Measurement and modeling of hydrogen transport through high-flux Pd membranes

The permeability of H2-selective palladium membranes fabricated with microsystem technology has been studied. The permeation experiments have been carried out at temperatures between 623 and 873 K at H2 feed partial pressures of 0.2–1.0 bar. At 823 K, a permeance based on the free membrane area of 18 mol H2/m2 s bar0.58 has been measured for a Pd membrane with a thickness of 0.5 μm. From the permeation experiments the rate determining transport step and the stability of the membranes have been determined. At 873 K the H2/He selectivity decreases rapidly, indicating the formation of pinholes at higher temperatures. From a combination of experiments and computer simulations the limiting transport step has been determined to be H-atom diffusion through the membrane at elevated temperatures and H2 surface reactions at the retentate side at lower temperature

High-flux palladium-silver alloy membranes fabricated by microsystem technology

In this study, hydrogen selective membranes have been fabricated using microsystem technology. A 750 nm dense layer of Pd (77 wt%) and Ag (23 wt%) is deposited on a non-porous 1 mm thick silicon nitride layer by cosputtering of a Pd and a Ag target. After sputtering, openings of 5 μm are made in the silicon nitride layer to create a clear passage to the Pd/Ag surface. As a result of the production method, these membranes are pinhole free and have a low resistance to mass transfer in the gas phase, as virtually no support layer is present. The membranes have been tested in a gas permeation system to determine the hydrogen permeability as a function of temperature, gas flow rate, and feed composition. In addition, the hydrogen selectivity over helium has been determined, which appears to be above 1500. At 0.2 bar partial hydrogen pressure in the feed, the hydrogen permeability of the membranes has been found to range from 0.02 to 0.95 mol.H2/m2×s at 350 and 450°C, respectively. It is expected that by improving the hydrodynamics and increasing the operation temperature, substantially higher fluxes will be attainable

Microsystem technology for high-flux hydrogen separation membranes

The application of thin hydrogen-selective membranes suffers from the occurrence of pinholes and a significant resistance to mass transfer in the porous support. To overcome these problems, Pd, Pd/Ag and Pd–Ta–Pd membranes with a thickness between 0.5 and 1.2 μm have been deposited on a dense and smooth surface of a silicon wafer. After membrane deposition, the underground has been etched to release the membrane surface for H2 permeation. Membranes have been prepared with a 1 μm thick microsieve as the support or without support. The prepared membranes have been characterized by the H2 and He flux as a function of temperature (623–723 K) and feed composition (0 < pH2 < 0.83 bar). The highest H2 flux, 3.6 mol H2/(m2 s), has been found with a microsieve-supported 1 μm thick Pd/Ag membrane at 723 K and 0.83 bar hydrogen partial pressure. The fluxes measured here are approximately one order of magnitude higher than the fluxes reported in the literature for Pd or Pd alloy membranes deposited on porous supports. Moreover, helium could not be detected in the permeate, thus indicating the absence of pinholes