High performance yttrium-doped BSCF hollow fibre membranes

Oxygen production from BSCF (Ba0.5Sr0.5Co0.8Fe0.2O3−δ) and yttrium-doped BSCF (Ba0.5Sr0.5Co0.8Fe0.175Y0.025O3−δ) hollow fibres was investigated, and the role of yttrium in the crystal structure was further explored using high-temperature X-ray diffraction. Yttrium substitution acted to increase the oxygen flux significantly, from 4.9 to 7.0 ml cm−2 min−1 at 900 °C for the BSCF and the BSCFY membranes, respectively. Permeation was particularly enhanced at lower temperatures, between 66% and 92% over the range 650–800 °C. The lattice expansion determined from high temperature X-ray diffraction measurements in air was similar for both compositions, suggesting that the higher oxygen fluxes obtained for BSCFY hollow fibres could be attributed to the higher non-stoichiometry due to yttrium addition to the BSCF crystal structure. In addition, the improvement of oxygen fluxes for small wall thickness (∼0.3 mm) hollow fibres operating below the critical length (i.e. limited surface kinetics regime) indicates that yttrium has enhanced the surface exchange rates. XRD patterns showed split peaks around 2θ 31° and 56° above 200 °C, likely corresponding to a coexisting hexagonal perovskite phase. This peak-splitting was more pronounced for BSCFY, suggesting that the kinetics of the hexagonal phase formation may be faster for the yttrium-doped perovskite. The lattice volume of BSCFY expanded more than BSCF when exposed to nitrogen at 900 °C, confirming a higher release of oxygen and enhanced oxygen non-stoichiometry.

► Yttrium addition to BSCF improved oxygen flux from 4.9 to 7.0 ml cm−2 min−1. ► HT-XRD suggests kinetics of hexagonal phase formation faster for BSCFY. ► Lattice expansion as a function of temperature is similar for BSCF and BSCFY. ► BSCFY expands more in nitrogen at 900 °C, suggesting increased non-stoichiometry. ► Increased fluxes in BSCFY also attributed to improved surface kinetics.