Spontaneous formation of electro-catalytically active molten Sn@SnOx nanoparticles on ceria Luca Bardini, ∗,† Alfonsina Pappacena, † Montserrat Dominguez, ‡ Jordi Llorca, ‡ Marta Boaro, ∗,† and Alessandro Trovarelli † †University of Udine, via del Cotonificio 8, 33010 Udine, Italy ‡Institut de T` ecniques Energ` etiques and Centre for Research in Nanoengineering, Universitat Polit` ecnica de Catalunya, 08028 Barcelona, Spain E-mail: luca.bardini@uniud.it; marta.boaro@uniud.it Abstract We perform a combined electrochemical and strucural investigation to understand the influence of tin oxide on the electrocatalytic activity of ceria in solid oxide fuel cell anodes. We show that the co-presence of the two pure materials improves the power output of fuel cells by up to a factor of 10 when compared to ceria alone. X- ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HR-TEM) show that the increase in electrocatalytic activity is due to core-shell nanoparticles comprised of a molten metallic tin core capped by a nanometric film of amorphous tin oxide. These nanoparticles form spontaneously upon reduction of an initially uniform tin dioxide film on cerium oxide, and are stable at 873 K thanks to oxygen transfer from the ceria supporting particles. It is highlighted how the SnO x amorphous shell acts both as a binding agent which stabilizes Sn 0 against segregation at 400 K above its melting temperature, as well as an electro-catalytically active interface. 1