Constructing an expeditious and durable composite as an air electrode of solid oxide cells through synergistic phase transformation and phase segregation engineering

The sluggish catalytic activity of iron-rich perovskite-based air electrodes at low temperatures (<650 °C) is a common problem faced by solid oxide cells (SOCs). Herein, an expeditious and durable iron-rich, multifunctional, composite material is reported as an outstanding air electrode for SOCs. Such a composite consists of a dominant cubic single perovskite (SP) phase, SrFe_1-x(Ta,Nb)_xO_3−δ, and a minor oxygen vacancy-rich double perovskite (DP) phase, Sr₂FeNbO_6−δ. The incorporation of pentavalent Ta and Nb effectively inhibits the formation of tetragonal SP and induces phase transformation to a cubic SP with high symmetry, while the in-situ separated DP phase synergistically boosts the performance of oxygen activation. Such multiple benefits result in the generation of an oxygen-ion conductor-based solid oxide fuel cell (O-SOFC) with the developed composite electrode that yields a superb maximum power density (P_max) of 1259 mW cm⁻² at 600 °C, ∼2.1 times that of an O-SOFC with SrFeO_3−δ parent electrode (595 mW cm⁻²). A reversible protonic ceramic cell (R-PCC) with such composite air electrode delivers a remarkable electrochemical performance, e.g., a P_max of 844 mW cm⁻² and an electrolysis current density of −957 mA cm⁻² @ 1.3 V at 650 °C. More attractively, the resulting cell exhibits an outstanding operating endurance of 500 h in fuel cell mode and 210 h in cycle mode (i.e., alternating between fuel cell and electrolysis cell modes).