Mechanically‐Enhanced, Single‐Phased, and Triple‐Conducting Air Electrode for Robust Oxygen‐Ion and Proton Conducting Ceramic Cells

Abstract Reversible proton‐conducting fuel cells (Re‐PCFCs) are poised to become the next generation of solid‐state ionic devices for direct conversion between hydrogen and electricity. However, their commercialization has been hindered by the absence of a high‐performance triple‐conducting air electrode that combines excellent electrochemical activity with superior mechanical robustness. Here, a robust single‐phased air electrode is successfully developed with its mechanical strength and electrochemical activity greatly co‐enhanced, by employing high valence Nb doping to stabilize the cubic perovskite lattice of pristine BaCe 0.2 Fe 0.8 O 3‐δ . The resulting BaCe 0.1 Fe 0.8 Nb 0.1 O 3‐δ (BCFNb) air electrode demonstrates exceptional mechanical properties in terms of Young's modulus (by 47%) and fracture toughness (by 67%). Meanwhile, the distribution of relaxation times (DRT) and Oxygen temperature‐programmed desorption (O 2 ‐TPD) characterization reveals the enhanced oxygen mobility, surface exchange kinetics, and the mixed conductivity of oxygen ions and protons that synergistically resulted in the remarkably enhanced electrochemical activity—only a low area‐specific resistances of 0.262 Ω·cm 2 at 550 °C, translated into a high power densities of 1.091 W cm − 2 at 650 °C with degradation rates <0.005 mV h − ¹ in fuel cell mode and 0.14 mV h − ¹ in electrolytic mode at 550 °C. These results highlight the potential of single‐phased perovskite as air electrode for mechanically and electrochemically robust Re‐PCFCs.