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. Xray 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.