Effect of doping, microstructure, and CO2 on La2NiO4+δ-based oxygen-transporting materials

Alkaline earth-free La2NiO4+δ based materials were synthesized by a sol–gel method and studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques as well as oxygen permeation experiments. Effects of doping the nickel position with a variety of cations (Al, Co, Cu, Fe, Mg, Ta, and Zr) were investigated with regards to oxygen flux and microstructure. Doping was always found to diminish the oxygen flux as compared to the reference composition. However, larger grains, which were achieved by longer annealing times at 1723 K have a minor negative impact on oxygen permeation flux in case of La2NiO4+δ and La2Ni0.9Fe0.1O4+δ system. Mössbauer spectroscopy shows that the iron-doped system exhibits a secondary phase, which was identified by high-resolution transmission electron microscopy (HRTEM) as a higher Ruddlesden-Popper phase. In-situ XRD in an atmosphere containing 50 vol% CO2 and long-term oxygen permeation experiments using pure CO2 as the sweep gas revealed a high tolerance of the materials towards CO2.