Application of XPS to study electrocatalysts for fuel cells

▶ We provide a review of the application of X-ray photoelectron spectroscopy (XPS) for fuel cells; in terms of the reactivity, selectivity, and catalytic ability of substances at catalytic fuel cell interfaces. ▶ We point out that XPS allows determination of changes in the electronic structure for different surface preparation and composition based on shifts of the binding energies of core-level electrons. ▶ (It is also an ideal method that allows identification of the surface or near surface species in relation to fuel cell catalysis.) ▶ We also point out that the fundamental theoretical concepts, which are used to analyze and interpret XPS spectra are sometimes not correctly understood or correctly applied. ▶ We provide rigorous definitions of fundamental concepts used to understand XPS spectra, including the separation of initial and final state effects and the relaxation of valence electrons to screen core-holes. ▶ An additional direction of our review is to show the relationships between XPS binding energy shifts and XPS satellite structure with chemical bonding and chemical interactions. ▶ Our primary concern however is to provide reviews of representative cases of the application of XPS to solving fuel cell and electrocatalysis-related problems, highlighting progress in this laboratory. Analysis of the surface is paramount to understanding the reactivity, selectivity, and catalytic ability of substances. In particular, this understanding is required to make an efficient use of the catalytic surfaces in fuel cells. X-ray photoelectron spectroscopy (XPS) allows determination of changes in the electronic structure for different surface preparation and composition based, mainly, on shifts of the binding energies of core-level electrons. It is also an ideal method that allows identification of the surface or near surface species in relation to fuel cell catalysis. However, the fundamental theoretical concepts, which are used to analyze and interpret XPS spectra are sometimes not correctly understood or correctly applied. In this review, we not only report on XPS operational parameters in use for fuel cell electrocatalysis, but, more significantly, we review and provide rigorous definitions of fundamental concepts used to understand XPS spectra, including the separation of initial and final state effects and the relaxation of valence electrons to screen core-holes. An additional direction of our review is to show the relationships between XPS binding ener