DOI: https://doi.org/10.29363/nanoge.interect.2022.019
Publication date: 11th October 2022
Understanding Pt surface oxidation is of key importance for the development of durable oxygen reduction reaction catalysts as used in low temperature fuel cells. The formation of an ultra-thin surface oxide on Pt electrodes causes atomic-scale restructuring of the electrode surface and Pt dissolution, which promotes the degradation of Pt-based catalysts. However, the precise role of the Pt surface oxides during Pt dissolution is still unclear, although a strong influence is evident, since dissolution mostly occurs transiently during oxide formation and reduction [1,2]. For a better understanding of Pt dissolution and Pt electrode restructuring, a detailed atomistic picture of the oxide structure is required, which only existed previously for the Pt(111) electrode [3,4].
In this study, we have used high energy surface X-ray diffraction [5,6] to analyse the atomic-scale surface structure of Pt(111) and Pt(100) during oxide formation and reduction in 0.1 M HClO4 and 0.1 M H2SO4. These surfaces exhibit distinct differences in stability versus restructuring after oxidation/reduction cycles. For example on Pt(111), almost no surface roughening can be observed even after oxidation at potentials of up to 1.15 V, while Pt(100) immediately degrades upon oxidation. The Pt dissolution mirrors this trend and is one order of magnitude higher on Pt(100) as shown in our complementary dissolution measurements using inductively coupled plasma mass spectrometry [6]. To elucidate this difference we performed a detailed analysis of the crystal truncation rods from the onset of oxide formation up to the onset of oxygen evolution to determine the location and potential-dependent coverage of the Pt atoms in the oxide on Pt(100). Two phases of Pt oxides were found, a stripe-like oxide consisting of Pt rows [6] and an amorphous oxide at a higher vertical distance from the surface. Based on the geometry of these oxides we were able to find an oxide growth mechanism for Pt(100) which inherently leads to surface roughening and thereby explains the difference in structural stability. Comparison of the coverage of the two oxide phases with the corresponding dissolution reveals a correlation of the dissolution during oxide formation and reduction with either of the two oxide phases.
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