(Invited) Membranes of Mixed Ionic and Electronic Conductors for Gas Separation: Effect of Surface and Interfaces

Dense membranes of mixed ionic-electronic conductors represent a highly efficient and economically viable approach to hydrogen and oxygen separation. For example, a “cermet” membrane consisting of a ceramic proton conductor and a metal (e.g., nickel) could be a promising technical solution for recovery of high-purity hydrogen from synthetic gas mixtures during various industrial processes such as coal gasification and water gas-shift reaction.  In particular, yttrium/ytterbium doped barium zirconate-cerate (BZCYYb) exhibits the highest proton conductivity in the temperature range suitable for hydrogen separation.  Further, Ni-BZCYYb cermets have high tolerance to sulfur poisoning and coking under typical conditions for hydrocarbon reformation.  To date, however, there is little fundamental understanding of the surface processes relevant to hydrogen separation.  A fundamental understanding of the charge and mass transport processes associated with H₂ dissociation and proton incorporation (or H₂ evolution) on the surfaces of a cermet hydrogen permeable membrane is vital to achieving rational design of a new generation of hydrogen permeable membranes.  In this presentation, we will highlight our recent progress in understanding the effects of surface and interfaces on gas separation processes, aiming at elucidating the mechanisms that control proton incorporation or H₂ dissociation at the surfaces of the membranes, the nature of the proton solubility sites, and the diffusion process through the membrane.