V, with a 1 s dwell time at 8 C. The result with a Pt/C catalyst is shown for comparison. S7

8 Supplementary Figure S8: High-angle annular dark field (HAADF)/scanning transmission electron microscopy (STEM) image of the Pt ML /Pd 9 Au 1 /C electrocatalyst after the 2, potential cycle (.6 V V) fuel-cell test. Bar size = 2 nm. S8

9 Current (ma) rpm 625 rpm 9 rpm 1225 rpm 16 rpm 225 rpm 25 rpm Potential (V RHE) Supplementary Figure S9: Rotating disk electrode (RDE) measurements of the oxygen reduction reaction (ORR) on the Pt ML /Pd 9 Au 1 /C electrocatalyst in a.1 M HClO 4 solution purged by O 2 at different rotation speeds (4 to 25 rpm). The Pt loading was determined to be 6.7 µg cm -2 (6.77 nmol) by the Induction-Coupled Plasma (ICP) analysis. The ORR activity of the electrocatalyst is good, as indicated by the high onset potential of O 2 reduction (ca 1. V) as well as the high half-wave potential (95 mv at 16 rpm). The kinetic currents for ORR were determined from the Koutecky-Levich plot. The Pt specific and mass activities of the Pt ML /Pd 9 Au 1 /C electrocatalyst at.9 V are determined to be.47 ma cm -2 and.9 A mg -1 Pt, which are approximately 2 and 4.6 times higher than those of a commercial Pt/C electrocatalyst of 4 nm diameter (.24 ma cm -2 and.2 A mg -1 Pt ). Its total precious-group-metal (Pt+Pd+Au) mass activity is.32 A mg -1 PGM, which is 1.6 times higher than that of the commercial Pt/C electrocatalyst. Sweep rate = 1 mv s -1. Electrode Area =.196 cm 2. S9

10 Supplementary Table S1: Structural parameters (bond distance R and bond length disorder parameter σ 2 ) of Pd 9 Au 1 nanoparticles determined by the EXAFS experiment. R Au-Au (Å) R Au-Pd (Å) R Pd-Pd (Å) σ 2 Au-Au (Å 2 ) σ 2 Au-Pd (Å 2 ) σ 2 Pd-Pd (Å 2 ) (±.29) (±.7) (±.2) (±.38) (±.8) (±.3) S1

11 Supplementary Table S2: Relative energies a of defective sites compared to the perfect Pt nanoparticle b. Defect site E rel (ev) D 1.23 D 2.17 D 3.17 D 4.32 D 5.89 D 6.63 a Relative energies were calculated according to E rel = E[defective Pt NP] + E[one Pt atom] E[perfect Pt NP], where the electronic energies of a defective Pt nanoparticle, one Pt atom calculated from bulk Pt, and a perfect Pt nanoparticle. b The structures are shown in Supplementary Figure S6. S11

12 Supplementary Table S3: Summary of calculated relative energies after Au or Pd diffusion from the Pd 9 Au 1 subcore to a vertex site on the Pt shell. site a metal for diffusion ΔE (ev) Au-Pd-Pd-Pd (I) Au.49 Au-Pd-Pd-Pd (II) Au.51 mean.5 Au-Pd-Pd-Pd (I) Pd Au-Pd-Pd-Pd (II) Pd Pd-Pd-Pd-Pd Pd mean.14 a Au-Pd-Pd-Pd and Pd-Pd-Pd-Pd represent a (1) facet with Au and Pd and only Pd atoms, respectively (see Supplementary Figure S5a). S12