Interfacial effects in electrochemical cells for oxygen ionic conduction measurements: III. Transference numbers vs. grain-boundary resistivity

Resistive grain boundaries may significantly influence transference numbers of oxygen ion-conducting ceramics due to a distinct blocking effect on the ionic transport with respect to the electronic conduction. This may lead to remarkable uncertainties, particularly in the boundary resistance estimates obtained using impedance spectroscopy without a separation of the ionic and electronic contributions, and in the interpretation of transport parameters determined by DC techniques. The role of grain boundaries was examined by the example of “pure” and SiO2-containing pyrochlore ceramics Gd2−xCaxTi2O7−δ (x=0.05–0.14), studied at 973–1223 K using impedance spectroscopy, faradaic efficiency and e.m.f. methods. The oxygen ion transference numbers of “pure” materials in air vary in the range 0.95–0.98, increasing when temperature decreases. As expected, minor additions of SiO2 result in segregation of siliceous phase and highly resistive grain boundaries. The total ion transference numbers of silica-enriched ceramics become considerably lower, 0.76–0.89, and increase with increasing temperature due to relatively high activation energy for the boundary resistivity; the bulk transference numbers extracted from the impedance spectroscopy and faradaic efficiency data are similar for all compositions, with and without SiO2 additions. The results, including dependence of the transport properties on oxygen pressure, suggest that a simplified description of the boundaries as a passive transport-limiting barrier is sufficiently adequate.