Niobia based rutile materials as SOFC anodes

Materials that exhibit considerable electronic conductivity and good stability in fuel atmospheres are interesting candidates to replace of Ni in its current collecting role in Ni-cermet solid oxide fuel cell (SOFC) anodes. The Nb_1-xTi_xO₂ rutile system offers particularly high electronic conductivity and has good chemical compatibility with SOFC components and seems an interesting candidate. Of the three stable phases in the Nb₂O_5-TiO₂ system, Nb₂TiO₇ was chosen as starting composition as it has the highest Ti content and so exhibits the highest conductivity when reduced to rutile. Nb₂TiO₇ was prepared by standard solid state reaction at 1250 °C for 48 hours in air. Under the reducing conditions of a SOFC anode at 930 °C, it is reduced to rutile Nb_0.67Ti_0.33O₂. The conductivity increases by about 4 orders of magnitude to about 300 Scm^-1 between air and fuel atmospheres and the p(O₂)^-1/6 dependence close to fuel conditions indicates n-type conductivity. Impedance spectra of samples in both oxidising and reducing atmospheres were consistent with electronic domination; any ionic contribution seemed
insignificant. Symmetrical cell measurements of Nb₂TiO_7-x screen-printed electrodes on yttria stabilised zirconia in air were dominated by the series resistance of the electrodes. Under reducing conditions the series resistance was very low; however, there were significant polarisation impedances and also a large diffusion limitation. The poor electrochemical performance is thought to be related to very slow ionic diffusion in the niobate. The polarisation resistance in air was similar in value to that in hydrogen, despite much lower electrode conductivity. Despite the poor electrochemical performance this material still presents many attributes attractive for a current collection function.