Mechanism for the fast ionic transport in nanostructured oxide-ion solid electrolytes

M.G. BELLINO; D.G. LAMAS; N.E. WALSÖE DE RECA

ADVANCED MATERIALS

Wiley

Año: 2006 vol. 18 p. 3005 - 3009

0935-9648

It is well-known that nanostructured materials have attracted great interest in recent years due to their new or enhanced properties, which have been exploited in very different applications. Particularly, "nanoionics" is one of the hottest fields related to these materials, since important advances have recently been found. However, it is important to note that the transport processes in nanomaterials is not completely understood yet. The aim of this paper is to elucidate the physical mechanism behind the remarkable enhancement of the ionic conductivity in nanoceramics, recently reported by us and other authors (I. Kosacki et al., Solid State Ionics 2000, 136-137, 1225-1233; T. Suzuki et al., Solid State Ionics 2002, 151, 111-121; M. G. Bellino et al., Adv. Funct. Mater. 2006, 16, 107-113). We demonstrate here, for the first time, that the transport process in these materials is governed by the fast diffusion of free (delocalized) oxygen vacancies through the grain boundaries, while in conventional microcrystalline materials it is due to the localized jump of the carriers. Interestingly, the behavior of these nanoceramics resembles that of other families of solid electrolytes (materials with delocalized carriers, such as NASICON-type materials, Na-B-Alumina, etc.), which are expected to have a different conduction mechanism. We also explain a transition observed in the ionic conductivity in the nanoceramics as a function of temperature, reported in the above-mentioned previous works. The nature of this transition was investigated using a general law found in most of thermally activated phenomena in physics, chemistry and biology, known as the Meyer-Neldel rule. We show that the mentioned transition is related to a competition between delocalized and localized carries. At low temperature, the fast diffusion of the delocalized carries dominates the transport process, resulting in an enhancement of the ionic conductivity in about one order of magnitude compared to that of the conventional microcrystalline materials. The clarification of the transport mechanism in the studied nanoceramics opens the way for searching new ultrafast solid electrolytes, which could be very important from the technological point of view in order to reduce the operating temperatures of several devices, such as solid-oxide fuel cells, gas sensors, electrochemical reactors, solid-state batteries, etc. Therefore, we believe that our paper will be very interesting for the scientific community.