Invited Paper EH-WeE7
In Situ Observation of Electrochemical Processes at Solid/Liquid Interfaces

Wednesday, December 14, 2016, 7:40 pm, Room Lehua

In order to design highly efficient and durable materials for energy conversion devices such as fuel cells and batteries, it is important to understand the key electrochemical processes occurring at solid/liquid interfaces. We have utilized various x-ray techniques to investigate those interfacial processes under electrochemical potential control. In this talk, in situ XAFS studies on electron transfer reactions on electrocatalysts and development of an in situ electrochemical XPS apparatus for the solid/liquid interfaces will be presented.

The rate of oxygen reduction reaction (ORR) was significantly enhanced when cerium oxide (CeO_x) was utilized as a co-catalyst together with Pt catalyst. The promotion effect of CeO_x for ORR at Pt was investigated by in situ XAFS measurements of the Pt-CeO_x/C and a conventional Pt/C catalyst. Upon the incorporation of CeO_x into Pt, not only Ce⁴⁺ species corresponding to CeO₂ but also Ce³⁺ species were observed due to the formation of the Pt/CeO_x interface. After cleaning the Pt surface by oxidation/reduction cycles in an aqueous solution, Ce⁴⁺ species is preferentially eluted in the solution, so that 3−5 nm Pt nanoparticles coated with a few CeO_x layers were formed. At the Pt L₃ edge of the Pt/C, the white line intensity increased as the potential was made more positive than 1.0 V (vs. Ag/AgCl), showing the formation of Pt oxide because the white line intensity reflects the d-band vacancies. In contrast, for the Pt-CeO_x/C, the formation of Pt oxide was significantly suppressed by the CeO_x layer. This suggests that the intrinsic ORR activity of bare Pt was maintained at the Pt-CeO_x/C while the Pt surface is partly covered by insulating Pt oxide at the pure Pt/C. At the Ce L₃ edge of the Pt-CeO_x/C, a singlet peak due to the Ce³⁺ species formed at the Pt surface turned into a doublet peak characteristic to the CeO₂, suggesting that Ce³⁺ species was oxidized to Ce⁴⁺ species to inhibit the formation of Pt oxide.

XPS is a powerful technique to analyze the surface compositions and oxidation states. However, it is difficult to carry out in situ XPS analysis of electrode surfaces in contact with electrolyte solutions under electrochemical potential control because of requirement of vacuum. Recently, we constructed an in situ XPS apparatus, which is applicable to electrochemical processes at solid/liquid interfaces, using a micro-volume cell equipped with an ultrathin photoelectron window. Electrochemical growth of Si oxide in contact with water was observed as a first demonstration of the capability of this system and effect of potential and time on the thickness of Si and Si oxide layers was quantitatively determined.