In response to escalating concerns about global warming, there is a growing demand for renewable energy conversion systems that utilize H₂ as the energy carrier. Among them, reversible protonic ceramic cells (RPCCs) have been considered highly attractive for energy conversion devices due to their ability to directly convert chemical energy into electrical energy, and vice versa [1, 2]. One exceptional advantage of RPCCs is their operation at lower temperatures (500-700 ℃). This is attributed to proton conductors exhibiting lower activation energy compared to conventional reversible solid oxide cells (RSOCs) using oxygen-ion conductors. These characteristics overcome limitations in ensuring the thermal stability of the cell [3]. In RPCCs, the air electrode requires outstanding activities for both oxygen reduction and evolution reactions (ORR/OER). In particular, to maximize cathodic performance, the air electrode requires triple-conducting (H⁺/O^2–/e^–) properties that can utilize the overall cathode surface [4, 5]. In other words, it can facilitate water formation and help increase the active sites for electrochemical water decomposition. Many efforts have been dedicated to developing various air electrode materials. However, the current level of cathodic activities for practical employment is still elusive, especially due to the lack of proton conduction properties.

BaFeO₃-based materials have received attention for their high oxygen permeability and cost-effectiveness compared to various cobaltite materials. Although they exhibit high oxygen reactivity, they have limited proton capability [6]. Thus, we improve the proton conduction properties of BaFeO₃-based materials using a doping strategy. Firstly, we selected the doping element to improve proton conductivity by reducing hydration energy [7]. Furthermore, since the element has a larger ionic radius than that of Fe³⁺/Fe⁴⁺ in BaFeO₃ lattices, doping it into the Fe-site can reduce the ionic radius difference with Ba²⁺. This enables the formation of a stable cubic perovskite structure rationalized by the Goldschmidt tolerance factor [8]. Achieving a single-phase cubic perovskite structure in BaFeO₃ is indispensable, as it prevents phase transition, leading to decreases in cathode performance. In our work, the doping effect on this material demonstrate a significant decrease in area-specific resistance (ASR) and an increase in conductivity under a humidified air atmosphere, indicating enhanced proton uptake. Besides, our developed air electrode shows remarkable performance in both fuel cell and electrolysis cell mode.