Temperature Dependent Operando High Energy Surface X-Ray Diffraction Studies of Pt(111) Oxidation

Abstract

Platinum electrocatalysts are still widely used as cathode material in proton-exchange membrane fuel cells (PEMFC) because of their high ORR activity. However, their long-term stability is limited due to degradation processes. This degradation is partly caused by Pt oxidation and oxide reduction, which lead to irreversible surface restructuring and subsequently Pt dissolution. To increase the lifetime of PEMFCs it is mandatory to understand the mechanisms behind the degradation on an atomistic level. This requires studies by structure sensitive in situ methods, in particular, also studies at the elevated temperatures typically employed in PEMFCs operation.Previous studies revealed that in the initial stage of oxide formation Pt atoms are extracted out of the surface and form an ultra-thin oxide layer directly above the surface. This extraction is long known as place exchange [1], but the atomic-scale details of the surface oxide structure, the extraction mechanism, and especially the kinetics of the initial oxide growth were only recently clarified [2-7]. It was found that the structural arrangement of the Pt atoms within the surface oxide on the Pt(111) and the Pt(100) surface is distinctly different, leading to differences in the oxidation/reduction kinetics, the reversibility of the surface oxidation/reduction process, and the amount of Pt dissolution into the electrolyte.Recently, we performed fast operando high-energy surface X-ray diffraction (HESXRD) studies, in which we investigated the initial extraction of Pt atoms on Pt(111) during potential cycles. Surprisingly, it was found that the Pt extraction during the anodic sweep is largely independent of the potential sweep rate, although the peak cyclic voltammograms (CVs) associated with the charge transfer during surface oxidation showed a pronounced shift [7-9]. This suggests that the Pt extraction is a fast process, rather driven by potential and not directly coupled to the slow formation of surface oxygen species. However, the Pt extraction during the anodic sweep and the re-insertion during the cathodic sweep still exhibit significant hysteresis, indicating a complex multistep oxidation kinetics.Previous electrochemical studies of Pt(111) electrodes revealed that the described influence of oxidation/reduction kinetics and the reversibility of the charge transfer processes increase at elevated temperature [10]. However, the temperature dependence of the Pt extraction behavior is currently unknown, because all previous structural studies of oxide growth and reduction have been performed exclusively at room temperature.For the experiments at elevated temperatures we developed a new setup for HESXRD studies of single crystals, which combines a hanging meniscus electrochemical cell for operando X-ray diffraction measurements with an inductive heating device in a joint Ar-filled compartment. This setup allows direct (re-)preparation of the aligned Pt single crystals on the diffractometer. By this, the time between preparation and measurement is reduced down to a few minutes and sample transfer through air is rigorously avoided.Temperature-dependent HESXRD measurements at temperatures up to 343 K were performed at beamline ID31 of the European Synchrotron Radiation Facility. The simultaneously measured electrochemical current and X-ray intensity indicate kinetic shifts in the charge transfer but a rather temperature independent Pt extraction process.We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft via project no. 418603497 and the BMBF via 05K19FK3 and 05K22FK1.