Lead and Yttrium co-doped bismuth oxide electrolytes for low temperature solid oxide fuel cells

Abstract

A co-dopant bismuth oxide system with Pb2+ and Y3+was developed. When Pb2+ was doped as a single dopant, a mixture of cubic and tetragonal phases was obtained, indicating that Pb2+ on its own does not stabilise δ-Bi2O3. On the other hand, using 5 mol% Pb2+ and 10 mol% Y3+ double dopants (5Pb10YSB, where SB represents stabilized Bi2O3), we were able to stabilize a high temperature cubic phase with a total dopant concentration as low as 15 mol%. Its total conductivity at 500°C was 0.1185S cm-1, comparable to 0.0984 S cm-1 obtained from the double doped system with 8 mol% Dy3+ and 4 mol% W6+ stabilizing δ-Bi2O3(8D4WSB) and 100 times more conductive than 10 mol% Y3+ stabilized zirconia (10YSZ) at the same temperature. Arrhenius plots of the total conductivity for the 1.5Pb25YSB (1.5mol% Pb2+ and 25 mol% Y3+ doped Bi2O3) and 5Pb10YSB systems exhibited discontinuities between 500-550°C characteristic of phase changes. Differential thermal analysis, variable temperature Raman spectroscopy and variable temperature powder X-ray diffraction revealed that these discontinuities in conductivity occurs in the same temperature range as the phase transitions (cubic to tetragonal phase) observed in these materials. On the other hand, the high total dopant concentration, 1.5Pb25YSB (i.e. 26.5 mol%) showed a single phase from ambient temperature to higher temperatures (800°C), suggesting phase stability of material but displayed lower conductivities compared to 5Pb10YSB. This decrease in conductivity could be explained by an increase in dopant concentration resulting with stronger association of O2- with the dopant ions (stronger bonds) reducing mobility.