Supporting Information

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1 Supporting Information Synchrotron-Based In Situ Characterization of Carbon-Supported Platinum and Platinum Monolayer Electrocatalysts Kotaro Sasaki 1*, Nebojsa Marinkovic 2, Hugh S. Isaacs 1, Radoslav R. Adzic 1 1 Chemistry Department, Brookhaven National Laboratory, Upton, NY Chemical Engineering, Columbia University, New York, NY *Corresponding Author: ksasaki@bnl.gov

2 Figure S1. (a) Low resolution TEM image of carbon-supported Pt ML /Pd nanoparticle catalyst and (b) its particle size distribution. The average size: 4.2 nm

3 Figure S2. Synchrotron XRD pattern of a commercial PtO 2 sample. The structure is determined to be a hexagonal phase of α-pto 2 (JCPDS ). The assignments of diffraction peaks are included. Wavelength: Å.

4 Figure S3. Synchrotron XRD pattern of PdO sample. The structure is determined to be a tetragonal phase (JCPDS ). The assignments of diffraction peaks are included. Wavelength: Å.

5 Figure S4. other potential ( µ E ). In situ XANES representing the intensity of the white line at 0.41 V ( µ 0.41V ) and

6 Figure S5. Ratio of the charge of reduction peak (Q o ) to the charge of hydrogen desorption (Q H ) plotted as a function of applied upper potentials (E up ). The Q o, Q H, measured with increasing E up are derived from cyclic voltammograms in Ar-purged 0.1 M HClO 4 at a sweep rate of 20 mv/s (one example is shown in the inset). The degree of oxidation can be estimated from the charge of reduction peak (Q o ) observed in cyclic voltammograms (CVs), while the surface area can be determined from the charge of hydrogen desorption area (Q H ). Figure S5 shows a ratio of Q o to Q H derived from CVs with increasing upper potentials (E up ) as shown in the inset of Figure S5. We presume that the Q o /Q H is equal to 4 if the monolayer of Pt surface is oxidized to PtO 2. From the figure, the E at Q o /Q H =4 is 1.64 V. At 1.51 V, the Q o /Q H value is 2.8, which correspond to the oxidation of 70 % of Pt surface. The result from the electrochemical measurement agrees reasonably well with those from in situ XANES and XRD measurements, and we thus believe that the oxidation of Pt nanoparticles is confined to the surface monolayers. Above 1.7 V, the Q o /Q H starts to rise sharply in Figure S5, and we consider that it is caused by the reduction of O 2 gas trapped under the electrode.

7 Figure S6. Typical raw XANES spectrum of Pt L 3 edge with indication of the edge height (at ev).

8 Figure S7. Schematic of the interface between PtO 2 and an electrolyte together with a qualitative concentration profile of H + along the electrolyte. C is the change in concentration of H +, δ is the diffusion layer thickness, n is the charge number of H + (n = 1), F is the Faraday constant, A is the surface area, D is diffusion coefficient of H + in the solution, and I is the anodic current. We employ a simple model using Nernst s approach to calculate a proton concentration near the interface as shown in Figure S7. When both the OER and carbon corrosion are taking place at the surface (Eqs. (5) and (6) in the manuscript, respectively), the concentration of H + increases and local ph is reduced. Substituting δ 0.05 cm, D 10-5 cm 2 /s, I 0.1 A, and A = 1 cm 2 into Eq. 1 in Figure S7, gives [H + ] to be approximately 5 M (the value of I was obtained from the measurement at 2.37 V in Figure 7). The estimated proton concentration under the OER and carbon corrosion could be 50 times larger than the bulk concentration (0.1 M HClO 4 ), and thereby the solubility of PtO 2 must also increase 50 times.

9 Figure S8. In situ XANES for Pt L 3 edge of Pt ML /Pd/C at potentials ascending from 0.41 to 1.51 V. Three distinct isosbestic points are observed at the same potentials for the Pt ML /Pd/C catalyst (at ev, ev, and ev as designated by arrows).

10 Figure S9. Comparison of PtO 2 percentages per surface Pt atoms between Pt/C (Figure 4b) and Pt ML /Pd/C (Figure 9) plotted as a function of applied potentials.

11 Figure S10. (a) In situ XANES and (b) Fourier transform (FT) EXAFS for Pd K edge of the Pt ML /Pd/C catalyst at different potentials (from 0.41 V to 1.31 V), together with those of the PdO sample. The peaks around 1.6 Å in Figure S10b reflect the formation of Pd oxide.

12 Table S1. Number of atoms, particle size (sphere) and average coordination numbers for icosahedron and cuboctahedron for platinum, calculated by the formulae given in reference 1. Number of shells Total atoms Surface atoms % of surface atoms Particle size (nm) (icos) (cuboct) Reference 1. Benfield, R. E. J. Chem. Soc. Faraday Trans. 1992, 88,