UTC Power, South Windsor, CT United Technologies Research Center, East Hartford, CT

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1 Supporting Information Electrocatalysis on Platinum Nanoparticles: Particle Size Effect on Oxygen Reduction Reaction Activity Minhua Shao,, * Amra Peles,, * Krista Shoemaker UTC Power, South Windsor, CT United Technologies Research Center, East Hartford, CT minhua.shao@utcpower.com; pelesa@utrc.utc.com Electrochemical measurements Approximately 15 mg of Pt/C (TKK, 10wt.%) were dispersed in a solvent consisting of 12 ml of water, 3 ml of isopropanol and 60 µl of 5% Nafion (Aldrich) by ultrasonic for 10 min. 10 µl of the suspension was deposited on the pre-cleaned glassy carbon substrate (RDE, Pine Instruments) and allowed to dry. Pt/C was cycled between 0.02 and 1.2 V (vs RHE) for 10 cycles in a N 2 saturated 0.1 M HClO 4 solution until a stable CV was obtained. The extra Pt layers were deposited by galvanic displacement with Pt atoms of an underpotentially deposited (upd) Cu monolayer on the Pt/C surface from a 50 mm H 2 SO mm CuSO 4 solution. The electrode covered with a Cu monolayer was rinsed and immersed in a 1.0 mm K 2 PtCl 4 (Johnson Matthey) + 50 mm H 2 SO 4 (GFS Chemicals) solution for about 2 min to displace the Cu with Pt. The

2 catalysts with an extra Pt layer was rinsed with water and transfered into the electrochemical cell containing N 2 saturated 0.1 M HClO 4 solution. The catalsyts were cycled between 0.02 and 1.05 V for 5 cycles at 100 mv/s and a final CV was recorded at 50 mv/s for ECA calculation. The oxygen reduction activity was measured in an oxygen saturated 0.1 M HClO 4 solution at 1600 rpm. This process was repeated on the electrode to increase the number of Pt layers and characterize their electrochemical behaviors. A reversible hydrogen electrode (RHE) was used as a reference electode in the electrochemical measurement except in the Cu deposition, where an Ag/AgCl/KCl (3 M) leak-free electrode was used. All potentials are quoted with respect to the reversible hydrogen electrode (RHE). The kinetic currents for ORR at 0.93 V were calculated based on eq. 1: = + (1) j j k j d where j and j d are the measured current and limited current, respectively. TEM measurements TEM images were taken using an FEI Tecnai F20 transmission 202 electron microscopy (TEM) with a Field Emission Source at 200 kev. The Pt/C catalysts deposited on glassy carbon with various extra Pt layers were dispersed in 1 ml of ethanol. A drop of the dispersion was placed on a 3 mm Cu grid coated with a carbon film and allowed to dry. DFT calculations We implement first principles calculations based on density functional theory (DFT) using a Generalized Gradient Approximation (GGA), [1] as implemented in the Vienna Ab-Initio Simulation Package (VASP). [2] The Projector Augmented-Wave (PAW) approach was used, [3]

3 with atomic configurations of 5d 9 6s 1 for Pt, and 2s 2 2p 4 for O. We used a plane-wave cutoff of 400 ev, and Brillouin zone is sampled at the Gamma point. We implement super cell approach with particles in the cubic cell isolated by the vacuum layer of 8-10 Å in all three directions to avoid mutual interactions between periodically repeated cells. All degrees of freedom were allowed to relax except for the oxygen adsorption on 807 atoms particle, where because of the computational intensity the optimized particle is cut in half and basal plane of the semi-particle was fixed while other atoms were allowed to relax in the study of oxygen adsorption. The adsorption energy on an extended (111) surface was calculated on a 7x7 periodic slab. References [1] J. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett [2] G. Kresse, J. Furthmuller, Phys. Rev. B [3] P. E. Blochl, Phys. Rev. B 1994,

4 Figure S1. Particle size distribution derived from TEM images of Pt/C after 0 (as-received), 2 and 4 extra Pt layers deposition. The one standard deviations for all three samples are in the range of nm suggesting that the high monodispersity was maintained during particle growth.

5 Figure S2. Pt particle size as a function of extra layer deposited on the as-received Pt/C catalyst. The projected particle size was obtained by assuming a full Pt monolayer (blue dots in the cartoon) deposited on the Pt particles in each Cu-UPD-Pt-replacement cycle. The particle size would increase by ~0.45 nm per full layer. The particle size values measured by the TEM are smaller than projected. This discrepancy can be explained by the fact that the number of Cu atoms deposited during UPD is equal to the surface atoms of the Pt core, rather than the number required to fully cover the core. For instance, about 70% of the total atoms are on the surface for a 1.3 nm cubo-octahedral Pt particle (79 atoms total). Thus, 55 Cu atoms are deposited on such a Pt particle resulting in a particle size of 1.56 nm after they are replaced by Pt. On the other hand, 110 atoms are required to fully cover the 1.3 nm particle.

6 ½ Id Half-wave potential higher than 0.9 V but lower than 0.93 V. Id Figure S3. The polarization curves of Pt/C with different number of extra Pt layers in an O 2 - saturated 0.1 M HClO 4 solution. Sweep rate = 10 mv s -1. The half-wave potentials for samples with more than 2 extra Pt layers are higher than 0.9 V but lower than 0.93 V. By using the current at 0.93 V rather than 0.9 V can minimize inaccuracy of the mass-transport correction on the calculation of kinetic currents based on the Koutecky-Levich equation. Id: diffusion limited current density.

7 Figure S4. The peak potentials of Pt oxides reduction for Pt/C with different number of extra Pt layers as a function of average coordination numbers.