Sihyuk Choi

Most protonic ceramic fuel cell (PCFC) cathodes rely on Co‐rich materials to achieve high electrochemical performance; however, their thermomechanical incompatibility with proton‐conducting electrolytes is the primary obstacle for the practical realization of PCFC systems. Despite significant progress over the past few years, simultaneously achieving thermomechanical stability and excellent electrochemical performance remains an unresolved challenge. Herein, a rationally designed composite cathode is presented that synergistically combines a robust Sr‐doped LaMnO ₃ (LSM) backbone with infiltrated electrocatalytically active PrBa _0.5 Sr _0.5 Co _1.5 Fe _0.5 O _5+δ (PBSCF) nanoparticles (NPs). The optimized cathode delivers an outstanding peak power density of 0.80 W cm ⁻² at 500 °C, while also maintaining stable performance over 100 cycles between 550 and 400 °C in power generation mode at 0.5 V. This high electrochemical performance is attributed to the newly established routes for H ⁺ /O ²⁻ transport through well‐interconnected PBSCF NPs. To elucidate the high level of thermomechanical stability, a computational thermal stress analysis is conducted, validating that the LSM backbone effectively offsets thermal expansion mismatch between the electrode and electrolyte. Additionally, this study provides practical design guidelines for optimizing material combinations and microstructural engineering to ensure reliable operation in PCFC systems.

Reversible protonic ceramic cells (R‐PCCs) offer a compelling solution for efficient energy conversion and storage at intermediate temperatures (400–600 °C); however, their practical implementation and overall electrochemical performance are severely constrained by sluggish electrochemical reaction kinetics at the air electrode. Herein, a novel triple ionic–electronic conducting material is presented, the Ni‐doped layered perovskite PrBa _0.5 Sr _0.5 Co _1.8 Ni _0.2 O _5+δ (PBSCN20), to be utilized as an air electrode in R‐PCCs. Thermogravimetric analysis and density functional theory calculations demonstrate that Ni doping at the Co site significantly promoted oxygen vacancy formation while simultaneously facilitating proton uptake and migration. Consequently, the R‐PCCs with a PBSCN20 air electrode exhibited outstanding electrochemical performance, attaining peak power densities of 1.30 and 0.60 W cm ⁻² in fuel cell mode, and current densities of −1.72 and −0.41 A cm ⁻² at 1.3 V in electrolysis mode at 600 and 500 °C, respectively, as well as superior long‐term stability for over 700 h at 500 °C.

Cr species and CO₂ rapidly degrade the PCFC electrolyte BaCe_0.4Zr_0.4Y_0.1Yb_0.1O_3−δ, while the air electrode PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ remains stable. A thin PBSCF layer enhances air electrode/electrolyte interface stability.

The suppressed mismatch in thermal expansion coefficient between a thermomechanically durable Sr/Co-free PrNi_0.5Fe_0.5O_3−δ air electrode and a typical BaZr_0.4Ce_0.4Y_0.1Yb_0.1O_3−δ electrolyte leads to significantly enhanced thermal cycling stability.

Systems at intermediate temperatures (400-600oC) due to their remarkable capability for reversible operation in both hydrogen production and electricity generation. To successfully implement this system, intensive research is mainly focused on the development of air electrodes with outstanding chemical stability, catalytic properties, and structural flexibility. This review provides a detailed overview and summary of the recent research trends on layered perovskite-type air electrodes for R-PCCs. In particular, the effects of A-, A’-, and B-site doping elements into a chemical formula of AA’B2O5+δ of layered perovskite structure, where A is a trivalent lanthanide, A’ is an alkaline earth element, and B is a transition metal ion are discussed. Furthermore, the final section presents suggestions for future studies and emphasizes the possibility of improving the electrochemical performance for highly efficient R-PCCs.

The proton‐conducting oxides, widely employed as electrolytes in ceramic electrochemical cells, exhibit remarkable proton conductivity that facilitates efficient energy conversion processes. However, their inherent refractory nature poses a challenge in producing chemically stoichiometric and physically dense electrolytes within devices. Here a novel approach is presented, dual‐phase reaction sintering, which can overcome the inherent low sintering ability of the representative BaCeO_3‐δ‒BaZrO_3‐δ proton conducting oxides. This approach involves the simultaneous transformation of a two‐phase mixture (comprising fast‐sintering and slow‐sintering phases) into a complete single‐phase solid solution compound, along with the densification of the electrolyte, all accomplished within a single‐step heating cycle. During the dual‐phase reaction sintering process, the grains of the fast‐sintering phase experience rapid growth owing to their intrinsic superior sintering ability. Additionally, this growth is augmented by the Ostwald ripening behavior manifested by the smaller slow‐sintering phase. This synergistic strategy is validated using BaCe_0.4Zr_0.4Y_0.1Yb_0.1O_3‐δ, and its applicability in electrochemical cells is demonstrated, resulting in a significant enhancement in performance. These findings offer insights into streamlining the preparation of refractory ion‐conducting ceramic electrolytes while maintaining their intrinsic properties for practical applications.