In the present talk we will describe the application of a recently developed XAS technique to the electrochemical growth of metal oxides. This technique, known as FEXRAV (Fixed Energy X-ray Absorption Voltammetry), is an experimental method that couples cyclic voltammetry (CV) with the acquisition of the XAS signal at a fixed energy diagnostic of a chemical and/or structural modification of the material. This approach is extraordinary powerful and we propose it as a new tool to understand the formation of oxide thin films on metals relevant to electrochemical energy conversion and storage.
This contribution reviews the fundamental principles, the electrochemical cell design, and the synchrotron beamline set-up that enable the collection of FEXRAV data. As a case study, we report the investigation of the formation of palladium oxides on palladium nanoparticles in alkaline media, a subject of extreme importance for alkaline fuel cells. With our investigation we have discovered that palladium oxidation occurs at potential larger than 0.65 V (RHE) and that the passivation layer blocks out the capability of palladium to oxidize alcohols. Remarkably, these conditions happen in real systems and are responsible of the loss of efficiency of direct alcohol fuel cells. We have also found that the presence of promoting metal oxide in contact with Pd accelerate the oxidation and, ultimately are detrimental for the stability of the devices.
Finally, the description of a novel experimental set-up with enanched surface sensitivity based on grazing incidence FEXRAV experiments will be given, together with preliminary results on the in-situ electro-oxidation of palladium thin films thae have been obtained at the LISA BM08 beamline of the European Synchrotron Radiation Facility.