Dense Ceramic Membranes for Hydrogen Separation

In the 1980s and 1990s, the development of oxygen ion conductors for solid electrolytes, as well as mixed oxygen–electron conductors as electrodes for solid oxide fuel cells, brought scientists to envision a possibility of mixed-conducting oxygenpermeable ceramic membranes. Today this field is well established, and ceramic membranes are close to implementation in processes for oxygen extraction from air as well as for direct partial oxidation of natural gas. The technology seems to offer superior routes for efficient power production from fossil fuels, combined with CO2 sequestration. Many materials with very high oxygen ion and electronic conductivities are available. It was not equally obvious that dense ceramic hydrogen-permeable membranes would be of similar interest. There are clearly needs for hydrogen purification membranes, but polymers and microporous materials as well as metals such as palladium and its alloys appeared to fill these needs. In addition, possible candidates for dense ceramic hydrogen-permeable materials were not as appealing as the oxygen-permeable ones in terms of performance and stability. As plans for fossil-fuel-based power plants incorporating oxygen-permeable membranes were developed, it became clear that hydrogen-permeable membranes might find use as well. In particular, this would be the case if the temperature of operation was high enough that hydrogen-permeable ceramic membranes could be thermally integrated with other processes, such as reforming, oxygen separation by membranes, gas turbines, or solid oxide fuel cells. Several research groups and industries have therefore focused on the possibility of developing materials and related technologies for ceramic hydrogen-permeable membranes. At present, the known hydrogen-permeable dense ceramic materials are oxides that are mixed proton–electron conductors. We would claim that ceramic hydrogen-separation membranes are in most aspects more challenging than their oxygen-permeable counterparts. Proton transport at high temperatures is fast, but thermodynamics speaks against a high concentration of protons in the materials at high temperatures. Combinations of both high protonic and electronic conduc-