Advances in the Development and Application of δ-Bi₂O₃-Based Ionic Conductors for Ceramic Electrochemical Cells

The increasing demand for sustainable energy solutions has driven interest in ceramic electrochemical cells, which are also known as solid oxide electrochemical cells, for high-efficiency power generation and hydrogen production. Ceramic electrochemical cells offer fuel flexibility and reduced CO₂ emissions. However, high operating temperatures (>700 °C) result in higher costs and performance degradation. Efforts to lower the operating temperatures have led to advancements in materials, particularly Bi₂O₃-based ionic conductors, which are known for their superior oxygen ion conductivity. Despite their potential, Bi₂O₃-based materials suffer from instability at the expense of facile ionic transport. This review examines recent research addressing these challenges, focusing on intrinsic properties, chemical compositions, cell designs, and fabrication methods to improve the stability and performance. Additionally, the potential of incorporating Bi³⁺ into other oxides is explored. The discussion and summary in this review aim to guide the rational design of ceramic electrochemical cells operating at low temperatures with Bi₂O₃-based ionic conductors.