H electro-insertion into Pd/Pt(1 1 1) nanofilms: an original method forisotherm measurement coupled to in situ surface X-ray diffractionstructural study

International audienceIn order to get a thorough comprehension of the mechanisms governing hydrogen insertion into nano-metric metallic films, we have studied ultra-thin Pd/Pt(111) layers. Here we propose an original method allowing the measurement of hydrogen insertion electrochemical isotherms. The use of a hanging meniscus rotating disc electrode and a new calculation approach permit to remove the contributions to the insertion charge of both hydrogen evolution and hydrogen oxidation reactions. We have measured hydrogen insertion isotherms for Pd/Pt(1 1 1) films from 14 ML down to 4 ML. Independently from the film thickness, the maximum hydrogen insertion rate (H/Pd)max is smaller than that of bulk Pd. Both hydrogen solubility and the two-phase domain width diminish with the decrease of the film thickness. The behaviour of hydrogen electrochemical insertion isotherms is interpreted in the light of the Pd nanofilms structure obtained with in situ surface X-ray diffraction. The lattice constraints induced by the substrate result in a lower insertion rate in the Pd deposit close to the Pt-Pd interface. Only the outermost region of the film is relaxed and behaves like bulk Pd