Superionic Conducting Bismuth Oxide-Based Electrolytes for Solid Oxide Fuel Cells

Abstract

The erbia-stabilized Bi2O3 (ESB) is known as a promising SOFC electrolyte material due to its excellent ionic conductivity compared to conventional oxide ion conductors such as doped CeO2 and stabilized ZrO2. Despite its superior ionic conductivity at reduced operating temperatures below 700 oC, ESB has not been a practical choice for an electrolyte material for SOFCs because its thermodynamic instability under reducing atmosphere and kinetic instability of time-dependent conductivity degradation. To address the thermodynamic instability of ESB, we developed a novel thin and dense ESB/YSZ bilayered electrolyte using nano-sized ESB powders on anode-supported cells, demonstrating high power density SOFC with high efficiency at intermediate temperatures (< ~750 oC). In addition, we dramatically enhanced the kinetic stability of stabilized bismuth oxides via substituting Bi3+ with very small amounts of the secondary aliovalent dopant. In order to investigate the effect of the aliovalent dopants on suppressing the time-dependent degradation mechanism, the cation diffusivity of the samples was evaluated by Boltzmann-Matano method, and chemical and structural changes by extended annealing were also investigated. The electrochemical performance and the stability of the SOFCs with our novel bismuth oxides were evaluated with I-V characteristics and impedance spectroscopy.