Mixed-conducting dense ceramics for gas separation applications

Mixed-conducting (electronic and ionic conducting) dense ceramics are used in many applications, including fuel cells, gas separation membranes, batteries, sensors, and electrocatalysis. This paper describes mixed-conducting ceramic membranes that are being developed to selectively remove oxygen and hydrogen from gas streams in a nongalvanic mode of operation (i.e., with no electrodes or external power supply). Ceramic membranes made of Sr-Fe-Co oxide (SFC), which exhibits high combined electronic and oxygen ionic conductivities, call be used for high-purity oxygen separation and/or partial oxidation of methane to synthesis gas (syngas, a mixture of CO and H-2) The electronic and ionic conductivities of SFC were found to be comparable in magnitude. Steady-state oxygen permeability of SFC has been measured as a function of oxygen-partial-pressure gradient and temperature. For a approximate to 3-mm-thick membrane, the oxygen permeability was approximate to 2.5 scc cm(-2) min(-1) at 900 degrees C. Oxygen permeation increases as membrane thickness decreases. Tubular SFC membranes have been fabricated and operated at 900 degrees C for approximate to 1000 h in converting methane into syngas. The oxygen permeated through the membrane reacted with methane in the presence of a catalyst and produced syngas. We also studied the transport properties of yttria-doped BaCeO3-delta (BCY) by impedance spectroscopy and open-cell voltage (OCV) measurement. Total conductivity of the BCY sample increased from approximate to 5 x 10(-3) Omega(-1).cm(-1) to approximate to 2 x 10(-2) Omega(-1).cm(-1), whereas the protonic transference number decreased from 0.87 to 0.63 and the oxygen transference number increased from 0.03 to 0.15 as temperature increased from 600 to 800 degrees C. Unlike SFC, in which the ionic and electronic conductivities ale nearly equivalent, BCY exhibits protonic conductivity that is significantly higher than its electronic conductivity. To enhance the electronic conductivity and therefore to increase hydrogen permeation, metal powder was combined with the BCY to form a cermet membrane. Nongalvanic permeation of hydrogen through the BCY-cermet membranes was demonstrated and characterized as a function of membrane thickness.