Novel dual-phase oxygen-transporting membranes with compositions 40 wt% Nd_0.6Sr_0.4Al_0.2Fe_0.8O_3−δ–60 wt% Ce_0.9Nd_0.1O_2−δ (NSAF6428–CN91) and 40 wt% Nd_0.5Sr_0.5Al_0.2Fe_0.8O_3−δ–60 wt% Ce_0.8Nd_0.2O_2−δ (NSAF5528–CN82) were successfully synthesized via a one-pot sol–gel method. The oxygen permeation performance and the structural properties of the membranes could be simultaneously improved owing to Al doping of the perovskite phase. The newly developed dense ceramic membranes (0.6 mm thick) displayed long-term stable oxygen permeation fluxes of 0.31 and 0.51 cm³ min⁻¹ cm⁻² under an air/CO₂ oxygen partial pressure gradient at 950 °C for NSAF6428–CN91 and NSAF5528–CN82, respectively. The NSAF6428–CN91 showed a stable oxygen flux of 0.15 cm³ min⁻¹ cm⁻² at 900 °C for 100 h, without any deterioration of the microstructure under pure CO₂ sweeping.