High-Performance Co-Free Ruddlesden–Popper-Type Perovskites by In Situ-Controlled Exsolution-Defined Nanocomposites for Protonic Ceramic Fuel Cell Cathodes

Evolving protonic ceramic fuel cell (PCFC) cathodes require excellent oxygen reduction reactivity (ORR), high triple (H⁺/O^2–/e^–) conductivity, and adequate operational stability at intermediate-to-low temperatures. In this work, a brand new nanocomposite compound was designed by applying the cathodic surface modification method on A-site-deficient Ruddlesden–Popper-type (RP) Pr_2.7Ni_1.6Cu_0.3Nb_0.1O_7‑δ (P2.7NCNO) possessing a triple-conducting property for PCFC cathodes. This nanocomposite contains the primary phase of the RP structure, with NiO nanoparticles evenly dispersed upon its surface. The ORR activity improves with polarization resistance reaching 0.25 Ω·cm² at 600 °C. The quick charge transfer and oxygen surface exchange benefit from the Nb and Cu codoping and surface NiO nanoparticles based on distribution of relaxation time (DRT) analysis. Furthermore, P2.7NCNO exhibits higher proton conductivity under different atmospheres owing to Nb and Cu codoping. Excellent results are observed at 600 °C when used as the cathode in PCFC single cells, achieving a peak power density of 1024 mW·cm^–2. Moreover, the P2.7NCNO sample exhibits appropriate endurance durability (600 mA·cm^–2 at 600 °C for ∼130 h) and acceptable thermal compatibility with proton conductor electrolytes BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3‑δ (BZCYYb) thanks to the Co-free structure. These results demonstrate that the surface modification strategy of proper composition manipulation on the RP structure provides useful guidance for future study and optimization for intermediate-to-low-temperature PCFC cathodes.