Why do BaCo0.4Fe0.4Zr0.1Y0.1O3–δ-derived complex oxides become one of the most promising electrodes for protonic ceramic electrochemical cells? An explanatory review

Efficient energy conversion through the use of fuel and electrolysis cells is crucial for addressing the challenges posed by hydrogen energy and carbon neutrality initiatives. Solid oxide electrochemical cells, which are based on proton-conducting electrolytes (known as protonic ceramic fuel cells (PCFCs) and electrolysis cells (PCECs)), are a viable solution due to their ability to achieve high performance and efficiency at reduced operating temperatures, below 600 °C. However, electrode kinetics become sluggish at low- and intermediate-temperature ranges, necessitating the search for new electrode materials with high electrochemical and electrocatalytic activity towards oxygen reduction and oxygen evolution reactions. While a wide range of single-phase and composite materials have been proposed as PCFC/PCEC electrodes, determining the optimal composition is often challenging due to the complex interplay of functional properties. BaCo0.4Fe0.4Zr0.2O3–δ (BCFZ), BaCo0.4Fe0.4Zr0.1Y0.1O3–δ (BCFZY), along with their derived compositions, are considered as highly promising oxygen electrodes for electrochemical cells with proton-conducting electrolytes. These materials offer unique advantages over widely studied materials, such as La1–xSrxCo1–yFeyO3–δ (labeled as LSCF), Ba1–xSrxCo1–yFeyO3–δ (BSCF), PrBa1.5Sr0.5Co1.5Fe0.5O6–δ (PBSCF), and Ln2NiO4+δ (LNO). The present review discusses why BCFZ, BCFZY, and their doped compositions have garnered increased attention from both fundamental and applied perspectives.