Current Status of Dense Cermet Membranes for Hydrogen

Argonne National Laboratory (ANL) and the National Energy Technology Laboratory (NETL) are developing dense membranes for separating hydrogen from mixed gases, particularly product streams generated during coal gasification, methane partial-oxidation, and water-gas shift reactions. Hydrogen separation with these membranes is nongalvanic, i.e., it does not use electrodes or an external power supply to drive the separation, and hydrogen selectivity is nearly 100% because the membranes contain no interconnected porosity. Novel cermet membranes (i.e., ceramic-metal composites) have been developed to separate hydrogen from gas mixtures at high temperature and pressure. These membranes (ANL-1, -2, and –3) are classified according to the hydrogen transport properties of their metal and ceramic phases. We report results regarding the thermodynamic stability and electrical conductivity of BaZr0.8-xPrxY0.2O3 (x £ 0.8), materials that are being considered as a potential ceramic matrix in ANL-1 and –2 membranes. The hydrogen flux through ANL-3e membranes is reported as a function of membrane thickness and partial pressure of hydrogen in the feed gas. The results indicate that the hydrogen flux through ANL-3e membranes is limited by bulk diffusion of hydrogen through the metal phase in the thickness range of 22-100 mm. The highest hydrogen flux for an ANL-3e membrane (19.0 cm (STP)/min-cm) was measured with a 22-mm-thick sample at 900°C using 100% H2 as the feed gas. This value is only slightly lower than the record flux measured with an ANL-3a membrane (20 cm3(STP)/min-cm2). The hydrogen flux of ANL-3e membranes was stable for up to 120 h in atmospheres containing up to 400 ppm H2S. While longer-term (>120 h) studies are needed, these results indicate that the membranes may be suitable for practical applications.