Analysis of Hydrogen Permeation Through Sub-Micron-Thick Palladium Alloy Membranes

Separation of hydrogen from the reaction products stream leaving fuel processor is an essential step prior to its introduction to the fuel cell. To this end, we are developing micromachined Pd/Ag alloy membranes for in-situ hydrogen separation suitable for integration with catalytic fuel reforming microreactors. In this work, we report an analysis of mass transport and kinetics of hydrogen permeation through a non-porous palladium membrane for the case of the sub-micron membrane thickness. A simplified model has been developed which divides the permeation into seven distinct regimes; gas phase mass transport to the surface, adsorption onto the surface, transition into the bulk material, solid-phase diffusion through the bulk material, transition to the effluent surface, desorption into the gas phase and diffusion away from the surface. Historically, this permeation process is limited by the bulk diffusion step and therefore membrane thickness controls permissible flux. Based on the present model applied to the sub-micron membrane, and utilizing accepted values from the literature, desorption of hydrogen from the Pd surface back into the gas phase has been identified as the rate-limiting step for this process below a ‘critical’ temperature. This result indicates that careful consideration of membrane packaging is critical for maximizing hydrogen flux for the sub-micron membrane and that further efforts to increase flux via thinner membranes would be futile.