Chemical stability and H sub(2) flux degradation of cercer membranes based on lanthanum tungstate and lanthanum chromite

Ceramic-ceramic composite (cercer) membranes of (Mo-doped) lanthanum tungstate, La sub(27)(W,Mo) sub(5)O sub(55.5- delta ), and lanthanum chromite, La sub(0.87)Sr sub(0.13)CrO sub(3- delta ), have recently been shown to exhibit H sub(2) permeabilities among state-of-the-art. The present work deals with the long-term stability of these cercer membranes in line with concern of flux degradation and phase instability observed in previous studies. The H sub(2) permeability of disc shaped membranes with varying La/W ratio in the lanthanum tungstate phase (5.35 less than or equal to La/W less than or equal to 5.50) was measured at 900 and 1000 degree C with a feed gas containing 49% H sub(2) and 2.5% H sub(2)O for up to 1500 h. It was observed that the H sub(2) permeability decreased by a factor of up to 5.3 over 1500 h at 1000 degree C. Post-characterization of the membranes and similarly annealed samples was performed by SEM, STEM and XRD, and segregation of La sub(2)O sub(3) was observed. The decrease in H sub(2) permeability was ascribed to the compositional instability of the cation-disordered lanthanum tungstate under the measurement conditions. Equilibration of the La/W ratio by segregation of La sub(2)O sub(3) leads to a lower ionic conductivity according to the materials inherent defect chemistry. Partial decomposition and reduction of the lanthanum tungstate phase, presumably to metallic tungsten, was also observed after exposure to nominally dry hydrogen.