Mixed-conducting dense ceramics for gas separation applications

摘要

Mixed-conducting (electronic and ionic conducting) dense ceramics are used in211u001emany applications, including fuel cells, gas separation membranes, batteries, 211u001esensors, and electrocatalysis. This paper describes mixed-conducting ceramic 211u001emembranes that are being developed to selectively remove oxygen and hydrogen from 211u001egas streams in a nongalvanic mode of operation (i.e., with no electrodes or 211u001eexternal power supply). Ceramic membranes made of Sr-Fe-Co oxide (SFC), which 211u001eexhibits high combined electronic and oxygen ionic conductivities, can be used 211u001efor high-purity oxygen separation and/or partial oxidation of methane to 211u001esynthesis gas (syngas, a mixture of CO and H(sub 2)). The electronic and ionic 211u001econductivities of SFC were found to be comparable in magnitude. Steady-state 211u001eoxygen permeability of SFC has been measured as a function of oxygen-partial-211u001epressure gradient and temperature. For an (approx)3-mm-thick membrane, the oxygen 211u001epermeability was (approx)2.5 scc(center-dot)cm(sup (minus)2)(center-dot)min(sup 211u001e(minus)1) at 900 C. Oxygen permeation increases as membrane thickness decreases. 211u001eTubular SFC membranes have been fabricated and operated at 900 C for (approx)1000 211u001eh in converting methane into syngas. The oxygen permeated through the membrane 211u001ereacted with methane in the presence of a catalyst and produced syngas. We also 211u001estudied the transport properties of yttria-doped BaCeO(sub 3(minus)(delta)) (BCY) 211u001eby impedance spectroscopy and open-cell voltage (OCV) measurement. Total 211u001econductivity of the BCY sample increased from (approx)5 x 10(sup (minus)3) 211u001e(Omega)(sup (minus)1)(center-dot)cm(sup (minus)1) to (approx)2 x 10(sup (minus)2) 211u001e(Omega)(sup (minus)1)(center-dot)cm(sup (minus)1), whereas the protonic 211u001etransference number decreased from 0.87 to 0.63 and the oxygen transference 211u001enumber increased from 0.03 to 0.15 as temperature increased from 600 to 800 C. 211u001eUnlike SFC, in which the ionic and electronic conductivities are nearly 211u001eequivalent BCY exhibits protonic conductivity that is significantly higher than 211u001eits electronic conductivity. To enhance the electronic conductivity and therefore 211u001eto increase hydrogen permeation, metal powder was combined with the BCY to form a 211u001ecermet membrane, Nongalvanic permeation of hydrogen through the BCY-cermet 211u001emembranes was demonstrated and characterized as a function of membrane thickness.