Surface Oxidation of Pt(111) Studied By Surface X-Ray Diffraction and Grazing-Incidence Small-Angle X-Ray Scattering
Authors: Ruge Martin , Rahn Björn , Reikowski Finn , Carlà Francesco , Felici Roberto , Stettner Jochim , Drnec Jakub , Magnussen Olaf M. , Harrington David A. 
Published: February 2020

Electrochemical growth of Pt oxide is important because of its role in the oxygen-reduction reaction (ORR) and in Pt dissolution, which affect the performance and durability of PEM fuel cell Pt electrocatalysts. It has been known for a long time that potential cycling leads to a restructuring of the surface, in which Pt atoms move away from their lattice sites. The details of this process have not been fully resolved, though early surface X-ray diffraction (SXRD) studies confirmed the key role of a "place exchange" in which Pt and O (or OH) species exchange places [1]. Reported here are in-situ SXRD and Grazing-Incidence Small-Angle X-ray Scattering (GISAXS) studies of Pt(111) electrodes in 0.1 M HClO4 carried out at the European Synchrotron Radiation Facility [2-6].

The atoms are located by a crystal-truncation rod analysis, which shows that the place-exchanged Pt is located directly above its original lattice site. Simultaneous cyclic voltammograms of current and the intensity of the (1,1,1.5) reflection associate the initiation of the place exchange with the "oxide" peak at 1.05 V vs RHE. The voltammogram is unchanged by cycling to 1.15 V, as the place exchange is reversed on the reduction cycle, albeit with significant hysteresis. Cycling to higher potentials leads to irreversible changes to the surface, which were studied in detail with GISAXS.

GISAXS probes the evolution of nanoscale features on the surface. Progressive surface roughening occurs with cycling, to an extent that is dependent on the upper reversal potential (E
up). The average spacing of the islands ranges from 40-70 Å for E
up from 1.4-1.6 V, with smaller spacing for higher potentials. The spacing increases slightly with cycle number but is mainly determined after the first cycle. The islands grow in height and become more prominent and homogeneous in size with the number of cycles. These changes are similar to the evolution of Pt surfaces in UHV undergoing deposition of Pt or surface erosion, showing that the underlying processes such as vacancy coalescence are common for the UHV and electrochemical cases. SXRD experiments in the presence of oxygen show no effect on the place exchange during cycling. The oxygen reduction current decreases at potentials more negative than the initiation of the place exchange, showing that it is limited by the adsorbed intermediate(s) in the ORR (likely OH) and not Pt oxide formation.

The authors thank the European Synchrotron Radiation Facility, Deutsche Forschungsgemeinschaft, and the Natural Sciences and Engineering Research Council of Canada for support. [1] H. You, D.J. Zurawski, Z. Nagy, R.M. Yonco, J. Chem. Phys., 100, 4699 (1994). [2] J. Drnec, M. Ruge, F. Reikowski, B. Rahn, F. Carlà, R. Felici, J. Stettner, O.M. Magnussen, D.A. Harrington, Electrochim. Acta, 224, 220 (2017).

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