Exploring the Processing of Tubular Chromite- and Zirconia-Based Oxygen Transport Membranes

Tubular oxygen transport membranes (OTMs) that can be directly integrated in high temperature processes have a large potential to reduce CO₂ emissions. However, the challenging processing of these multilayered tubes, combined with strict material stability requirements, has so far hindered such a direct integration. We have investigated if a porous support based on (Y₂O₃)_0.03(ZrO₂)_0.97 (3YSZ) with a dense composite oxygen membrane consisting of (Y₂O₃)_0.01(Sc₂O₃)_0.10(ZrO₂)_0.89 (10Sc1YSZ) as an ionic conductor and LaCr_0.85Cu_0.10Ni_0.05O_3−δ (LCCN) as an electronic conductor could be fabricated as a tubular component, since these materials would provide outstanding chemical and mechanical stability. Tubular components were made by extrusion, dip coating, and co-sintering, and their chemical and mechanical integrity was evaluated. Sufficient gas permeability (≥10⁻¹⁴ m²) and mechanical strength (≥50 MPa) were achieved with extruded 3YSZ porous support tubes. The high co-sintering temperature required to densify the 10ScYSZ/LCCN membrane on the porous support, however, causes challenges related to the evaporation of chromium from the membrane. This chemical degradation caused loss of the LCCN electronic conducting phase and the formation of secondary lanthanum zirconate compounds and fractures. LCCN is therefore not suitable as the electronic conductor in a tubular OTM, unless means to lower the sintering temperature and reduce the chromium evaporation are found that are applicable to the large-scale fabrication of tubular components.