CHEMICAL STABILITY OF DOPED δ-Bi2O3 AS AN ELECTROLYTE FOR SOLID OXIDE FUEL CELLS

Abstract

The high temperature phase of bismuth oxide (δ-Bi2O3) is a promising material for application as an electrolyte for solid oxide fuel cells (SOFCs), due to its high oxygen ion conductivity. Doping with rare earth cations stabilizes δ-Bi2O3 phase down to room temperature. According to literature [1], the ionic conductivity of such δ-Bi2O3 is not significantly decreased even at 600 °C. This opens the possibility to lower SOFC operating temperature from 1000 °C to intermediate temperatures. The main drawbacks of this material are the instability in reducing atmosphere and reactivity toward electrode materials. Bismuth ruthenate (Bi2Ru2O7) was chosen as a potential electrode material because of its chemical stability, compatibility with δ-Bi2O3 and metal-like electronic conductivity. Stoichiometric mixtures of Bi2O3 with Tm2O3 or Lu2O3 were dry homogenized and heat treated at 750 °C for 3 h in order to obtain δ-Bi2O3 with following compositions: (Bi0.8Tm0.2)2O3 and (Bi0.75Lu0.25)2O3, respectively. Bi2Ru2O7 was synthesized similarly, i.e. homogenized mixture of Bi2O3 and RuO2·xH2O was heated at 900 °C for 3 h. The obtained powders were pressed into disc-shaped pellets and sintered at 920 °C in case of δ-Bi2O3 and 880 °C in case of Bi2Ru2O7. Chemical stability of these materials was investigated by exposing the pellets to the hydrogen and butane atmospheres. Compatibility of electrode and electrolyte materials was tested by heating a homogenized mixture of Bi2Ru2O7 and (Bi0.8Tm0.2)2O3 (mass ratio 50:50) at 600 °C. Moreover, a mixture of (Bi0.75Lu0.25)2O3 and Bi2Ru2O7 (mass ratio 30:70) was pressed into pellet, sintered at 880 °C, and exposed to hydrogen atmosphere in order to evaluate chemical stability of the mixture under reducing conditions. Both electrolyte- and electrode-supported configurations were considered with the aim to form a functional fuel cell. 1. A. Dapčević, D. Poleti, J. Rogan, A. Radojković, M. Radović, G. Branković, Solid State Ionics, 280 (2015) 18