Dense ceramic oxygen separation membranes can pass oxygen perm-selectively at
elevated temperature and have potential for improving the performance and reducing
the cost of several industrial processes: such as the conversion of natural gas to
syngas, or to separate oxygen from air for oxy-fuel combustion in electricity
generation (to reduce NOx emissions and facilitate CO2 sequestration). These
pressure-driven solid state membranes are based on fast oxygen-ion conducting
ceramics, but also need a compensating flow of electrons. Dual-phase composites are
attractive since they provide an extra degree of freedom, compared with single phase
membranes, for optimising the overall membrane performance. In this study,
composites containing gadolinia doped ceria (CGO, Ce0.9Gd0.1O2- ) and either
strontium-doped lanthanum cobaltite (LSC, La0.9Sr0.1CoO3- or La0.6Sr0.4CoO3- ) or
silver (Ag) were investigated for possible application as oxygen separation
membranes in oxy-fuel combustion system. These should combine the high oxygen
ion conductivity of CGO with the high electronic conductivity and fast oxygen
surface exchange of LSC or silver.
Dense mixed-conducting composite materials of LSC/CGO (prepared by powder
mixing and sintering) and Ag/CGO composites (prepared by silver plus copper oxide
infiltration method) showed high relative density (above 95%), low background gas
leakage and also good electrical conduction. The percolation threshold of the
electronic conducting component was determined to be approximately 20 vol.% for
both LSC compositions and 14 vol.% for Ag. Isotopic exchange and depth profiling
by secondary ion mass spectrometry was used to investigated the oxygen tracer
diffusion (D*) and surface exchange coefficient (k*) of the composites. Composites
just above the electronic percolation threshold exhibited high solid state oxygen
diffusivity, fast surface exchange activity moderate thermal expansion and sufficient
mechanical strength thus combining the benefits of their constituent materials. The
preliminary work on oxygen permeation measurement showed that the reasonable
magnitude of oxygen fluxes is possible to be achieved. This indicates that the
composites of LSC/CGO and Ag/CGO are promising for further development as
passive oxygen separation membranes
