Low oxygen partial pressure increases grain boundary ion conductivity in Gd-doped ceria thin films

Abstract

Grain boundaries play an important role in the transport properties of oxide ion conducting electrolytes and mixed ionic electronic conductors. Nevertheless, very little is known about the electrical grain boundary properties in thin films. In these, the separation of in-plane grain and grain boundary conductivity is more complicated due to the large capacitive effect of the insulating substrate. This can be overcome by using interdigitating electrodes with separation of few micrometres. By comparing grain and grain boundary conductivities of Gd-doped Ceria (GDC) thin films with 5 and 10 mol % Gd content, we can show that the much lower conductivity of 5% doped GDC is almost exclusively caused by a significantly higher grain boundary resistance. In reducing atmosphere, GDC becomes mixed ion and electron conducting and in such conditions, the employed Pt thin film electrodes are virtually blocking for oxygen anions and reversible for electrons. With impedance spectroscopy we can therefore simultaneously measure ionic and electronic conductivities under reducing conditions. Although the bulk vacancy concentration remains dominated by the extrinsic acceptor doping, the ionic conductivity of the films increases by up to one order of magnitude when going from oxidising to reducing atmosphere. This result is—although in such a clear manner not observed or predicted before—in line with the widely accepted grain boundary space charge model. It is concluded that an accumulation of Ce3+ in the space charge zone weakens the oxygen vacancy depletion and therefore increases the grain boundary conductivity. The results are of high relevance for understanding and optimising the properties of GDC in anodes and electrolytes for solid oxide fuel cells, and potential new uses such as electrostrictive and memristive devices, for which oxygen partial pressure dependent ionic conductivity is an important new aspect.