A mixed ionic and electronic conducting membrane includes a two-phase
solid state ceramic composite, wherein the first phase comprises an
oxygen ion conductor and the second phase comprises an n-type
electronically conductive oxide, wherein the electronically conductive
oxide is stable at an oxygen partial pressure as low as 10.sup.-20 atm
and has an electronic conductivity of at least 1 S/cm. A hydrogen
separation system and related methods using the mixed ionic and
electronic conducting membrane are described.