For practical utilization of proton-conducting ceramic fuel cells and electrolyzers, it is essential to lower the sintering temperature and processing time of BaZrO₃-based proton conductors. We investigated the effect of sintering temperature and time on the structural and electrochemical properties of dense BaZr_0.8Y_0.2O_3−δ (BZY) prepared by a solid-state reactive sintering process, using NiO as a sintering aid. The sintered BZY prepared from the micronized precursor powder exhibited a density higher than 93%, and an average grain size in the range of 0.6 to 1.4 μm. The orthorhombic BaY₂NiO₅ phase was also observed in the sintered BZY from the combined conventional and synchrotron X-ray diffraction measurements. Electrochemical impedance spectroscopy showed that the total proton conductivities of BZY can be modulated by sintering temperature in a wet reducing atmosphere. The maximum total ion transport number achieved was 0.89 at 600 °C, and the maximum power density of the symmetric BZY electrolyte supported cell with Pt electrodes was 5.24 mW·cm⁻² at 900 °C.

Recently, a number of studies on oxygen transport membranes employing zirconia composites as the oxygen ion conductor have been reported, given that stabilized zirconia has superior robustness due to its high mechanical strength, a low sintering temperature, and a chemical expansion coefficient that makes it suitable for commercialization. However, the drastic decrease in the oxygen permeability due to the inter-diffusion reaction between zirconium and rare-earth metals in zirconia–perovskite composites remains a conundrum for the industrial application of oxygen permeation membranes. In this study, we propose a breakthrough strategy for dramatically enhancing the oxygen flux of a zirconia-based dual-phase membrane by applying a new coating material and minimizing the perovskite content. The membrane comprises a mixture of scandia-stabilized zirconia and strontium-doped lanthanum manganate with a volume ratio of 70 : 30. The Ruddlesden–Popper oxide structure (Nd₂NiO_4+δ), which exhibits low reactivity with zirconium and a high surface exchange coefficient, as well as high chemical stability in the presence of CO₂, was used as a new coating material to improve the exchange reaction on the membrane surface. The highest oxygen permeation flux of up to 1.65 mL cm⁻² min⁻¹ was accomplished with the zirconia-based membrane at 900 °C under an air/He atmosphere. This value is much higher than that of La_0.6Sr_0.4CoO_3−δ (LSC), a representative perovskite coating material with high surface exchange kinetics, used as a membrane coating. Superior stability was also observed in a long-term test where pure CO₂ was used as the sweep gas.

A mixed ionic electronic conductor (MIEC) membrane with thermo-mechanical and chemical stability has been developed. A fluorite-rich dual-phase composite (80 vol% Ce_0.9Gd_0.1O_2−δ–20 vol% La_0.7Sr_0.3MnO_3−δ) based on the chemically stable pure electronic conductor oxide (LSM) and doped-ceria (GDC) was used as the membrane material. By introducing a thermo-mechanically and chemically stable coating material (Pr₂NiO_4+δ) to the membranes, this study proposes a new strategy for enhancing the overall stability of the oxygen permeation ceramic membrane. The stability of the dual-phase membrane was assessed in the presence of CO₂ at intermediate temperatures, and thermal cycling tests were performed to evaluate its performance under extreme conditions. The dual-phase membrane with Pr₂NiO_4+δ coating layer not only showed remarkable stability in a pure CO₂ atmosphere but also exhibited thermo-mechanical stability during rapid thermal cycling tests (cooling and heating rate: 30 °C min⁻¹).