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 Ce 3+ 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.

Silica: ubiquitous poison of metal oxide interfaces

“…Thus, increasing the bulk dopant concentration leads to a shrinkage in the size of the space charge and its width. This can be done extrinsically through doping or intrinsically by reducing the electrolytes under H2/Ar environments in order to generate additional negative charges that compensate the space charge potential, though the later has limited use as it also reduces the ionic transference of solid electrolytes 144 . Moreover, the segregation of the negatively charged dopant into the grain boundary core can help counteract the naturally present positive charges (Figure 11, b).…”

The Relation of Microstructure, Materials Properties and Impedance of SOFC Electrodes: A Case Study of Ni/GDC Anodes

“…This already indicates that the grain boundary contribution to the ion conduction, which has activation energies of 0.8-1 eV [70][71][72], is relatively low. A low-grain boundary resistance of GDC is expected in reducing conditions [73]. In absolute numbers, the conductivity of a dense GDC pellet is 9 ± 1 times larger than the effective conductivity of the cermet.…”

Active Tuning of Optical Constants in the Visible–UV: Praseodymium‐Doped Ceria—a Model Mixed Ionic–Electronic Conductor

“…That will enable a more direct connection to the specific charge carrier type, e.g., a mobile oxygen vacancy, and separating the slower ionic contribution from the more mobile electronic one in MIECs. Furthermore, measuring the ionic transport optically in addition to electrical characterization methods, e.g., impedance spectroscopy for measuring electrical conductivities, [ 56 ] could assist in extracting information on the ionic defect concentrations.…”