Perovskite-spinel heterostructure enabled air electrode with superior performance and durability for protonic ceramic...

Protonic ceramic fuel cells (PCFCs) are regarded as highly promising next-generation energy conversion systems. Nevertheless, their practical application is severely hindered by the sluggish kinetics of the cathodic oxygen reduction reaction (ORR) and insufficient durability. In this work, to construct a high-performance and durable air electrode, the A-site of Sr₄Co₂Fe₄O_13-δ (SCF) was partially substituted by Ba²⁺ (10, 20, 25, and 50 mol%) using a self-assembly approach. The incorporation of Ba not only markedly enhanced the electrochemical performance but also induced a phase transition. Among all compositions, Ba₁Sr₃Co₂Fe₄O_13-δ (25 mol%, B1SCF) exhibited the highest electrocatalytic activity. At 550 °C, it achieved a polarization resistance of 0.25 Ω cm² and peak power density (PPD) of 541 mW cm^-2, merely 20% of the SCF resistance and 228% higher in power output. Structural characterizations confirmed that SCF adopts a cubic perovskite phase and an orthorhombic perovskite phase, while B1SCF forms a unique perovskite-spinel heterostructure composed of a cubic perovskite matrix and a spinel phase. Results from oxygen temperature-programmed desorption (O₂-TPD), thermogravimetry (TG), iodometric titration, and X-ray photoelectron spectroscopy (XPS) indicated that the perovskite-spinel heterostructure effectively increases oxygen vacancy concentration, promotes oxygen adsorption and dissociation, and enhances proton hydration capability. These findings are consistent with distribution of relaxation times (DRT) analysis, which revealed that the heterostructure accelerates charge transfer and surface oxygen exchange processes, thereby mitigating the sluggish ORR kinetics. In addition, in a symmetrical cell configuration, B1SCF with the perovskite-spinel heterointerface maintained excellent long-term operational stability for over 240 h at 550 °C, and exhibited robust thermomechanical endurance after 36 harsh thermal cycles (∼180 h) in the range of 450-700 °C, with a minimal degradation rate of 1.23 × 10^-5 Ω cm² h^-1. Collectively, the perovskite-spinel heterointerface enabled B1SCF is an excellent PCFC air electrode with superior performance and durability.