Protonic ceramic electrochemical cells (PCECs) have attracted considerable attention owing to their ability to reversibly convert chemical fuels into electricity at low temperatures below 600 °C. However, extreme sintering conditions during conventional convectionbased heating induce critical problems for PCECs such as nonstoichiometric electrolytes and microstructural coarsening of the electrodes, leading to performance deterioration. Therefore, we fabricated PCECs via a microwave-assisted sintering process (MW-PCEC). Owing to the ultrafast ramping rate (∼50 °C/min) with bipolar rotation and the resistive heating nature of microwave-assisted sintering, undesirable cation diffusion and grain growth were effectively suppressed, thus producing PCECs with stoichiometric electrolytes and nanostructured fuel electrodes. The MW-PCEC achieved electrochemical performance in both in fuel cell (0.85 W cm −2 ) and in electrolysis cell (1.88 A cm −2 ) modes at 600 °C (70% and 254% higher than the conventionally sintered PCEC, respectively) demonstrating the effectiveness of using an ultrafast sintering technique to fabricate high-performance PCECs.

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Solid oxide electrochemical cells (SOCs) are promising energy conversion and storage systems owing to their high efficiency and low environmental impact. To lower operating temperatures, the state‐of‐the‐art SOCs with highly active cobaltite‐based oxygen electrodes essentially require doped‐ceria interlayers to avoid undesirable reactions with commercially available zirconia electrolytes. However, the inherent cation interdiffusion between ceria and zirconia materials at high temperatures (>1300 °C) has retarded the construction of highly dense and stoichiometric ceria/zirconia bilayers. This study reports the fabrication of a highly conductive, ultra‐thin (250 nm), and defect‐free Sm0.075Nd0.075Ce0.85O2‐δ (SNDC) interlayer via readily processable gelatin‐assisted deposition. The SOC with the gelatin‐derived SNDC interlayer achieved exceptionally high electrochemical performances both in the fuel cell (≈3.34 W cm‐2) and electrolysis mode (≈2.1 A cm‐2 at 1.3 V) at 750 °C—one of the best records for SOCs with similar configuration to date—along with excellent long‐term durability (1500 h). Mechanistic analysis reveals that the ultra‐thin and dense structure of the SNDC interlayer provides a faster route for oxygen‐ion conduction and more active sites for both oxygen reduction and oxygen evolution reactions at the oxygen electrode/electrolyte interface. The findings suggest that the thin and dense gelatin‐derived SNDC interlayer has great potential for use in high‐performance reversible SOCs.

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Fast oxygen-ion conductors for use as electrolyte materials have been sought for energy conversion and storage. Bi2O3-based ionic conductors that exhibit the highest known oxygen-ion conductivities have received attention for use in next-generation solid electrolytes. However, at intermediate temperatures below ~600 °C, their conductivities degrade rapidly owing to a cubic-to-rhombohedral phase transformation. Here, we demonstrate that physical manipulation of the grain structure can be used to preserve the superior ionic conductivity of Bi2O3. To investigate the effects of microstructural control on stability, epitaxial and nanopolycrystalline model films of Er0.25Bi0.75O1.5 were fabricated by pulsed laser deposition. Interestingly, in situ impedance and ex situ XRD analyses showed that the grain boundary-free epitaxial film significantly improved the stability of the cubic phase, while severe degradation was observed in the conductivity of its polycrystalline counterpart. Consistently, the cation interdiffusion coefficient measured by the Boltzmann–Matano method was much lower for the epitaxial thin film compared to the polycrystalline thin film. Furthermore, first-principles calculations revealed that the presence of grain boundaries triggered the structural resemblance between cubic and rhombohedral phases, as evidenced by radial distribution functions. Additionally, phase transition energetics predicted that the thermodynamic stability of the cubic phase with respect to the rhombohedral counterpart is reduced near grain boundaries. Thus, these findings provide novel insights into the development of highly durable superionic conductors via microstructural engineering.