Reversible solid oxide cells (SOCs) have drawn much attention as alternative energy conversion and storage devices owing to their high efficiency and environmental benignity. However, considering their high operating temperature involves unwanted chemical reactions, thermal stress, and high system costs it is essential to lower their operating temperature for commercial feasibility. At reduced temperature, on the other hand, the catalytic activity of oxygen reduction reactions (ORRs), oxygen evolution reactions (OERs), and oxygen ionic transport are diminished due to their thermally activated nature. For example, the most widely used yttria-stabilized zirconia (YSZ) shows significantly decreased ionic conductivity at reduced temperature (e.g. 0.003 S/cm at 600 °C), which is not sufficient for high performing SOCs.

Among various alternative oxygen ionic conducting materials, rare-earth stabilized bismuth oxides have been well known as super ionic conductors which exhibit 30 times higher conductivity than that of YSZ. However, the ionic conductivity of stabilized bismuth oxides is significantly decreased due to their phase transformation from cubic phase to rhombohedral phase at 600 °C. In this regard, it is crucial to carefully utilize proper doping material for enhancing their stability while retaining their high ionic conductivity.

In this study, we newly developed stabilized bismuth oxide with superior ionic conductivity and durability at 600 °C via triple-doping strategy. Initial ionic conductivity of the stabilized bismuth oxide was maintained for more than 1000 hours at 600 °C with no phase transformation. This stabilized bismuth oxide was combined with conventional La_0.8Sr_0.2MnO_{3- δ} (LSM) as a novel oxygen electrode. Furthermore, a SOC with this novel air electrode showed high electrochemical performance both in the fuel cell (FC) and the electrolysis (EC) modes (e.g. 2.5 W/cm² and 1.4 A/cm² at 1.3V at 700 °C in the FC and the EC modes, respectively).