Supporting Information For Pt Monolayer on Porous Pd-Cu Alloys as Oxygen Reduction Electrocatalysts

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1 Supporting Information For Pt Monolayer on Porous Pd-Cu Alloys as Oxygen Reduction Electrocatalysts Minhua Shao, *, Krista Shoemaker, Amra Peles, Keiichi Kaneko #, Lesia Protsailo UTC Power, South Windsor, CT United Technologies Research Center, East Hartford, CT # Toyota Motor Engineering & Manufacturing North America Inc., Ann Arbor, MI Minhua.shao@utcpower.com Experimental procedures Pd-Cu nanoparticles supported on Vulcan XC-72 with a molar ratio of Pd:Cu from 1:2 to 1:12 were prepared. Two grams of 19 wt % Pd/C obtained from BASF and calculated amount of Cu(NO 3 ) 2 2.5H 2 O (Aldrich) were mixed with 250 ml of distilled water and sonicated for 30 minutes then dried at 80 C. The dried mixture was then heated at 700 C in Ar atmosphere for 4 hours and allowed to cool down. Approximately 20 mg of the particles so prepared were dispersed in a solvent consisting of with 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. Pd-Cu/C was cycled between 0.02 and 1.2 V (vs RHE) for 50 cycles in a N 2 saturated 0.1 M HClO 4 solution to achieve a porous structure by leaching out Cu. The Pt monolayer was prepared by galvanic displacement with Pt atoms of an underpotentially deposited (upd) Cu monolayer on the dealloyed Pd-Cu/C or Pd/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. An Ag/AgCl/KCl (3 M) leak-free electrode and Pt mesh were used as a reference and counter electrode, respectively. All potentials are quoted with respect to the reversible hydrogen electrode (RHE). All of these operations were carried out in a nitrogen atmosphere. The kinetic currents were calculated based on eq. 1: = + (1) j j k jd where j and j d are the measured current and limited current, respectively. Powder x-ray diffraction patterns (XRD) were taken by a commercial diffractometer (Bruker AXS D8 Discover) using Cu Ka radiation (λ= Å). Samples for analyses were obtained by filtering the Pd-Cu/C suspension on filter paper and transferring it to carbon paper (7 cm x 7 S1

2 cm) by pressing. After drying, the carbon paper was cut into small pieces with an area of 1 cm 2 to undergo dealloying and Pt monolayer deposition. Diffraction patterns were collected from 20 to 90 with a step size of TEM images were taken using an FEI Tecnai F20 transmission electron microscopy (TEM) with a Field Emission Source at 200 kev. A dispersion of the catalyst in ethanol was placed on a 3 mm Ni grid coated with a carbon film and allowed to dry. The DFT calculations were performed using the Vienna ab initio simulation package (VASP) (1). The projector augmented wave (PAW) method (2) was implemented. The exchangecorrelation effects were described within the generalized gradient approximation (GGA) using the Perdew-Wang (PW-91) functional (3). The Brillouin zone was sampled using the Monkhorst- Pack technique (4); an grid for 2x2 periodic slabs was found to be sufficient. The kinetic energy cutoff for plane-wave expansions was 400 ev for all calculations. The surface structure was modeled by a slab of four layers of metal and 12 Å vacuum between periodically repeated surfaces to avoid spurious surface interactions. The bottom layer of the slabs was kept fixed at their corresponding calculated bulk positions, while upper layers were fully relaxed. All adsorption energies reported were obtained as follows: 1 EO = Eslab+ O Eslab Egas ( O 2 ) 2 (1) G. Kresse and J. Furthm uller, Phys. Rev. B 54, (1996); Computat. Mater. Sci. 6, 15, (1996). (2) P. E. Bl ochl, Phys. Rev. B 50, (1994). (3) J. P. Perdew et al., Phys. Rev. B 46, 6671 (1992). (4) H. J. Monkhorst and J. D. Pack, Phys. Rev. B 13, 5188 (1976). S2

3 PdCu 6 Figure S1. Lattice constant as a function of atomic percentage of Cu atoms in a Pd-Cu alloy according to Vegard s Law. S3

4 (B) (C) (A) Figure S2. HAADF images of dealloyed PdCu 6 /C (A), typical EDS spectra for a porous particle (B) and a solid particle (C). Ni signal is from the TEM grid. Table S1. Normalized wt.% Cu and Pd for particles shown in Figure S2. Spectrum Cu Pd Total Spectrum Porous Spectrum Porous Spectrum Porous Spectrum Solid Spectrum Solid S4

5 Figure S3. Polarization curves for the ORR on Pt monolayer on Pd/C, and dealloyed PdCu 6 /C in an oxygen saturated 0.1 M HClO 4. Sweep rate = 10 mv s -1 ; room temperature. The Pd loadings in Pt/Pd/C and Pt/PdCu 6 /C are 12.8 and 8.0 µg cm -2. The Pt loading in Pt/Pd/C and Pt/PdCu 6 /C are 2.8 and 1.5 µg cm -2. S5

6 I Q Figure S4. Cu UPD curve on the dealloyed PdCu 6 /C, scanning rate = 5 mv/s in 0.05 M H 2 SO M CuSO 4 solution. The weight of Pt was calculated based on the Cu UPD charge:, where Q (mc) is the total charge during Cu UPD, I (ma) is the background current, t (s) is the time during UPD, 2 is number of electrons associated with UPD for a Cu atom, (C/mol) is the Faraday's constant and 195 (g/mol) is the molar mass of Pt. S6

7 (B) (A) Figure S5. HAADF image of Pt monolayer on dealloyed PdCu 6 /C (A) and EDS spectrum (B) for all the particles in the red square in (A). Ni signal is from the TEM grid. Table S2. Normalized wt.% Cu, Pd and Pt in Figure S4. Spectrum Cu Pd Pt Total Spectrum S7

8 (A) (B) Figure S6. Kinetic currents at 0.9 V normalized to the weight of Pt + ¼ Pd assuming the cost of Pd is a quarter of Pt (A) and normalized to the weight of total precious metals (B). S8

9 Pt/C Pt/Pd-Cu/C Figure S7. Comparison of long term stability of Pt/C and Pt monolayer on the dealloyed PdCu 6 /C. The catalysts were cycled by keeping the potential at 0.65 V and 1.0 V for 5 s each. The mass activity was normalized to the initial activity before potential cycling. S9

10 Figure S8. Changes in oxygen binding energy with respect to the d-band center in top most catalyst layer. The PdCu core in Pt/Pd/PdCu has a atomic ratio of Pd:Cu =0.875: 0.125, similar to that of a dealloyed PdCu 6 particle. A pure Pd layer was included to simulate the Pd shell formed during dealloying. S10