Combined investigation of bulk diffusion and surface exchange parameters of silver catalyst coated yttrium-doped BSCF membranes

The combined effect of minor yttrium doping and silver catalyst deposition on the surface kinetics (kchem) and bulk diffusion (D-chem) of BSCF (Ba0.5Sr0.5Co0.8Fe0.2O3-d) perovskite membranes was explored using electrical conductivity relaxation (ECR) and validated using oxygen permeation measurements. Yttrium doping of BSCF to form Ba0.5Sr0.5Co0.8Fe0.175Y0.025O3-d (BSCFY) improved both the surface exchange kinetics and the bulk diffusion by an average of 44% and 177% respectively, supporting improved oxygen permeation measurements. The deposition of a silver catalyst on BSCFY further improved the surface kinetics by 63-450% at intermediate operating temperatures (600-750 degrees C), and reduced the activation energy from 163 to 90 kJ mol(-1). Interestingly, these improvements did not translate into enhanced oxygen fluxes for the silver coated thicker 0.5 and 1 mm membranes, indicating that the oxygen ion transport was limited by bulk diffusion. However, oxygen permeation measurements on catalyst-coated 0.3 mm-thick membranes yielded improvements of 20-35% in the range 600-900 degrees C. The silver catalyst was beneficial in overcoming surface kinetic limitations for the thinner 0.3 mm BSCFY membranes, thus suggesting that the critical thickness of BSCFY membranes lies around similar to 0.4 mm and validating the ECR measurements.