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

Abstract

The combined effect of minor yttrium doping and silver catalyst deposition on the surface kinetics (k_chem) and bulk diffusion (D_chem) of BSCF (Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3–δ) perovskite membranes was explored using electrical conductivity relaxation (ECR) and validated using oxygen permeation measurements. Yttrium doping of BSCF to form Ba_0.5Sr_0.5Co_0.8Fe_0.175Y_0.025O_3-δ (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 °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 °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 ∼0.4 mm and validating the ECR measurements.