In situ X-ray Scattering Studies of the Electrooxidation of Platinum Single Crystal Electrodes

Pt electrooxidation and reduction is linked to the degradation of Pt-based electrocatalysts for electrochemical energy conversion.
The atomic-scale mechanisms of Pt electrooxidation, dissolution and restructuring of Pt single crystal electrodes are studied with surface X-ray scattering techniques. High energy surface X-ray diffraction (HESXRD) was employed for the first time to study the atomic-scale structure of single crystal surfaces in electrochemical environment.
A study of the initial electrooxidation of Pt(111) and Pt(100) using HESXRD revealed different surface stability of both electrodes. This difference was explained on the basis of the differing location of the extracted Pt atoms in the oxides. A specific stripe oxide is formed on Pt(100), which produces unstable atoms at the stripe ends. At higher potentials a second amorphous Pt (hydr-)oxide was found, which is located further apart from the surface. Pt dissolution during oxide formation was attributed to the nucleation & growth of the stripe oxide, while dissolution during oxide reduction is linked to the Pt (hydr-)oxide.
Fast simultaneous measurements of the electrochemical charge transfer and the coverage of extracted Pt atoms on Pt(111) revealed that Pt extraction is a fast, potential-driven process, whereas formation of adsorbed oxygen-containing species occurs on a much slower time scale and is evidently uncoupled from the extraction process. The formation of nanostructures upon repeated oxidation and reduction was studied with grazing incidence small angle X-ray scattering. The structural evolution of the lateral and vertical nanostructure size is similar on Pt(111), Pt(100) and Pt(110). Rapid nanostructure steepening was followed by a growth mode where the ratio of vertical and horizontal feature size remains constant. This was attributed to a dynamic equilibrium of up- and downhill currents of Pt (hydr-)oxide atoms, which are detached from a step edge oxide.