X-ray absorption spectroscopy: Application to electrode surface and interface analysis

Although x-ray absorption spectroscopy (XAS) is a bulk spectroscopy, proper design of experiments permit its application to surface and interface analysis. Examples are the use of glancing angle incidence x-rays and fluorescence detection to study underpotential deposited (UPD) monolayers on single crystal surfaces and the study of passive films on AlCr alloys by the use of thin film (similar to20Angstrom) alloy electrodes deposited on a Ta coated Mylar window that is in contact with the cell electrolyte. XAS can be used to study surface processes on fuel cell electrocatalysts. The particle size of these electrocatalysts is typically 2-4 nm. A significant fraction of the atoms (30-50%) in the electrocatalyst are surface atoms. Transmission or fluorescence XAS can detect surface processes. XAS can also be used to investigate corrosion of metal hydride battery electrodes. This is possible because the hydride alloy electrodes consist of small particles (similar to3mu) and the corrosion of cycled electrode can consume 5%, or more, of the alloy. These alloys can contain as many as nine components. The element specific nature of XAS make it a powerful technique for corrosion studies of these alloys. The paper will show examples of the application of XAS to the study of carbon supported Pt and PtSn electrocatalysts and corrosion processes in metal hydride electrodes.