Kavli Institute at Cornell Seminar by Timo Fuchs (Kiel University)
The degradation of Pt single crystal electrodes caused by electro-oxidation: From the atomic-scale to the nanoscale
Understanding the 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 is causing 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 largely unclear, although a strong influence is evident, since dissolution mostly occurs transiently during oxide formation and reduction [1]. For a better understanding of Pt dissolution and Pt electrode restructuring, a detailed atomistic picture of the oxide structure is required, which only existed for the Pt(111) electrode [2,3].
In this work, high energy surface X-ray diffraction [4-6] was used to analyse the atomic-scale structure of Pt single crystal surfaces during oxide formation and reduction in acidic electrolyte. Depending on the surface orientation, distinct differences in stability versus restructuring after oxidation/reduction cycles was found. For example on Pt(111), almost no surface roughening can be observed even after oxidation at potentials of up to 1.15 V, whereas Pt(100) immediately degrades upon oxidation. The Pt dissolution mirrors this trend and is one order of magnitude higher on Pt(100) [5]. To elucidate this difference, a detailed analysis of the crystal truncation rods is presented to determine the location and potential-dependent coverage of the Pt atoms in two distinct oxide phases on Pt(100). Based on the geometry of these oxides, an oxide growth mechanism for Pt(100) is presented 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 that the dissolution during oxide formation and reduction is linked to different oxide phases [6].
Repeated oxidation/reduction cycles lead to the growth of Pt nanoislands (mounds) with a well-defined lateral size of about 30 – 50 Å [7] which coarsen with continued oxidation/reduction. Differences in mound shape and growth behaviour on the low index planes of Pt determined by grazing incidence small angle X-ray scattering will be discussed.
References
[1] S. Cherevko et al., Nano Energy 2016, 29 275–98 [2] H. You et al., J. Chem. Phys. 1994, 100, 4699, [3] J. Drnec et al., Curr. Opin. Electrochem. 2017, 4, 69, [4] J. Gustafson et al., Science 2014, 343, 758, [5] T. Fuchs et al., Nat. Catal. 2020, 3, 754 [6] T. Fuchs et al., Angew. Chem. Int. Ed. 2023, e202304293. [7] L. Jacobse et al., Nat. Mater. 2018 17 (3), 277-282
Hosted by Andrej Singer. [email protected] for Zoom information.
