Conductivity and stability of bismuth oxide-based electrolytes and their applications for IT-SOFCs

Cubic stabilized ((DyO1.5)x-(WO3) y-(BiO1.5)1-x-y) electrolytes (DWSB) having higher ionic conductivity than (ErO1.5)0.2(BiO1.5) 0.8 (20ESB) were developed to achieve higher conductivity. An optimal 2:1 dopant content ratio (Dy:W) was determined based on the solid solubility limit, X-ray diffraction (XRD) pattern and Arrhenius behavior. Various compositions with the same 2:1 dopant content ratio were tested to obtain the conductivity dependence on total dopant concentration. With double doping, stabilization of the cubic phase was achieved with as little as 12 mol% total dopant concentration. Overall, DWSB has a closer inherent structure to pure delta-Bi2O 3 than any singly doped compositions. Both lattice parameter and conductivity linearly extrapolate with total dopant concentration to that of pure delta-Bi 2O3, resulting in the ability to stabilize delta-phase at lower dopant concentration thus achieving higher conductivity.;However, this DWSB composition experienced conductivity degradation like other cubic stabilized bismuth oxides at intermediate temperatures (IT), i.e. 500∼700°C. Several DWSB compositions with the same 2:1 dopant content ratio (Dy:W) were annealed to observe isothermal conductivity behavior in the IT range with time. All DWSB compositions maintained their initial conductivity at 700°C, but underwent conductivity degradation at ≤600°C. The effect of total dopant concentration on conductivity degradation behavior was investigated at 600°C and 500°C. Notably, the effect of dopant composition on conductivity behavior with time at 500°C demonstrates that there is a trade-off between initial conductivity and long term stability at this temperature. Therefore, it is necessary to find an optimal total and relative concentration of dopants to provide the enhanced long term stability needed to make the DWSB electrolyte system feasible for 500°C operation. To this end, it was found that (DyO1.5)0.25-(WO 3)0.05-(BiO1.5)0.70, 25D5WSB, maintained a conductivity of 0.0068 S/cm without appreciable degradation after annealing at 500°C for 500 hours.;Cathode performance can be improved by the introduction of a second ionic conducting phase with high conductivity due to increased triple phase boundary (TPB) lengths. In this study, bismuth oxide-based electrolytes were combined with La1-xSrxMnO3-delta (LSM) to obtain composite cathodes. This composite cathode had better performance on ESB ((Er 2O3)0.20(Bi2O3)0.80 ) than on GDC (Gd0.1Ce0.9O2-d).