Enhanced Performance of La2NiO4+δ Oxygen-Transporting Membranes Using Crystal Facet Engineering via Microemulsion-Based Synthesis

La₂NiO_4+δ nanorods, synthesized via reverse microemulsion─a crystal facet engineering method─served as building blocks for developing oxygen transport membranes. Comparisons were drawn with ceramic membranes derived from commercial La₂NiO_4+δ nanoparticles. The membrane manufacturing process involved either conventional sintering or the field-assisted sintering technique/spark plasma sintering. The microstructure analysis of the initial powders and the resulting ceramics was thoroughly assessed by X-ray diffraction, scanning and transmission electron microscopy as well as energy-dispersive X-ray spectroscopy. As a consequence of the reaction conditions, the nanorods possess an orthorhombic crystal structure, with LaOBr present as a minor phase. Furthermore, the surface structure of the La₂NiO_4+δ nanorods was discerned via selected area electron diffraction, revealing a composition of (001)_o-type and (11¯$\overline{\mathrm{1}}$$\overline{\mathrm{1}}$0)_o-type facets on the sides and (110)_o-type facets at the end, with additional facets observed between these surfaces. Among the sintering techniques, spark plasma sintering demonstrated superior performance, when applied to La₂NiO_4+δ nanorods, as it effectively preserved their rod-like nanostructure during the sintering process. The resulting nanorod-derived La₂NiO_4+δ ceramics exhibited excellent oxygen permeation, largely due to the large proportion of orthorhombic (11¯$\overline{\mathrm{1}}$$\overline{\mathrm{1}}$0)_o-type surfaces in the rod-shaped grains, which correspond to tetragonal (010)_t and (01¯$\overline{\mathrm{1}}$$\overline{\mathrm{1}}$0)_t surfaces. The (11¯$\overline{\mathrm{1}}$$\overline{\mathrm{1}}$0)_o-type facets facilitated the oxygen surface exchange, leading to improved oxygen permeation fluxes between 1023 and 1123 K compared to membranes derived from nanoparticles.