A central component of a solid oxide fuel cell is the electrolyte which consists of an oxygen ion conducting ceramic material. Such ion conducting materials can also be used for membranes. We are studying both oxygen ion conducting and proton conducting materials.
Materials exhibiting both high oxygen ion conductivity and electronic conductivity (so-called mixed conduction) can, if they posses sufficient thermodynamic stability, be used for oxygen separation membranes. A specific use of such mixed conducting membranes is in a membrane reactor for production of synthesis gas (a mixture of CO and H2, which is the starting point for a number of industrially important syntheses, e.g. of methanol and ammonia). The membrane route has some environmental advantages and has the potential of becoming cost competitive with the existing industrial routes. The technology is currently pursued both by several multinational industrial consortia and by many university groups. The materials considered for this application further have potential use as electrode materials for solid oxide fuel cells. We are currently studying possible membrane materials among the class of perovskites (materials with a crystal structure that resembles the structure of the mineral perovskite, CaTiO3).
Proton conducting materials can, when having additional electronic conductivity, function as hydrogen permeable membranes. Many chemical reactions in the petrochemical industry, such as cracking and reforming of hydrocarbons, depend on the transfer of hydrogen. It would be advantageous to make this transfer via a hydrogen permeable membrane: Hydrogenation would be performed using impure hydrogen while, in dehydrogenation, hydrogen would be available in a pure form. Presently available membranes based on palladium support a high flux of hydrogen, but their mechanical stability is limited and their catalytic properties favour the formation of coke. It is desirable to find alternative oxide based materials with hydrogen permeability.
Work over the last 20 years has shown that small amounts of hydrogen dissolve in oxides, especially oxides with basic cations, resulting in appreciable conductivity of H+ ions (protonic conductivity). To achieve hydrogen permeability, the oxide must have a component of electronic conductivity in addition to the ionic one. This could be done by doping a single phase oxide or by forming a two-phase composite material. We are investigating the structure and transport properties of a number of doped oxides, mainly perovskites.