In order to improve the efficiency of the energy conversion devices such as fuel cells, rechargeable batteries, and photocatalysts, it is important to understand the mechanisms of key electrochemical processes taking place at the solid/liquid interfaces. We have utilized various in situ characterization techniques to investigate those interfacial processes under electrochemical potential control.1-6 Here, I will present our recent progresses regarding in situ x-ray absorption fine structure (XAFS) study on role of ceria in the enhancement of oxygen reduction reaction activity in polymer electrolyte membrane fuel cells (PEMFCs)2 and development of an in situ electrochemical x-ray photoelectron spectroscopy (XPS) apparatus for the solid/liquid interfaces.3 XAFS is a powerful technique for the determination of the local geometric and electronic structures of materials of interest with very high sensitivity and elemental specificity by using x-rays as a probe. Recently, we utilized a Pt-CeOx nanocomposite for oxygen reduction reaction (ORR) in PEMFCs and found that the nanocomposite showed very high ORR activity as compared with a conventional pure Pt catalyst. The promotion effect of CeOx in the Pt-CeOx/C catalyst for ORR is investigated by in situ XAFS measurements at Pt LIII and Ce LIII edges of the Pt-CeOx/C catalyst as well as at Pt LIII edge of the conventional Pt/C catalyst in oxygen saturated H2SO4 solution. XPS is a crucial technique for the investigation of surface compositions, oxidation states and electronic structures of constituent elements with very high surface sensitivity. Although surface structures of the materials in vacuum are significantly different from those under operations, it is a very difficult challenge to conduct an in situ XPS analysis of the solid/liquid interfaces during electrochemical processes because XPS requires vacuum. Recently, we constructed an in situ electrochemical XPS apparatus for the measurements at solid/liquid interface under electrochemical condition using hard x-rays from synchrotron radiation light sources and a micro-volume cell equipped with an ultrathin silicon membrane as a window for x-rays and photoelectrons, separator between vacuum and ambient, and working electrode. Electrochemical growth of Si oxide in contact with water was observed and the effect of potential and time on the thickness of Si and Si oxide layers was quantitatively determined at a sub-nanometer resolution.

In situ XAFS and XPS for various electrochemical processes at solid/liquid interfaces

251st National Meeting (2016)

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